Документ взят из кэша поисковой машины. Адрес оригинального документа : http://hea-www.harvard.edu/~garcia/astroph.0203243.ps
Дата изменения: Wed Feb 9 00:13:34 2011
Дата индексирования: Mon Oct 1 21:32:04 2012
Кодировка:
Поисковые слова: m 35

arXiv:astroph/0203243
v1
15
Mar
2002
X-ray Point Sources in The Central Region of M31 as seen by Chandra
Albert K.H. Kong, Michael R. Garcia, Francis A. Primini, Stephen S. Murray, Rosanne
Di Stefano 1 and Jerey E. McClintock
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
ABSTRACT
We report on Chandra observations of the central region of M31. By combining
eight Chandra ACIS-I observations taken between 1999 and 2001, we have identied
204 X-ray sources within the central 17 0 17 0 region of M31, with a detection limit
of 2 10 35 erg s 1 . Of these 204 sources, 21 are identied with globular clusters, 2
with supernova remnants, 8 with planetary nebula, and 11 with supersoft sources. By
comparing individual images, about 50% of the sources are variable on time scales of
months. We also found 14 transients, with light curves resembling those seen in our
Galaxy. We also extracted the energy spectra of the 20 brightest sources; they can be
well t by a single power-law with a mean photon index of 1.8. The spectral shapes of
12 sources are shown to be variable, suggesting that they went through state changes.
The luminosity function of all the point sources is consistent with previous observations
(a broken power-law with a luminosity break at 1:7 10 37 erg s 1 ). However, when
the X-ray sources in dierent regions are considered separately, dierent luminosity
functions are obtained. This indicates that the star-formation history might be dierent
in dierent regions.
Subject headings: galaxies: individual (M31) | X-rays: galaxies | X-rays: stars
1. Introduction
At a distance of 800 kpc, M31 provides us with a prime opportunity to study the global
properties of a galaxy which is similar to our Milky Way. As M31 and the Milky Way share similar
morphology and size, comparisons between their X-ray properties can be enlightening. Even though
M31 is further away, studies of X-ray sources within it are simplied relative to the Galaxy for
several reasons. Firstly, the distance is well known, allowing us to determine X-ray luminosities
accurately. Secondly, the extinction of X-ray sources within M31 is much lower than that typical of
sources in the Galaxy, which enables us to study the X-ray properties of M31 over a wider energy
band. Finally, the nearly face-on orientation of M31 allows us to more easily determine the location
of X-ray sources within (or outside of) spiral arms, the galactic bulge, or halo.
1 also Department of Physics and Astronomy, Tufts University, Medford, MA 02155

{ 2 {
M31 has been observed extensively by several earlier X-ray missions. The rst detailed ob-
servations were made with the Einstein IPC and HRI (Trinchieri & Fabbiano 1991) and over 100
sources were detected at a detection limit of 10 36 erg s 1 (0.2{4.0 keV). Subsequent ROSAT HRI
observations of the central 34 0 revealed 86 sources at a similar detection limit (Primini, Forman,
& Jones 1993, hereafter PFJ93). A comparison of the Einstein and ROSAT source lists showed that
42% of the sources within the central 7:5 0 region were variable. Recently, two deep and extensive
ROSAT PSPC surveys have identied 560 point sources in the entire disk of M31 (Supper et al.
1997, 2001; hereafter Su97, Su01). The detection limit of this survey was 5 10 35 erg s 1 (0.1{2
keV). These 560 sources include 33 globular clusters, 16 supernova remnants, 15 supersoft sources,
55 foreground stars and 10 background objects.
M31 was observed by Chandra and XMM-Newton soon after these observatories were launched.
In the rst observation (8.8 ks) of the core region by Chandra in 1999 October, 121 point sources
were identied within the central 17 0 17 0 region and the nucleus, which had been seen as one
source by previous missions, was nicely resolved into ve point sources (Garcia et al. 2000a).
Moreover, a bright transient was discovered 26 00 from the nucleus. A relatively deeper XMM-
Newton observation (34.8 ks) was made in 2000 June; 116 sources were detected down to a limiting
luminosity of 6 10 35 erg s 1 (0.3{12 kpc; Shirey et al. 2001) and a pulsating supersoft transient
with a periodicity of 865 s was discovered (Osborne et al. 2001). Moreover, both Chandra
(Garcia et al. 2001a; Primini et al. 2000) and XMM-Newton (Shirey et al. 2001) observations
conrmed that the unresolved X-ray emission in the core region is much softer than most of the
resolved X-ray sources in that region. Fifteen X-ray point sources are newly associated with globular
clusters (Di Stefano et al. 2002), which when combined with the previous ROSAT results (Su01)
brings the total number of M31 globular clusters with detected X-ray emission to 48. The X-ray
luminosity of these globular clusters appears to be substantially higher than clusters in the Galaxy;
for example, in M31 1=3 of the clusters have luminosities LX > 10 37 erg s 1 (0.5{7.0 keV).
Chandra and XMM-Newton also discovered several bright (LX > 10 37 erg s 1 ) transients in M31
and M32 (Garcia et al. 2000a; Garcia et al. 2000b; Osborne et al. 2001; Shirey 2001; Kong et al.
2001; Garcia et al. 2001b). The brightest of these reached a peak luminosity of LX 3 10 38
erg s 1 .
We report herein the properties of the point sources in the central 17 0 17 0 region of M31
as deduced from eight separate 5ks ACIS-I observations spanning 1:5 years. The detection
limit (while variable) is 2 10 35 erg s 1 (0.3{7 keV) across most of this region. In addition to
a source catalog, we discuss the overall spectral properties, temporal variability and luminosity
function of these sources. The brighter sources have suфcient counts to allow meaningful searches
for spectral variability, allowing comparison to Galactic sources. The main focus of this paper
is on the complete source list and the overall properties of point sources in M31. In a series of
companion papers, we will discuss the diuse emission (for instance, see Primini et al. 2000 and
Dosaj et al. 2001 for a preliminary analysis), the X-ray emission from the central supermassive
black hole (Garcia et al. 2002), temporal and spectral variability (Kong et al., in preparation) and

{ 3 {
supersoft source populations (Di Stefano et al., in preparation)
In this paper, we adopt a distance of 780 kpc (Stanek & Garnavich 1998; Macri et al. 2001)
and assume a hydrogen column density equal to the Galactic value of 7 10 20 cm 2 (Dickey
& Lockman 1990) unless otherwise specied. The quoted errors throughout this paper are at 90%
condence, unless otherwise specied.
2. Observations and Data Reduction
M31 was observed with Chandra regularly as part of the AO-1 and AO-2 GTO program during
1999{2001. The program was designed to search for transients. The observations consist of a series
of HRC snapshots ( 1 ks) that cover the entire galaxy. If a transient is discovered in the HRC
mosaic, then a follow-up ACIS image ( 5 ks) of the transient is obtained; otherwise an ACIS
image of the nucleus is obtained. In this paper we focus only on the ACIS-I (I0, I1, I2 and I3) data
obtained for the central 16:9 0 16:9 0 region of M31. These data consist of 8 separate observations,
with exposure times ranging from 4 to 8.8 ks. The details of the observations are given in Table
1. The nuclear region of M31 was placed near the aim-point of the ACIS-I array. The focal-
plane temperature was 110 ф C during the rst four observations, and 120 ф C for the others. The
observations were made at various spacecraft roll angles; consequently, the total region covered by
the observations is slightly larger than 16:9 0 16:9 0 , and sources near the outer edge of ACIS are
not observable in all eight exposures.
All data were telemetered in Faint mode and were collected with a frame transfer time of 3.2
s. In order to reduce the instrumental background, only data with ASCA grades of 0, 2, 3, 4, and
6 were included. We selected data free from bad columns, hot pixels and columns close to the
borders of each ACIS chip node. The standard 0:5 00 pixel randomization was also removed. Only
events with photon energies in the range of 0.3{7.0 keV were included in our analysis. We inspected
the background count rates from the ACIS-S3 chip for all of the observations; except for the rst
observation, no signicant background ares were found. For the rst observation, periods with
high background were rejected, resulting in an eective exposure time of 8.8 ks out of 17.5 ks (see
Garcia et al. 2000a for details of this observation).
The data reduction and analysis was done with CIAO v2.1 2 . Some of the image analysis was
done with IRAF, while the spectra were analyzed with XSPEC v11 3 .
2 http://asc.harvard.edu/ciao/
3 http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/index.html

{ 4 {
3. Analysis
3.1. X-ray Images
In order to create a deep image suitable for the detection of faint sources, the eight observations
were combined into a single stacked image. The rms uncertainty of the aspect solution of Chandra is
1 00 , but there can be systematic errors (up to 2 00 ) for specic ACIS-I observations 4 . These errors
are larger than the PSF, and therefore could degrade the PSF of the stacked image. We removed
these errors by registering all the data sets to the coordinate frame of OBSID00312 using 12 bright
X-ray sources within 5 0 of the aim-point. The positions of these sources were determined from
the weighted centroid of the counts in a 0:5 00 pixel (= one ACIS pixel) image. The cross registration
of the eight data sets is accurate to 0:4 00 .
Because the nuclear region of M31 is particularly crowded (see Garcia et al. 2000a), we were
concerned that the 0:4 00 accuracy achieved above would be insuфcient. We therefore repeated
the procedure above on an image of the central 2 0 2 0 , but used a pixel size of 1=8 00 (= 1/4 ACIS
pixel) scale. This improved the registration in the nuclear region to 0:25 00 .
The absolute astrometry of these images is still limited by the Chandra aspect solution to 1 00 .
In order to allow comparison to HST images (which also have absolute astrometric errors of 1 00 ),
we registered the combined Chandra images to a merged HST image using two globular clusters
detected in both data sets (see Garcia et al. 2002 for details). The exposure of the combined images
ranges between 4 ks (at the extreme edges) and 39.7 ks (in the center). More than 170 sources
have a total integration time of 30 ks.
Figure 1 shows the stacked \true color" image of M31, which is a composite of images from soft
(0.3{1.0 keV), middle (1{2 keV) and hard (2{7 keV) bands. Soft sources appear red, moderately
hard sources appear green, and the hardest sources appear blue. The image has been corrected for
exposure and smoothed slightly with a Gaussian ( = 0:5 00 ) in order to improve the appearance of
point sources. In Figure 2, we show the \true color" image of the central 2 0 2 0 region of M31 in
1=8 00 pixel resolution, with the possible nuclear counterpart (M31 ) marked (Garcia et al. 2002).
3.2. Source Detection
Discrete sources in the stacked image were found with WAVDETECT (Freeman et al. 2002),
a wavelet detection algorithm implemented within CIAO. The central 2 0 2 0 region was treated
separately by using the 1=8 00 pixel image. The two source lists were combined into the master
source list shown in Table 2. We set the detection threshold to be 10 6 and carried out the wavelet
analysis at 9 scales (1,
p
2, 2, 2
p
2, 4, 4
p
2, 8, 8
p
2 and 16). This detection threshold corresponds
4 see http://asc.harvard.edu/mta/ASPECT/

{ 5 {
to < 1 false detection due to statistical uctuations in the background. We then repeated this
procedure on the annular square region between 2 0 and 8 0 in size using an image with 1=2 00 pixels.
On the region outside of the 8 0 8 0 square we used an image with 2 00 pixels. A total of 205 sources
were detected. Source count rates were determined via aperture photometry. The radius of the
aperture was varied with average o axis angle in order to match the 90% encircled energy function.
The average o axis angle was computed for each source based on the o axis angles of the eight
observations weighted by their exposure times. The extraction radius varies from 1 00 near the
aim-point to 17 00 for the sources with the largest o axis angles.
Background was extracted from an annulus centered on each source. In some cases, for example
in the nuclear region, we modied the extraction region to avoid nearby sources. It was also
necessary to modify the extraction radius for some faint sources that are close to more luminous
sources. Every extraction region was examined carefully in the image. The count rate was corrected
for exposure, background variation and instrumental PSF. We examined the image of each detected
source, and found only one case in which the detection was clearly spurious (this source had signal-
to-noise ratio S/N < 2).
Table 2 lists the 204 sources in our catalog, sorted in order of increasing R.A. The columns give
the source number, the IAU approved name, the position (J2000), the net counts, the count rate
and 1- error, hardness ratios (see x3.4) and the 0.3{7.0 keV luminosity. The source numbers have
a prex of r1, r2 and r3. The r1 sources are located in the central 2 0 2 0 or \inner bulge" region.
The r2 sources are located within the central 8 0 8 0 excluding the inner bulge region; we refer to
this annular region as the \outer bulge". The r3 sources are located outside the central 8 0 8 0 in
the \disk" region. This nomenclature is based on optical studies (Morton, Andereck & Bernard
1977); a similar classication was also used by Trinchieri & Fabbiano (1991). The conversion to
luminosities assumes an absorbed power-law spectrum with a photon index of 1.7 and NH = 10 21
cm 2 , which is the typical spectrum of a point source in M31 (Shirey et al. 2001). All sources in
the catalog have S/N > 2:5 and only 5 have S/N < 3. The detection limit for the sources varies
across the image due to the variations of exposure time, background and instrumental PSF, and is
highest near the edges where the PSF broadens rapidly and exposure time is lowest. Over the inner
4 0 of the eld, the detection limit is 2:1 10 4 counts s 1 , which is equivalent to LX 2 10 35
erg s 1 .
3.3. Source Identication
Tables 3{5 summarize the results of matching our new ACIS source catalog with existing
catalogs of M31 objects. Table 3 lists the number of matches found in each of the catalogs we
searched, and also the number of accidental (spurious) matches expected. Table 4 lists the catalog
identications of the ACIS sources and the oset between the cataloged object and the ACIS source.
Table 5 itemizes the matches between our ACIS catalog and the ROSAT HRI source list (PFJ93).
We varied the search radius based on the accuracy of the various catalogs (increasing it as needed),

{ 6 {
and also based on the density of sources in the catalogs (reducing it in order to limit accidental
matches).
We used the following catalogs and corresponding search radii:
ROSAT sources: the ROSAT HRI catalog (PFJ93) | 6 00 search radius.
Globular clusters: the Bologna catalog (Battistini et al. 1987), the catalog by Magnier (1993),
and a recent catalog based on HST data by Barmby (2001) | 3 00 search radius for all three.
Supernova remnants: the lists by d'Odorico et al. (1980), Braun & Walterbos (1993), and
Magnier et al. (1995) | 10 00 search radius.
Planetary nebulae: the catalogs by Ford & Jacoby (1978) and Ciardullo et al. (1989)| 3 00
search radius.
Extragalactic objects: the NASA/IPAC Extragalactic Database (NED) and SIMBAD | 3 00
search radius.
M31 stars: the catalog of 485,425 objects (mainly stars) by Haiman et al. (1994) and the
SIMBAD | 1 00 search radius.
Stellar Nova: as reported in the IAUC during the period covered by our observations | 3 00
search radius.
OB Associations: the catalog of 174 OB associations within M31 identied by Magnier et al.
1993) | 3 00 search radius.
In order to estimate the accidental correlation rate, we used a technique similar to that de-
scribed by Hornschemeier et al. (2001). We shifted all the Chandra sources by 10 00 to the northeast,
northwest, southeast, and southwest, and ran the search for each of the catalogs. The results were
averaged to estimate the accidental matching rate listed in Table 3. With the exception of the
large catalog of M31 stars, the accidental matching rates are generally small, therefore justifying
our choice of search radius. For example, the average accidental matching rate with ROSAT sources
is 7.5, which is < 10% of the total number of ROSAT matches found (77).
We nd two matches with SNR, but predict a random match rate (1.5) that is similar to
what we nd. However, the unusual nature of the two sources which match with the SNR catalog
leads us to believe that these matches are real. Both of these Chandra sources have relatively low
hardness ratio values (see x3.4) and Chandra source r3-63 is resolved into a ring-like object (Kong
et al. 2002). Both of these identications are listed in Table 4, and the total number of true SNR
IDs is listed in Table 3 as two.
We found 40 Chandra sources within 1 00 of objects in the Haiman et al. (1994) catalog, which
consists mainly of stars in the eld of M31. The number of accidental matches even with this
small 1 00 search radius is 34.75, nearly equal to the number found! This is due to the very high
density of objects in this catalog, and leads us to expect that most of the 40 matches are spurious.

{ 7 {
However, one can test these possible identications with the spectral (or hardness ratio) data. X-
ray emission from stars is relatively soft; for instance, the PSPC survey (Su97) showed that the
energy spectrum of a foreground star can be best tted by a Raymond-Smith model with kTRS 1
keV or a power-law model with a photon index > 3. However the Chandra sources which have a
suфcient number of counts for spectral analysis (see x3.5) have much harder spectra ( 1 2)
and are therefore unlikely to be stars. In the remaining cases where there are too few counts we
examined the hardness ratio (HR2; see x3.5). If the ratio is inconsistent with a soft spectrum (i.e.,
if HR2 is greater than 0:5), we similarly rule out a stellar association. In this way we rule out 35
of the possible 40 matches. Table 4 lists the remaining 5 possible stellar identications. We note
the (fortuitous?) agreement in numbers between the number of sources with soft spectra (5) and
the accidental matching rate which leads us to expect that 5 out of our possible 40 matches may
be real. These numbers are summarized in Table 3.
We nd that 77 Chandra sources have counterparts in the ROSAT HRI catalog (see Table 5).
The remaining 127 (= 204 77) Chandra sources were not detected in the ROSAT HRI catalog,
presumably because they are below the ROSAT detection limit or are variable. The Chandra
catalog extends 5 fainter than the ROSAT HRI catalog. We have not compared our Chandra
catalog to the new complete ROSAT PSPC catalog (Su01) because the later catalog covers a much
larger area and has substantially lower resolution, making it less appropriate to compare to this
ACIS data than the HRI catalog. We note that in 5 cases the ROSAT HRI sources match up
with 2 or more Chandra sources, indicating either that these ROSAT sources have been resolved
by Chandra or that one (or more) of the Chandra sources is transient. Alternatively, we note that
we expect 7:5 spurious matches with ROSAT sources, so something like 10% of these multiple
matches may be spurious. The expected numbers of spurious matches leads us to list the \true"
number of matches as 69 in Table 3.
We identify 22 Chandra sources with globular clusters (see Table 4). Twelve of these globular
clusters are identied as X-ray sources for the rst time. Because globular clusters are sometimes
used to register M31 images taken at dierent wavelengths, it may be worth noting that the average
oset between the X-ray and optical positions is 1:6 00 . Chandra source r3-74 is within 3 00 of both
mita165 and mita166, but we note that these clusters themselves are separated by only 1:5 00 . We
therefore allow the possibility that in reality they constitute a single cluster. This possibility, along
with the number of expected spurious matches, leads us to list the number of detected globular
clusters as 21 in Table 3.
We found 10 matches between Chandra sources and planetary nebula (PN), and expect that
2:5 of these are spurious matches. Ford 316 is within 2:4 00 of both r1-21 and r1-33, both of which
are in the very crowded nuclear region of M31. It seems likely that Ford 316 is associated with only
one (but not both) of these Chandra sources. Chandra source r1-26 is within 3 00 of both Ford 21 and
Ford 74, but it seems unlikely that both PN would be associated with a single X-ray source. While
it is unclear which associations might be spurious, these multiple identications and the expected
number of spurious sources leads us to estimate that there are 8 true associations between PN and

{ 8 {
Chandra sources (see Tables 3 and 4).
Planetary nebula in the Milky Way are in general rather weak and soft X-ray sources, with
LX 10 30 erg s 1 and kT 0:5 keV (Guerrero et al. 2000, 2001). The most luminous PN within
our galaxy has LX 1:3 10 32 erg s 1 and kT 0:3 keV (Kastner, Vrtilek, & Soker 2001). These
Chandra sources have luminosities ranging from 2 10 35 erg s 1 to 5 10 37 erg s 1 and (with the
exception of r2-56) X-ray colors much harder than the Milky Way PN, raising the possibility that
these sources are either very unusual or something other than PN. We note that the Ford & Jacoby
(1978) survey identied PN on the basis of on and o band imaging within the 5007 A O [III] line.
Symbiotic stars often show strong 5007 A emission and are generally soft X-ray sources of modest
intensity, with the single exception of GX 1+4, which has a hard spectrum and LX 10 37 erg s 1 .
This suggests that these \PN" may in fact be GX 1+4 analogs in M31. Optical spectroscopy could
help determine the nature of these sources.
We searched for matches between stellar novae as listed in the IAUC that were contempora-
neous with our Chandra observations we found no matches within 3 00 . Because novae may appear
as X-ray sources some time after their optical appearance, we started our search with the nova
reported in IAUC 7093 (Stagni, Buonomo, & di Mille 1999 Jan), which appeared 10 months
before our rst Chandra observation. Other novae include those in IAUC 7218 (Modjaz and Li
1999 July), 7236 (Johnson, Modjaz, & Li 1999 Aug), 7272 (Filippenko et al. 1999 Oct), 7477 (Li,
2000 Aug), 7516 (Donato et al. 2000 Nov) and 7709 (Fiaschi, Di Mille, & Cariolato 2001 Sept).
While outside of our search radius (3 00 ), a nova found in 2001 September (IAUC 7709, Fiaschi, Di
Mille & Cariolato 2001) is within 5 00 of source r2-29. This Chandra source is positionally coincident
with XMMU J004234.1+411808, which Osborne et al. (2001) suggested may be an X-ray nova.
This source was not detected in the ACIS observation of 2000 June 1 (OBSID 309); it reached a
peak ux in an HRC-I observation on 2000 June 6 (OBSID 273), more than one year before the
optical nova (see Figure 5). We know of no cases in which the X-ray outburst of an optical nova
(or an X-ray nova) proceeded the optical outburst by 1 year, which therefore suggests that the
spatial proximity of the two events is a random coincidence. The expectation value of such events
is 0.25, so this is moderately possible.
We also searched for matches with OB associations, as O and B stars may be moderate X-ray
sources (LX < 10 33 erg s 1 , Berghofer et al. 1997). While this is well below our detection limit,
a group of O and/or B stars may reach our detection threshold, and star forming regions could
conceivably harbor massive X-ray binaries. However, we found no matches within our search radius
of 3 00 .
We searched for extragalactic emission line objects as listed in NED and SIMBAD, and found a
single match (r3-83, see Table 4). AGN are likely to be obscured by the bulge of M31 and therefore
we expect that we may detect many AGN without optical counterparts. A better estimate to the
number of such object is made based on deep eld observations in the next section.

{ 9 {
3.4. Hardness Ratios
Many of the sources in our catalog have < 100 counts, which makes it diфcult to derive spectral
parameters with meaningful constraints. However, hardness ratios can give a crude indication of
the X-ray spectra in these cases. We therefore computed the hardness ratios for all the detected
sources. These ratios were based on the source counts in three energy bands: S (0.3{1.0 keV),
M (1{2 keV) and H (2{7 keV). The two hardness ratios are dened as HR1=(M-S)/(M+S) and
HR2=(H-S)/(H+S). Table 2 lists both HR1 and HR2. Figures 3 and 4, respectively, show the
color-color diagram (CD) and hardness-intensity diagram (HID) for sources with over 20 counts.
We have overlaid the CD with 6 lines showing the tracks followed by representative spectra with
diering values of NH . Power-law spectra tend to occupy the top right section of the diagram,
while soft blackbody models occupy the lower left. For example, a \supersoft source" (SSS) having
a blackbody spectrum with a temperature of 70 eV and NH of 10 21 cm 2 would be in the lower
left with HR1= 0:98 and HR2= 1. A typical AGN or X-ray binary with a power-law spectrum
and a photon index = 1:7 would be in the extreme upper right if NH 10 22 cm 2 .
From the CD in Figure 3, it is clear that there are three sources with more than 20 counts and
with both HR1 and HR2 consistent with 1 (r3-84, r2-12, and r3-8). This indicates that they have
no counts above 1 keV, and therefore are very likely SSSs. The brightest of these three sources
(r2-12) is within the central 8 0 8 0 region and has LX > 10 37 erg s 1 . This source was also detected
in the ROSAT surveys and identied as a SSS (Su01).
We searched our catalog for other candidate SSSs, following a method analogous to that of
Su97 and Kahabka (1999). These surveys identied 15 SSSs (Su97) or perhaps 16 additional SSSs
(Kuhabka 1999), depending upon the selection criteria used. Candidate Chandra SSSs are those
with HR2+HR2 1 and HR1 < 0, or HR1+HR1 0:8. There are 14 sources satisfying these
conditions: r1-25, r2-12, r2-19, r2-42, r2-46, r2-56, r2-57, r3-5, r3-8, r3-63, r3-69, r3-77, r3-84, and
r3-96. A single one of these (r1-25) is in the central 2 0 2 0 region. Source r2-12 was noted as a bright
SSS in both the Einstein and PSPC surveys. Source r3-8 was not classied as a SSS in the PSPC
surveys (Su97, Su01) but was subsequently identied as a SSS based on the less tight selection
criteria of Kahabka (1999). Sources r3-63 and r3-69 were both detected in the PSPC survey and
were identied to be SNRs (see Table 4). Source r2-42 may be associated with an 18.6 magnitude
star listed in the Ha94 catalog. Removing this star and the two SNR from consideration, we are
left with 11 SSS candidates, 9 of which are newly discovered.
In the CD and HID, we have marked the three regions of M31 we have dened in dierent
colors: the central 2 0 2 0 (region 1) is blue, the central 8 0 8 0 excluding the central 2 0 2 0 (region 2)
is red, and the entire eld excluding the central 8 0 8 0 (region 3) is green. All three regions show
extensive overlap in both diagrams, with one clear exception: the top right hand corner of the CD
is populated mainly with sources from region 3. The appearance of this gure is born out by the
average hardness ratios for the three regions as listed in Table 6. Both HR1 and HR2 of region 3
are signicantly ( 5) higher than the values for regions 1 and 2, which are themselves consistent

{ 10 {
at the 2 level. Although these sources may be intrinsically hard and within M31, many are
probably strongly absorbed background AGN.
These harder sources may form a distinct group in the CD, i.e., there may be a separate clump
of sources with HR1 > 0:55 and HR2 > 0:45. There are 42 sources that meet these hardness ratio
criteria, 34 of them are in region 3. Based on the Chandra deep eld observations (Brandt et al.
2001), we estimate that there should be 30 serendipitous sources in our eld. Thus, the majority
of these will be background AGN and would therefore be strongly absorbed by the dust and gas in
M31.
3.5. Temporal Variability
The eight Chandra ACIS-I observations described herein span nearly 2 years from 1999{2001.
This is substantially longer than previous surveys by ROSAT (2 observations separated by 1
year). In order to study long-term X-ray variability, we computed a variability parameter following
PFJ93:
S(F max F min ) = jF max F min j
q
2
Fmax + 2
Fmin
; (1)
where F min and F max are the minimum and maximum X-ray ux during the 2 years of observations,
and min and max are the corresponding errors. We dene a source to be a variable if S > 3.
The 99 sources we found to be variable using this criteria are indicated with a \v" in Table 2.
We note that 6 of the sources are excluded from this analysis as they were only observed once.
Thus, the variable sources represent 50% of the total. The minimum amount of variability found
corresponds to a factor of 1.5.
We note that the fraction of variable sources depends upon the region of M31 considered. In
the central 2 0 2 0 region, 73% of the sources are variable, while this fraction drops to 58% and
39% in regions 2 and 3, respectively (see Table 7). One might worry that this apparent trend is an
artifact of the variation in PSF and/or diuse background with o axis angle. The average number
of background counts per source in regions 1 and 2 is < 3, while in region 3 the average is 12. These
background counts could mask small variations in source ux. In order to determine the eect of
the variable PSF, we used MARX 5 to simulate this eect. The maximum and minimum intensity
images of the 24 variable sources found in region 1 were re-projected to random positions within
region 3. The observed background (with Poisson noise) was included. We then re-computed S for
these sources, and nd that the average change from the original numbers is 2%. This indicates
5 http://space.mit.edu/ASC/MARX/

{ 11 {
that the measurement of variability is robust, and gives us some assurance that the dierence in
variability between the three regions is real.
Previous observations may be consistent with this apparent trend. For example, by comparing
ROSAT observations to Einstein observations made 10 years earlier, PFJ93 found that 42%
of the X-ray sources in the central 7:5 0 region were variable. By comparing two XMM-Newton
observations separated by six months Osborne et al. (2001) found that > 15% of the sources in the
central 30 0 were variable.
We have also discovered 13 bright transients which are indicated with a \t" in Table 2. We
dene a bright transient as follows: i) the source has S > 3 and ii) the source is found in at
least one observation with a luminosity of > 5 10 36 erg s 1 and is not detected in at least one
of the other observations. The non-detection must be well above (> 6) the detection limit of
the observation. Note that the luminosity limit covers typical outburst luminosities of soft X-ray
transients and Be/X-ray binaries in our Galaxy.
One important transient was missed by this analysis because it had a peak luminosity below our
\bright transient" threshold during the eight ACIS-I observations considered here. This object is
XMMU J004234.1+411808 (= CXOM31 J004234.3+411809 = r2-29), which Osborne et al. (2001)
and Trudolyubov, Borozdin, & Priedhorsky (2001) suggest is an X-ray nova. Observations with
the HRC-I (OBSIDs 273, 275, and 276), the ACIS-S (OBSIDs 309 and 310) and XMM (Osborne
et al. 2001) allow us to reconstruct the lightcurve (Figure 5), showing that this object had a peak
LX 2:4 0:5 10 37 erg s 1 and an exponential decay with a 20 day e-folding time. This source
increases the number of bright transients discovered in the last two years within M31 to 14.
Two of these transients show typical lightcurves with a fast rise followed by an exponential
decay (r2-29 and r2-28; see Figure 5) like those seen in our Galaxy (Chen, Shrader & Livio 1997). A
dierent sort of behavior is shown by the transient discovered with Chandra in the rst observation
(Garcia et al. 2000a), which remained in outburst for more than one year and nally turned o
in 2001 June (Figure 5). We note that Figure 5 includes data from the HRC-I and ACIS-S which
we do not analyze herein beyond measuring counting rates and luminosities for these few sources.
The temporal variability of the 14 transients and other sources will be described on a future paper
which will fully utilize the HRC and ACIS data (Kong et al., in preparation).
One source, while not a bright transient, deserves special mention because of it's unusual long-
term variability. Source r3-44 (Figure 5) is in the M31 globular cluster Bo86 and shows a possible
200 day modulation. This is reminiscent of the Galactic source 4U 1820{30 in the globular cluster
NGC 6624, which has a 176-d long-term modulation (e.g. Bloser et al. 2000).

{ 12 {
3.6. X-ray Spectra and Spectral Variability
We extracted the energy spectra of the brightest 20 sources from the rst observation (OBSID
303) and t them to simple one-component models consisting of absorbed power-law, blackbody
and Raymond-Smith (RS) shapes. In this paper we limit ourselves to the spectra as seen in this
longest single observation (8.8 ks) because of the complications involved in tting the spectrum of a
source which appears at dierent detector positions and is observed at diering ACIS temperatures
(i.e., with diering detector response matrices). These brightest 20 sources all have > 300 detected
counts, and are well distributed across the eld. They range from within 20 00 of the nucleus
to over 7 0 distant. Circular extraction regions centered on the source positions were applied and
correspond to 90% encircled energy. In order to employ 2 statistics to be used, all spectra were
grouped into at least 20 counts per spectral bin.
All the sources were satisfactorily t with simple absorbed power-law or RS models. Blackbody
models gave very poor ts ( 2
> 2) in most cases, except for sources r3-42, r3-52 and r3-61. Table 8
lists the best tting parameters determined by the power-law and RS ts to these 20 sources. Except
for r2-26 which suers about 20% of pile-up, pile-up can be ignored for other sources. Pile-up will
cause the energy spectrum harder than expected and we therefore extract the spectrum of r2-26
with an annulus region around the source (as the core is heavily aected by pile-up). The photon
index softens to 1.6 comparing to = 1 before pile-up correction (see Table 8), suggesting pile-up
aects the energy spectrum. Fitting the piled-up spectrum with pile-up model developed by Davies
(2001) shows that both results are in good agreement. In addition, a high S/N spectrum obtained
by XMM-Newton also shows similar results, conrming that the correct spectrum of r2-26 is very
typical among M31 X-ray sources. The power-law photon indices of the 20 sources range from 1 to
3 with a mean of 1.8. The average and minimum RS temperatures (kT 16 keV and kT> 2:4 keV,
respectively) are both higher than that of coronal sources (Dempsey et al. 1993), indicating that
none of these 20 sources is likely associated with a foreground star.
The column densities range from eectively zero to 9 10 21 cm 2 and has an average value
of NH = 2 10 21 cm 2 . The Galactic absorption along the line of sight to M31 is 7 10 20 cm 2
(Dickey & Lockman 1990), indicating that most of these bright sources have some additional local
absorption. Figure 6 shows the spectra of the softest ( = 3) and the hardest ( = 1:1) X-ray
sources among these 20 brightest objects.
We searched for spectral variability in all of the sources using a method analogous to that
described in x3.5, but replacing F in equation (1) with HR2. If this newly dened S is greater than
3, the source is identied as a spectral variable and noted as \sv" in Table 2.
Only 12 sources are found to meet our criteria for spectral variability, corresponding to 6% of
the total population. This is of course a lower limit, as small changes in the spectra of weak sources
are undetectable with the number of counts accumulated in our 40 ks of merged data.
In order to further investigate the nature of the spectral variations, we t simple spectra to two

{ 13 {
of the brighter spectral variables. The ts show that as the counting rate increases, the spectrum
becomes harder (see Figure 7). This is reminiscent of atoll and Z sources in our Galaxy (see e.g.
Hasinger & van dar Klis 1989). The luminosity of these two sources ranges from (0:4 1:0) 10 38
erg s 1 , which is higher than the typical luminosity of atoll sources (< 10 37 erg s 1 ). However,
this luminosity is similar to that of the Z sources, which are believed to reach the Eddington limit
(Psaltis, Lamb, & Miller 1995). The luminosity and spectral changes appear consistent with that
seen in Z sources as they move along the \normal branch". These are probably the rst extragalactic
Z sources to be identied, except for LMCX{2 (Smale & Kuulkers 2000). Continued monitoring
may conrm the nature of these sources by revealing the full Z-shape of the spectral variations
observed for galactic Z sources.
4. Luminosity Function
The count rates for sources were converted into unabsorbed 0.3{7.0 keV luminosities by assum-
ing an absorbed power-law model with NH = 10 21 cm 2 and = 1:7. This is a median spectrum, as
can be seen from Figure 3. The resulting conversion between count rate and luminosity is 7:510 38
erg count 1 . Luminosities using this conversion are listed in Table 2. The derived luminosities are
not very sensitive to the assumed spectral parameters, for example varying NH from (5 15)10 21
cm 2 and from 1.2 to 2.0 results in a 20% change in the conversion factor. An upper limit to
the error in this conversion factor might be that found by using the thermal bremsstrahlung model
of PFJ93, which gave luminosity dierences up to 80%. For the brightest 20 X-ray sources, we
derived the 0.3{7.0 keV luminosity directly from spectral ts (see x3.6).
In Figure 8 we plot the cumulative luminosity function (CLF) for all detected sources in
the stacked image, and also plot separately the CLFs of the inner bulge (region 1), outer bulge
(region 2), disk (region 3), and bulge (regions 1+2 combined). Histograms of the number of sources
detected against S/N peak at S/N= 3:5 and fall o below this. Clearly we are incomplete below
this level, and therefore limit our measurements of the LFs to sources with S/N> 3:5. We tted
a broken power-law model to the CLFs using standard 2 minimization techniques and found the
slopes listed in Table 9. The CLF for all sources has a break at (1:69 0:24) 10 37 erg s 1 , with
1 = 0:38 0:01 before the break and 2 = 1:63 0:2 after the break. This result is in good
agreement with previous ROSAT (PFJ93) and XMM-Newton (Shirey et al. 2001) measurements
of the CLF.
The CLF for the inner bulge is signicantly dierent, with a break at a lower luminosity
( 1:5 10 36 ) and a signicantly atter distribution at the faint end. We performed a two-sample
Kolmogorov-Smirnov (K-S) test for the luminosity functions of regions 1 and 3 and found that
there is only a 3% probability that they are drawn from the same distribution.
In order to test if the attening of the CLF of the inner bulge is due to incompleteness, we
performed simulations using MARX. We assumed that the luminosity function of the sources in

{ 14 {
the inner bulge is a single power-law that matches the best-t function above the break ( = 0:7)
and extends to the faint end, and we generated sources at random positions and luminosities which
conformed to this distribution. The diuse emission (and background) were modeled by taking
out all the detected sources, smoothing with a Gaussian and adding Poisson noise. The sources
in the simulated observation were detected using the identical method used previously. Detected
source counts were converted to luminosity using the same conversion factor. We found that the
luminosity function did not show attening below 10 36 erg s 1 . A K-S test indicates that the
simulated luminosity function is dierent from the actual one at a condence level > 99:99% level.
It is therefore likely that the attening below the break is intrinsic and not due to incompleteness.
We determined the slopes for the CLFs (above) via minimum 2 tting to these functions.
However, the counts in a CLF are not independent and therefore this tting method will underes-
timate the errors and may produce a biased best estimate of the slope. In order to more accurately
estimate the errors and slopes, we used a maximum likelihood method (e.g. Crawford, Jauncey &
Murdoch 1970) to determine the slopes in the dierential luminosity functions (DLFs). The slopes
determined by this method are shown in Table 9, where we have added one to the dierential slopes
in order to convert them into the equivalent cumulative slopes.
The slopes above the break seem insensitive to the analysis method. However at the faint end of
the LF, below the apparent breaks, the two methods sometimes yield signicantly dierent slopes.
This is particularly true in the inner bulge region, where the maximum likelihood method indicates
a much smaller change in slope that we found by 2 tting to the CLF. While the appearance of the
LFs (Figure 8) and the results of our MARX simulation (above) indicate the presence of a break to
a atter LF at the lowest uxes, the maximum likelihood method suggests that the statistics in the
inner bulge region are insuфcient to either conrm the presence or constrain the size of a break.
However, there are suфcient counts in the full catalog and in the disk region alone to conrm the
changes in slope using the maximum likelihood method.
The black holes and neutron stars that power many of the M31 X-ray sources have formed
through the evolution of initially massive stars. Because of this, the X-ray LF traces the history
of star formation and evolution of these massive stars in binary systems. Breaks in the LF may
indicate indicate an impulsive star formation event. As the X-ray binaries age, their average
luminosity shifts to lower values and therefore the location of the break may be an indication of
how long ago the star formation event occurred (Wu et al. 2002, Kilgard et al. 2002). Luminosity
functions which do not show a break may indicate that star formation is still occurring. Chandra
observations have measured the breaks in the LFs of several nearby galaxies, e.g. M81 (Tennant et
al. 2001), NGC 1553 (Blanton, Sarazin, & Irwin 2001), NGC 4697 (Sarazin, Irwin, & Bregman 2001)
and M83 (Soria & Wu 2002), and these breaks have been interpreted as evidence for impulsive star
formation. Within M31, we nd that the LF of three regions we studied has a break at a dierent
luminosity. The inner bulge has this break at the lowest luminosity, and the luminosity of the break
increases monotonically as we go out from the inner bulge. If the breaks do indicate the epochs
of star formation events, then these events occurred most recently in the disk of M31 and further

{ 15 {
back in time as we move towards the nucleus of M31.
As well as the monotonic shift in the break luminosity, there is a monotonic shift in the slopes
of the LFs. As we move in towards the nucleus these slopes become progressively atter. This is
somewhat diфcult to understand in the context of the discussion above, because the most luminous
sources would be expected to have the shortest lifetimes. Loss of these sources as they age would
tend to steepen the luminosity function, but we nd atter luminosity functions in the apparently
older populations.
It is interesting to compare the LF of these three regions of M31 to those of other galaxies.
M31 is not the rst galaxy found to show a break in its LF nor is it the rst to show dierent LFs
in dierent regions; both M81 (Tennant et al. 2001) and M83 (Soria & Wu 2002) show similar
behavior. In cases where a single slope is t to the LF (i.e., where there is no clear break) the LFs
of early type galaxies and the bulges of spirals tend to be steeper ( 1:7) than those of spiral
disks and galaxies with active star forming regions ( 0:8, e.g, Prestwich 2001, Kilgard et al.
2002). The opposite seems to be the case within M31: the disk has a steeper LF than the bulge
region. We speculate that this dierence may be related to the location of the breaks in the M31
LF, which are at a somewhat lower luminosity than those seen in other galaxies. At these lower
luminosities we may be sampling a dierent class of source, and the steepness of the LF may be due
to inclusion of this new class of faint sources rather than a loss of bright sources. We note that there
is some evidence that we are sampling a dierent class of sources as we move out from the bulge
because the fraction of sources which show variability decreases monotonically. If these sources
have an intrinsically steeper LF than bright accreting binaries, then they may be responsible for
the steepening of the LFs as we move from the bulge towards the disk.
5. Summary
By using a stacked image (39.7 ks) of M31 from Chandra ACIS-I data taken between 1999 and
2001, we have detected 204 X-ray sources in the central 17 0 17 0 region of M31, with luminosity
above 1:6 10 35 erg s 1 . Of these 204 sources, we identied 21 globular clusters, 2 supernova
remnants, 8 planetary nebula, 11 SSSs and 1 background object. We suggest that an additional 5
source are normal stars based on both their positional coincidence and soft spectra, and another
30 sources may be background AGN based on deep eld observations and their hard (possibly
absorbed) spectra. We do not detect any OB associations or stellar nova.
We nd 10 positional matches with M31 planetary nebular, but expect that 2 of these may
be random coincidences. The X-ray luminosity of these sources is an astounding 5 to 7 orders of
magnitude higher than planetary nebula in the Galaxy. We suggest that these may not be planetary
nebula at all, but analogs to GX 1+4 (i.e., symbiotic stars with a neutron star primary). Optical
spectroscopy of these sources could conrm or refute this suggestion.
About 50% of all the detected sources are variable on time scales of months. We also found

{ 16 {
14 transients, corresponding to 7% of the total population. These transients show a variety
of lightcurves, including the classical \FRED" (fast rise and exponential decay) with an e-folding
decay time of 30 days, an outburst lasting > 1 year, and a possible periodicity of 200 days in
one case. The rst of these behaviors is analogous to X-ray nova in our Galaxy, the last reminiscent
of the Galactic globular cluster source 4U 1820-30.
The median energy spectrum of point sources can be represented by a single power-law with a
photon index of 1:7. There are 11 sources with hardness ratios indicative of SSSs. The spectra
of 12 sources are shown to be variable. The spectral variations and luminosity of the brightest of
these sources is reminiscent of Galactic Z sources on the normal branch.
The luminosity function of all the X-ray point sources is consistent with the ndings of ROSAT
and XMM-Newton observations (PFJ93, Su97 and Shirey et al. 2001), with a break at 1:7 10 37
erg s 1 above which the function steepens. We also found that the luminosity functions of three
separate regions (roughly corresponding to the inner bulge, outer bulge and disk) are dierent. In
particular, the inner bulge shows a break near 10 36 erg s 1 and it shifts monotonically to higher
luminosities when moving outward from the nucleus to the outer bulge and disk. In addition, the
slopes become steeper, indicating that the star formation and evolution histories might be dierent
for the bulge and disk sources. Hence, our Chandra observations reveal dierent star formation
histories even within the central 17 0 17 0 ( 3:9 kpc) region of M31. Future Chandra and XMM-
Newton observations along the disk will denitely improve our knowledge of the star formation
history of the whole galaxy.
We are grateful to Kinwah Wu, Andrea Prestwich and Phil Kaaret for stimulating discussion
and comments. AKHK was supported by a Croucher Fellowship. MRG acknowledges the support
of NASA LTSA Grant NAG5-10889 and NASA Contract NAS8-39073 to the CXC. The HRC GTO
program is supported by NASA Contract NAS-38248. This research has made use of the SIMBAD
database, operated at CDS, Strasbourg, France, and the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with NASA.
REFERENCES
Barmby, P. 2001, PhD thesis, Harvard Univ
Battistini, P., Bonoli, F., Braccesi, A. Federici, L., Fusi Pecci F., Marano, B., & Borngen, F. 1987,
A&AS, 67, 447
Berghofer, T.W., Schmitt, J.H.M.M., Danner, R., & Cassinelli, J.P. 1997, A&A 322, 167
Blanton, E.L., Sarazin, C.L., & Irwin, J.A. 2001, ApJ, 552, 106
Bloser, P.F., Grindlay, J.E., Kaaret, P., Zhang, W., Smale, A.P., & Barret, D. 2000, ApJ, 542, 1000
Brandt, W.N., et al. 2001, AJ, 122, 2810

{ 17 {
Braun, R., & Walterbos, R.A.M. 1993, A&AS, 98, 327
Chen, W, Shrader, C.R., & Livio, M. 1997, ApJ, 491, 312
Ciardullo, R., Jacoby, G.H., Ford, H.C., & Neill, J.D. 1989 ApJ 339, 53
Crawford, D.F., Jauncey, D.L., & Murdoch, H.S. 1970, ApJ, 162, 405
Davies, J.E. 2001, ApJ, 562, 575
Dempsey, R.C., Linsky, J.L., Schmitt, J.H.M.M., & Fleming, T.A. 1993, ApJ, 413, 333
Dickey, J.M., & Lockman, F.J. 1990, ARA&A, 28, 215
Di Stefano, R., Kong, A.K.H., Garcia, M.R., Barmby, P., Greiner, J., Murray, S.S., & Primini, F.A.
2002, ApJ, in press (astro-ph/0106254)
d'Odorico, S., Dopita, M.A., & Benvenuti, P. 1980, A&AS, 40, 67
Donato, L., Garzia, S., Gonano, V., & Sostero, G. 2000 IAUC 7516
Dosaj, A. , Garcia, M.R., Forman, W. Jones, C., Kong, A., DiStefano, R., Primini, F.A., & Murray,
S.S. 2001 in Proceedings \The High Energy Universe in Sharp Focus", in press, ASP, eds.
S. Vrtilek, E.M. Schegel & L. Kuhi
Fiaschi, M., Di Mille, F., & Cariolato, R. 2001 IAUC 7709
Filippenko, A.V., Chornock, R.T., Coil, A.L., Leonard, D.C., & Li W.D. 2000 IAUC 7272
Ford, H.C. & Jacoby, G. 1978 ApJ, 219, 437
Freeman, P.E., Kashyap, V., Rosner, R., & Lamb, D.Q. 2002 ApJS, 138, 185
Garcia, M.R., Murray, S.S., Primini, F.A., Forman, W.R., McClintock, J.E. & Jones, C. 2000a,
ApJ, 537, L23
Garcia, M.R., Murray, S.S., Primini, F.A., McClintock, J.E., & Callanan, P.J. 2000b, IAUC 7498
Garcia, M.R., Murray, S.S., Primini, F.A., Forman, W.R., Jones, C., & McClintock, J.E. 2001a, in
proceeding of IAU205, Galaxies at the Highest Angular Resolution (astro-ph/0012387)
Garcia, M., Kong, A., Primini, F., McClintock, J., Murray, S., & Distefano, R. 2001b, ATel, 79
Garcia, M.R, et al. 2002, in preparation
Guerrero, M.A., Chu, Y.-H., Gruendl, R.A., Williams, R.M., & Kaler, J.B. 2001, ApJ, 553, L55
Guerrero, M.A., Chu, Y.-H., & Gruendl, R.A. 2000, ApJS, 129, 295
Johnson, R., Modjaz, M., & Li, W.D. 1999 IAUC 7236
Haiman, Z., et al. 1994, A&A, 286, 725
Hasinger, G., & van der Klis, M. 1989, A&A, 225, 79
Hornschemeier, A.E., et al. 2001, ApJ, 554, 742
Kahabka, P. 1999, A&A, 344, 459
Kastner, J.H., Vrtilek, S.D., Soker, N. 2001, ApJ, 550, L189

{ 18 {
Kilgard, R.E., Kaaret, P., Krauss M.I., Prestwich A.H., Raley, M.T., Zezas, A. 2002, ApJ, submit-
ted
Kong, A., Garcia, M., Murray, S., Primini, F., McClintock, J., & Di Stefano, R. 2001, ATel 76
Kong, A.K.H., Garcia, M.R., Primini, F.A., Di Stefano, R., Murray, S.S. 2002, in the Proceedings
of the Symposium \New Visions of the X-ray Universe in the XMM-Newton and Chandra
Era", 26-30 November 2001, ESTEC, The Netherlands (ESA SP-488). (astro-ph/0202065)
Li, W.D. 2000, IAUC 7477
Macri, L.M., et al. 2001, ApJ, 549, 721
Magnier, E.A. 1993, PhD Thesis, Massachusetts Institute of Technology
Magnier, E.A., Prins, S., van Paradijs, J., Lewin, W.H.G., Supper, R., Hasinger, G., Pietsch, W.,
& Truemper, J. 1995, A&AS, 114, 215
Magnier, E.A., Battinelli, P., Lewin, W. H. G., Haiman, Z., van Paradijs, J., Hasinger, G., Pietsch,
W., Supper, R., & Truemper, J. 1993 A&A, 278, 36
Modjaz, M., & Li, W.D. 1999 IAUC 7218
Morton, D.C., Andereck, C.D., & Bernard, D.A. 1977, ApJ, 212, 13
Osborne, J.P., et al. 2001, A&A, 378, 800
Prestwich, A.H. 2001, in Proceedings \The High Energy Universe in Sharp Focus", in press, ASP,
eds. S. Vrtilek, E.M. Schegel & L. Kuhi (astro-ph/0108523)
Primini, F., Garcia, M., Murray, S., Forman, W., Jones, C., & McClintock, J. 2000, in proceedings
232 WE-Heraeus Seminar, 22-25 May 2000, Bad Honnef, Germany. Edited by Elly M.
Berkhuijsen, Rainer Beck, and Rene A. M. Walterbos. Shaker, Aachen, p. 145-148
Primini, F.A., Forman, W., & Jones, C. 1993, ApJ, 410, 615 (PFJ93)
Psaltis, D., Lamb, F.K., & Miller, G. S. 1995, ApJ, 454, L137
Sarazin, C.L., Irwin, J.A., & Bregman, J.N. 2001, ApJ, 556, 533
Shirey, R., et al. 2001, A&A, 365, L195
Shirey, R. 2001 IAUC 7659
Smale, A.P., & Kuulkers, E. 2000, ApJ, 528, 702
Stagni, R., Buonomo, U., & di Mille, F. 1999, IAUC 7093
Stanek, K.Z., & Garnavich, P.M. 1998, ApJ, 503, L131
Soria, R., & Wu, K. 2002, A&A, in press (astro-ph/0201059)
Supper, R., Hasinger, G., Pietsch, W., Trumper, J., Jain, A., Magnier, E.A., Lewin, W.H.G., &
van Paradijs, J 1997, A&A, 317, 328 (Su97)
Supper, R., Hasinger, G., Lewin, W.H.G., Magnier, E.A., van Paradijs, J., Pietsch, W., Read,
A.M., & Trumper, J. 2001. A&A, 373, 63 (Su01)

{ 19 {
Tennant, A.F., Wu, K., Ghosh, K.K., Kolodziejczak, J.J., & Swartz, D.A. 2001, ApJ, 549, L43
Trinchieri, G., & Fabbiano, G. 1991, ApJ, 382, 82
Trudolyubov, S.P., Borozdin, K.N., Priedhorsky, W.C. 2001 ApJ, 563, L119
Wu, K., Tennant, A.F., Swartz, D.A., Ghosh, K.K., & Hunstead, R.W. 2001, ApJ, submitted
This preprint was prepared with the AAS L A T E X macros v5.0.

{ 20 {
Table 1. Journal of Chandra Observations
Date Obs ID Exposure time (s)
Oct 13 1999 303 8830
Dec 11 1999 305 4129
Dec 27 1999 306 4132
Jan 29 2000 307 4113
Feb 16 2000 308 4012
Jul 29 2000 311 4894
Aug 27 2000 312 4666
Jun 10 2001 1583 4903

{ 21 {
Table 2. Chandra ACIS catalog of the central region of M31
ID IAU Name R.A. Dec. Positional Net Count Rate HR1 a HR2 b LX c Notes
(CXOM31) (h:m:s) ( ф : 0 : 00 ) Error ( 00 ) Counts (10 2 s 1 ) (10 37 )
r3-110 J004150.2+411337 00:41:50.258 +41:13:37.16 1.7 91 0:22 0:02 0:74 0:43 1:0 1:67 0.17
r3-109 J004150.4+412114 00:41:50.436 +41:21:14.66 1.8 93 0:24 0:03 0:45 0:42 0:42 0:44 0.18
r3-81 J004151.5+411438 00:41:51.504 +41:14:38.29 1.1 164 0:41 0:03 0:70 0:26 0:56 0:31 0.31 v
r3-108 J004154.9+412303 00:41:54.932 +41:23:03.49 1.3 148 0:37 0:04 0:80 0:41 0:81 0:42 0.28
r3-62 J004204.0+411531 00:42:04.005 +41:15:31.97 0.6 81 0:20 0:02 0:72 0:19 0:36 0:30 0.15
r3-107 J004204.3+410930 00:42:04.357 +41:09:30.63 1.7 75 0:19 0:03 0:40 0:32 0:13 0:49 0.14
r3-80 J004205.6+411133 00:42:05.632 +41:11:33.87 1.1 45 0:11 0:02 0:87 0:36 0:59 0:79 0.08
r3-79 J004207.0+411719 00:42:07.019 +41:17:19.17 0.6 44 0:11 0:02 0:88 0:34 0:86 0:39 0.08
r3-94 J004207.4+411025 00:42:07.422 +41:10:25.63 1.0 71 0:18 0:02 0:50 0:27 0:24 0:35 0.13 v
r3-61 J004207.6+411815 00:42:07.619 +41:18:15.17 0.1 1611 4:06 0:10 0:62 0:04 0:49 0:04 3.02 r,v
r3-93 J004208.2+411250 00:42:08.200 +41:12:50.92 1.0 32 0:08 0:02 1:00 0:46 1:00 0:45 0.06
r3-60 J004208.9+412048 00:42:08.952 +41:20:48.42 0.3 435 1:10 0:05 0:73 0:09 0:73 0:10 0.82
r3-59 J004209.3+411745 00:42:09.372 +41:17:45.63 0.2 443 1:12 0:05 0:66 0:07 0:48 0:08 0.83 g,r,v
r3-102 J004209.6+412008 00:42:09.614 +41:20:08.89 1.0 19 0:05 0:01 1:00 0:49 1:00 0:79 0.04
r3-58 J004210.1+411509 00:42:10.164 +41:15:09.90 0.3 148 0:37 0:03 0:52 0:12 0:20 0:15 0.28 r
r3-57 J004210.8+411248 00:42:10.839 +41:12:48.05 0.4 123 0:31 0:03 0:11 0:12 0:10 0:13 0.23
r3-78 J004210.9+410646 00:42:10.954 +41:06:46.95 1.3 213 0:54 0:04 0:09 0:19 0:23 0:19 0.40 v
r3-56 J004211.6+411048 00:42:11.633 +41:10:48.70 0.4 295 0:74 0:04 0:61 0:12 0:57 0:12 0.55 r
r3-55 J004211.8+411648 00:42:11.866 +41:16:48.49 0.3 154 0:39 0:03 0:85 0:15 0:79 0:17 0.29 v
r3-54 J004212.0+411758 00:42:12.026 +41:17:58.86 0.1 538 1:36 0:06 0:72 0:07 0:67 0:08 1.01 g,r,v
r3-92 J004212.6+411244 00:42:12.670 +41:12:44.79 1.0 20 0:05 0:01 1:00 0:58 1:00 0:46 0.04
r3-53 J004212.9+411628 00:42:12.944 +41:16:28.18 0.6 30 0:07 0:01 0:48 0:24 0:08 0:36 0.06 v
r3-52 J004213.0+411836 00:42:13.017 +41:18:36.73 0.1 2805 7:07 0:13 0:72 0:03 0:44 0:04 5.27 r,v
r3-77 J004214.1+412105 00:42:14.195 +41:21:05.82 0.7 13 0:03 0:01 0:83 0:60 0:30 0:39 0.02 v
r3-51 J004214.9+412122 00:42:14.951 +41:21:22.04 0.6 15 0:04 0:01 0:37 0:50 0:27 0:31 0.03
r3-50 J004215.0+411234 00:42:15.011 +41:12:34.23 0.1 1017 2:56 0:08 0:43 0:04 0:10 0:05 1.91 r,v,sv
r3-49 J004215.1+411802 00:42:15.138 +41:18:02.25 0.5 81 0:20 0:02 0:82 0:18 0:56 0:28 0.15
r3-48 J004215.3+412032 00:42:15.350 +41:20:32.27 0.3 205 0:52 0:04 0:57 0:10 0:27 0:12 0.39 r
r3-47 J004215.5+411721 00:42:15.571 +41:17:21.11 0.1 783 1:97 0:07 0:54 0:05 0:24 0:06 1.47 r,v
r3-106 J004215.6+412216 00:42:15.694 +41:22:16.18 1.4 21 0:05 0:01 0:73 0:54 0:18 1:22 0.04 v
r3-76 J004215.9+411552 00:42:15.988 +41:15:52.91 0.6 16 0:04 0:01 0:26 0:63 0:78 0:39 0.03 v
r3-75 J004216.4+411610 00:42:16.462 +41:16:10.55 0.7 15 0:04 0:01 0:37 0:39 0:34 0:40 0.03
r3-91 J004216.8+411856 00:42:16.808 +41:18:56.89 1.0 17 0:04 0:01 0:45 0:39 0:21 0:47 0.03
r3-46 J004216.9+411508 00:42:16.919 +41:15:08.54 0.3 114 0:29 0:03 0:70 0:16 0:54 0:18 0.21 v,t
r3-45 J004218.2+411223 00:42:18.241 +41:12:23.53 0.1 2011 5:07 0:11 0:38 0:03 0:25 0:04 3.78 r,v
r3-44 J004218.5+411401 00:42:18.534 +41:14:01.69 0.1 2428 6:12 0:12 0:43 0:03 0:36 0:03 4.56 g,r,v,sv
r3-90 J004218.8+412004 00:42:18.859 +41:20:04.38 0.7 17 0:04 0:01 0:68 0:38 0:25 0:61 0.03
r3-74 J004219.5+412154 00:42:19.579 +41:21:54.07 0.7 38 0:10 0:02 1:00 0:50 1:00 0:32 0.07 g
r3-89 J004220.2+411313 00:42:20.235 +41:13:13.98 0.8 15 0:04 0:01 0:51 0:46 0:10 0:78 0.03
r3-101 J004220.2+410824 00:42:20.290 +41:08:24.78 1.4 34 0:09 0:02 0:12 0:33 0:04 0:37 0.06
r3-43 J004220.9+411808 00:42:20.997 +41:18:08.55 0.3 88 0:22 0:02 0:81 0:16 0:50 0:24 0.17 v,t
r3-42 J004221.3+411601 00:42:21.382 +41:16:01.32 0.1 1447 3:65 0:10 0:61 0:04 0:42 0:04 2.72 r,v
r3-41 J004221.4+411419 00:42:21.435 +41:14:19.54 0.2 155 0:39 0:03 0:23 0:10 0:12 0:12 0.29 v
r3-40 J004222.3+411333 00:42:22.316 +41:13:33.99 0.1 986 2:49 0:08 0:39 0:04 0:12 0:05 1.85 r,v
r3-88 J004222.5+412234 00:42:22.541 +41:22:34.92 0.9 31 0:08 0:02 0:09 0:28 0:04 0:31 0.06
r3-39 J004222.8+411535 00:42:22.841 +41:15:35.14 0.04 2959 7:46 0:14 0:50 0:03 0:36 0:03 5.56 r,v
r3-73 J004222.8+410738 00:42:22.880 +41:07:38.21 0.8 129 0:33 0:03 0:40 0:18 0:07 0:24 0.24
r3-38 J004223.0+411407 00:42:23.047 +41:14:07.49 0.1 699 1:76 0:07 0:43 0:05 0:17 0:06 1.31 r,v
r2-57 J004224.0+411733 00:42:24.079 +41:17:33.58 0.4 16 0:04 0:01 0:84 0:43 0:94 0:46 0.03
r2-52 J004224.1+411535 00:42:24.109 +41:15:35.84 0.3 27 0:07 0:01 0:16 0:24 0:20 0:24 0.05 v
r2-45 J004225.0+411340 00:42:25.034 +41:13:40.40 0.1 207 0:52 0:04 0:19 0:09 0:03 0:10 0.39 r
r2-36 J004225.9+411915 00:42:25.941 +41:19:15.27 0.1 494 1:25 0:06 0:48 0:06 0:23 0:07 0.93 g,r,v
r3-87 J004226.0+412552 00:42:26.047 +41:25:52.74 1.0 431 1:09 0:05 0:62 0:25 0:66 0:24 0.81 r
r3-37 J004227.5+412048 00:42:27.541 +41:20:48.69 0.7 25 0:06 0:01 0:20 0:30 0:23 0:30 0.05
r3-36 J004228.0+410959 00:42:28.089 +41:09:59.84 0.1 1170 2:95 0:09 0:48 0:04 0:26 0:05 2.20 v
r2-35 J004228.1+411222 00:42:28.190 +41:12:22.76 0.04 2865 7:22 0:14 0:43 0:03 0:24 0:03 5.38 r,v,sv
r3-111 J004228.7+410434 00:42:28.789 +41:04:34.98 1.0 1783 4:49 0:11 0:64 0:09 0:15 0:12 3.35 r
r2-44 J004230.1+411653 00:42:30.166 +41:16:53.41 0.2 15 0:04 0:01 0:70 0:47 0:64 0:52 0.03
r2-43 J004230.8+411910 00:42:30.865 +41:19:10.32 0.2 32 0:08 0:01 0:41 0:22 0:03 0:28 0.06
r2-34 J004231.0+411621 00:42:31.045 +41:16:21.74 0.02 2409 6:07 0:12 0:44 0:03 0:24 0:03 4.52 r,v,sv
r2-33 J004231.1+411939 00:42:31.154 +41:19:39.19 0.1 608 1:53 0:06 0:53 0:06 0:32 0:07 1.14 g,r,v
r2-51 J004231.1+412008 00:42:31.193 +41:20:08.30 0.3 25 0:06 0:01 0:42 0:29 0:14 0:33 0.05
r3-86 J004231.8+412306 00:42:31.898 +41:23:06.19 1.0 27 0:07 0:02 0:99 0:48 0:98 0:79 0.05
r2-32 J004231.9+411314 00:42:31.979 +41:13:14.24 0.04 1487 3:75 0:10 0:22 0:03 0:25 0:03 2.79 r,sv
r2-55 J004232.4+411545 00:42:32.437 +41:15:45.62 0.3 12 0:03 0:01 0:85 0:79 0:93 0:50 0.02
r2-31 J004232.6+411310 00:42:32.648 +41:13:10.77 0.1 306 0:77 0:04 0:44 0:08 0:29 0:09 0.57 v

{ 22 {
Table 2|Continued
ID IAU Name R.A. Dec. Positional Net Count Rate HR1 a HR2 b LX c Notes
(CXOM31) (h:m:s) ( ф : 0 : 00 ) Error ( 00 ) Counts (10 2 s 1 ) (10 37 )
r2-30 J004233.8+411619 00:42:33.806 +41:16:19.88 0.05 362 0:91 0:05 0:36 0:07 0:19 0:08 0.68 r,v
r3-35 J004234.0+412150 00:42:34.069 +41:21:50.23 0.3 134 0:34 0:03 0:77 0:16 0:75 0:17 0.25
r2-29 J004234.3+411809 00:42:34.361 +41:18:09.60 0.1 39 0:10 0:02 0:27 0:21 0:12 0:22 0.07 v,t
r2-28 J004234.6+411523 00:42:34.693 +41:15:23.32 0.1 165 0:42 0:03 0:16 0:09 0:45 0:13 0.31 v,t
r2-27 J004235.1+412006 00:42:35.121 +41:20:06.09 0.1 385 0:97 0:05 0:63 0:09 0:61 0:09 0.72 r,v
r2-42 J004236.5+411350 00:42:36.514 +41:13:50.20 0.2 39 0:10 0:02 0:14 0:16 0:84 0:27 0.07 f
r3-100 J004237.8+410526 00:42:37.862 +41:05:26.07 1.8 69 0:17 0:03 0:65 0:37 0:02 0:78 0.13
r2-26 J004238.5+411603 00:42:38.503 +41:16:03.80 0.01 7749 19:53 0:22 0:53 0:02 0:64 0:02 14.55 v,sv
r2-54 J004238.6+411526 00:42:38.692 +41:15:26.44 0.3 12 0:03 0:01 0:07 0:39 0:62 0:64 0.02 f
r2-25 J004239.4+411428 00:42:39.451 +41:14:28.52 0.1 287 0:72 0:04 0:35 0:07 0:24 0:09 0.54 r,v
r1-15 J004239.9+411547 00:42:39.908 +41:15:47.68 0.02 1100 2:77 0:08 0:29 0:04 0:01 0:04 2.07 p,r,v,sv
r2-24 J004240.1+411845 00:42:40.121 +41:18:45.38 0.1 316 0:80 0:05 0:48 0:08 0:38 0:09 0.59 v
r2-23 J004240.4+411355 00:42:40.446 +41:13:55.33 0.1 18 0:05 0:01 0:62 0:40 0:51 0:44 0.03 v
r2-22 J004240.5+411327 00:42:40.566 +41:13:27.30 0.04 716 1:81 0:07 0:39 0:05 0:13 0:05 1.34 r,v
r3-34 J004240.6+411032 00:42:40.610 +41:10:32.32 0.5 74 0:19 0:02 0:37 0:18 0:38 0:18 0.14 g,v
r3-33 J004240.8+412216 00:42:40.834 +41:22:16.58 0.7 22 0:06 0:01 0:33 0:30 0:58 0:38 0.04 f
r3-85 J004240.9+410701 00:42:40.938 +41:07:01.24 1.1 31 0:08 0:02 1:00 0:51 1:00 0:87 0.06
r3-32 J004240.9+411101 00:42:40.983 +41:11:01.72 0.7 10 0:02 0:01 1:00 0:60 1:00 0:85 0.02
r1-32 J004241.3+411523 00:42:41.355 +41:15:23.94 0.1 130 0:33 0:03 0:42 0:14 0:08 0:17 0.25 g,v
r3-31 J004241.5+412106 00:42:41.566 +41:21:06.02 0.3 142 0:36 0:03 0:52 0:13 0:44 0:13 0.27 v
r1-31 J004241.9+411532 00:42:41.998 +41:15:32.18 0.1 90 0:23 0:02 0:35 0:14 0:32 0:21 0.17 v
r1-5 J004242.0+411608 00:42:42.082 +41:16:08.42 0.01 2779 7:00 0:13 0:44 0:03 0:35 0:03 5.22 v,t
r2-53 J004242.0+411914 00:42:42.093 +41:19:14.20 0.4 8 0:02 0:01 1:00 2:22 1:00 0:56 0.02
r2-21 J004242.2+411445 00:42:42.249 +41:14:45.54 0.05 545 1:37 0:06 0:33 0:06 0:03 0:07 1.02 r,v
r1-14 J004242.3+411553 00:42:42.386 +41:15:53.82 0.02 772 1:95 0:07 0:43 0:05 0:26 0:05 1.45 r,v
r1-30 J004242.4+411659 00:42:42.437 +41:16:59.59 0.1 70 0:18 0:02 0:25 0:16 0:04 0:17 0.13 r
r2-20 J004242.6+411455 00:42:42.632 +41:14:55.63 0.1 41 0:10 0:02 0:13 0:21 0:55 0:28 0.08 v
r1-13 J004242.9+411543 00:42:42.907 +41:15:43.26 0.03 600 1:51 0:06 0:43 0:05 0:27 0:06 1.13 r
r3-99 J004242.9+410830 00:42:42.972 +41:08:30.48 1.1 24 0:06 0:01 0:91 0:58 0:89 0:72 0.04
r1-24 J004243.1+411640 00:42:43.114 +41:16:40.31 0.1 55 0:14 0:02 0:43 0:22 0:48 0:21 0.10 p,v
r2-19 J004243.2+411319 00:42:43.225 +41:13:19.48 0.1 166 0:42 0:03 0:30 0:09 0:96 0:14 0.31
r1-12 J004243.6+411632 00:42:43.661 +41:16:32.60 0.02 620 1:56 0:06 0:39 0:05 0:01 0:06 1.16 r,v
r1-28 J004243.7+411514 00:42:43.715 +41:15:14.54 0.1 16 0:04 0:01 0:30 0:41 0:26 0:40 0.03 v
r1-29 J004243.7+411629 00:42:43.718 +41:16:29.39 0.1 72 0:18 0:02 0:30 0:14 0:15 0:17 0.14
r1-27 J004243.7+411611 00:42:43.770 +41:16:11.37 0.1 39 0:10 0:02 0:37 0:24 0:23 0:25 0.07 v
r1-23 J004243.7+411604 00:42:43.778 +41:16:04.05 0.04 174 0:44 0:03 0:26 0:09 0:04 0:11 0.33 v,t
r1-11 J004243.8+411629 00:42:43.805 +41:16:29.97 0.03 273 0:69 0:04 0:48 0:08 0:31 0:09 0.51 v,t
r1-33 J004244.0+411604 00:42:44.001 +41:16:04.23 0.1 13 0:03 0:01 0:26 0:31 0:53 0:38 0.02 v
r1-22 J004244.2+411614 00:42:44.205 +41:16:14.25 0.1 71 0:18 0:02 0:52 0:17 0:07 0:21 0.13
r1-10 J004244.2+411608 00:42:44.273 +41:16:08.90 0.03 287 0:73 0:04 0:11 0:07 0:14 0:08 0.54 r,v
r1-21 J004244.2+411605 00:42:44.277 +41:16:05.56 0.04 197 0:50 0:04 0:32 0:08 0:09 0:10 0.37 p,v
r1-9 J004244.2+411607 00:42:44.287 +41:16:07.65 0.03 286 0:72 0:04 0:05 0:06 0:86 0:10 0.54 v,t
r3-30 J004244.3+411157 00:42:44.326 +41:11:57.90 0.2 209 0:53 0:04 0:32 0:09 0:02 0:10 0.39 v
r1-8 J004244.5+411618 00:42:44.589 +41:16:18.28 0.04 234 0:59 0:04 0:12 0:07 0:36 0:10 0.44 r
r3-29 J004244.7+411137 00:42:44.766 +41:11:37.76 0.1 1306 3:29 0:09 0:31 0:03 0:14 0:04 2.45 r,v
r2-18 J004244.8+411739 00:42:44.817 +41:17:39.82 0.1 185 0:47 0:03 0:42 0:10 0:18 0:11 0.35
r1-26 J004244.9+411523 00:42:44.995 +41:15:23.30 0.1 111 0:28 0:03 0:17 0:12 0:14 0:14 0.21 p
r2-17 J004245.0+411407 00:42:45.014 +41:14:07.13 0.1 106 0:27 0:03 0:29 0:13 0:08 0:14 0.20 r
r1-4 J004245.0+411621 00:42:45.031 +41:16:21.90 0.02 908 2:29 0:08 0:31 0:04 0:11 0:05 1.70 v
r2-16 J004245.1+411722 00:42:45.141 +41:17:22.49 0.1 308 0:78 0:04 0:41 0:08 0:29 0:09 0.58 v,t
r1-20 J004245.1+411611 00:42:45.148 +41:16:11.31 0.1 125 0:32 0:03 0:06 0:10 0:32 0:13 0.24 v
r1-7 J004245.5+411608 00:42:45.508 +41:16:08.79 0.04 164 0:41 0:03 0:36 0:10 0:09 0:12 0.31 v
r3-72 J004245.6+412434 00:42:45.673 +41:24:34.46 0.8 50 0:13 0:02 0:19 0:34 0:37 0:31 0.09
r1-19 J004245.9+411619 00:42:45.918 +41:16:19.77 0.1 105 0:26 0:03 0:25 0:12 0:05 0:14 0.20 v,t
r2-15 J004245.9+411736 00:42:45.989 +41:17:36.35 0.1 48 0:12 0:02 0:40 0:22 0:36 0:23 0.09 g,v
r1-18 J004246.0+411543 00:42:46.066 +41:15:43.37 0.1 88 0:22 0:02 0:47 0:15 0:06 0:18 0.17 r
r3-28 J004246.8+412119 00:42:46.834 +41:21:19.65 0.3 79 0:20 0:02 0:71 0:20 0:68 0:21 0.15
r1-3 J004246.8+411615 00:42:46.885 +41:16:15.76 0.02 934 2:35 0:08 0:39 0:04 0:05 0:05 1.75 r,v
r1-2 J004247.0+411628 00:42:47.089 +41:16:28.65 0.01 2600 6:55 0:13 0:49 0:03 0:36 0:03 4.88 p,r,v,sv
r3-27 J004247.1+411157 00:42:47.154 +41:11:57.75 0.3 84 0:21 0:02 0:38 0:15 0:06 0:17 0.16 v
r3-26 J004247.7+411052 00:42:47.785 +41:10:52.42 0.5 23 0:06 0:01 0:58 0:36 0:37 0:44 0.04
r1-17 J004247.7+411623 00:42:47.787 +41:16:23.19 0.05 138 0:35 0:03 0:48 0:11 0:18 0:13 0.26 v
r1-25 J004247.8+411549 00:42:47.810 +41:15:49.85 0.1 26 0:07 0:01 0:79 0:28 1:00 0:33 0.05
r1-6 J004247.8+411533 00:42:47.813 +41:15:33.04 0.02 1002 2:53 0:08 0:18 0:04 0:18 0:05 1.88 r
r3-25 J004248.4+412523 00:42:48.423 +41:25:23.10 0.3 574 1:45 0:06 0:60 0:07 0:44 0:08 1.08 r,v
r1-1 J004248.4+411521 00:42:48.450 +41:15:21.31 0.01 3360 8:47 0:15 0:42 0:02 0:28 0:03 6.31 r,v,sv

{ 23 {
Table 2|Continued
ID IAU Name R.A. Dec. Positional Net Count Rate HR1 a HR2 b LX c Notes
(CXOM31) (h:m:s) ( ф : 0 : 00 ) Error ( 00 ) Counts (10 2 s 1 ) (10 37 )
r1-16 J004248.6+411624 00:42:48.637 +41:16:24.64 0.05 144 0:36 0:03 0:22 0:10 0:12 0:12 0.27 v
r3-84 J004248.8+412406 00:42:48.892 +41:24:06.87 0.8 59 0:15 0:02 0:86 0:21 1:00 0:22 0.11 v
r2-41 J004249.0+411742 00:42:49.063 +41:17:42.25 0.2 15 0:04 0:01 0:05 0:33 0:11 0:35 0.03
r2-14 J004249.1+411816 00:42:49.155 +41:18:16.26 0.1 322 0:81 0:05 0:45 0:08 0:34 0:08 0.60 r,v
r3-98 J004249.3+410635 00:42:49.311 +41:06:35.69 1.1 37 0:09 0:02 0:66 0:40 0:09 0:93 0.07
r3-24 J004249.9+411108 00:42:49.913 +41:11:08.64 0.5 38 0:09 0:02 0:14 0:43 0:75 0:24 0.07
r2-40 J004250.1+411813 00:42:50.150 +41:18:13.11 0.3 11 0:03 0:01 0:46 0:54 0:56 0:50 0.02
r2-56 J004250.3+411556 00:42:50.398 +41:15:56.15 0.3 12 0:03 0:01 0:71 0:48 1:0 0:60 0.02 p
r3-71 J004250.6+411033 00:42:50.654 +41:10:33.58 0.7 28 0:07 0:01 0:23 0:28 0:05 0:26 0.05 g
r3-70 J004251.4+412633 00:42:51.461 +41:26:33.58 1.0 80 0:69 0:08 0:30 0:36 0:41 0:37 0.51
r2-39 J004251.5+411302 00:42:51.555 +41:13:02.65 0.2 46 0:12 0:02 0:58 0:23 0:35 0:26 0.09
r2-50 J004252.2+411735 00:42:52.203 +41:17:35.02 0.2 22 0:05 0:01 0:53 0:29 0:09 0:38 0.04
r2-49 J004252.4+411835 00:42:52.448 +41:18:35.24 0.3 12 0:03 0:01 1:00 0:81 1:00 0:54 0.02
r2-12 J004252.4+411540 00:42:52.450 +41:15:40.20 0.03 918 2:31 0:08 0:99 0:05 1:0 0:05 1.72 r,v
r2-13 J004252.4+411854 00:42:52.450 +41:18:54.75 0.02 3943 9:94 0:16 0:49 0:02 0:36 0:02 7.40 r,v
r2-38 J004252.5+411328 00:42:52.543 +41:13:28.72 0.2 13 0:03 0:01 0:09 0:36 0:01 0:37 0.02
r3-69 J004253.5+412553 00:42:53.578 +41:25:53.56 0.7 188 0:47 0:04 0:43 0:12 0:95 0:18 0.35 r,s
r2-11 J004254.8+411603 00:42:54.859 +41:16:03.46 0.02 3282 8:27 0:14 0:42 0:03 0:27 0:03 6.16 r,v,sv
r2-10 J004255.1+411836 00:42:55.108 +41:18:36.33 0.1 179 0:45 0:03 0:39 0:09 0:11 0:12 0.34 v
r3-23 J004255.3+412556 00:42:55.316 +41:25:56.60 0.3 1011 2:55 0:08 0:64 0:08 0:51 0:08 1.90 r,v
r2-9 J004255.5+411835 00:42:55.550 +41:18:35.44 0.1 108 0:27 0:03 0:26 0:12 0:08 0:14 0.20 g,r,v
r2-8 J004256.8+411844 00:42:56.836 +41:18:44.37 0.1 286 0:72 0:04 0:32 0:07 0:06 0:08 0.54 v,t
r3-22 J004257.8+411104 00:42:57.854 +41:11:04.59 0.1 1646 4:15 0:10 0:39 0:03 0:08 0:04 3.09 r
r2-7 J004258.2+411529 00:42:58.257 +41:15:29.46 0.1 487 1:23 0:06 0:36 0:06 0:07 0:07 0.91 r,v
r2-59 J004258.5+411200 00:42:58.543 +41:12:00.14 0.6 15 0:04 0:01 0:23 1:07 0:84 0:39 0.03
r2-58 J004259.0+411158 00:42:59.015 +41:11:58.75 0.6 14 0:03 0:01 1:00 0:71 1:00 0:50 0.03
r3-97 J004259.0+412613 00:42:59.082 +41:26:13.76 1.2 130 0:33 0:03 0:70 0:35 0:18 0:69 0.24
r2-48 J004259.4+411242 00:42:59.480 +41:12:42.26 0.3 41 0:10 0:02 0:33 0:23 0:32 0:24 0.08
r2-6 J004259.5+411919 00:42:59.594 +41:19:19.72 0.04 1678 4:23 0:10 0:38 0:03 0:13 0:04 3.15 g,r,v,sv
r2-5 J004259.8+411606 00:42:59.803 +41:16:06.01 0.03 1356 3:42 0:09 0:43 0:04 0:36 0:04 2.55 g,r,v
r2-37 J004301.0+411351 00:43:01.041 +41:13:51.69 0.2 65 0:16 0:02 0:50 0:17 0:26 0:20 0.12 r
r2-47 J004301.6+411814 00:43:01.638 +41:18:14.98 0.2 15 0:04 0:01 0:41 0:40 0:21 0:62 0.03 v
r3-96 J004301.6+411052 00:43:01.676 +41:10:52.63 0.8 15 0:04 0:01 0:40 0:36 0:67 0:46 0.03 f
r2-46 J004301.7+411726 00:43:01.743 +41:17:26.78 0.3 12 0:03 0:01 0:32 0:38 1:00 0:64 0.02 f
r3-68 J004302.3+411203 00:43:02.343 +41:12:03.71 0.9 17 0:04 0:01 1:00 0:71 1:00 0:44 0.03
r2-4 J004302.8+411522 00:43:02.877 +41:15:22.82 0.04 1430 3:60 0:10 0:40 0:03 0:19 0:04 2.69 g,r,v
r3-21 J004302.9+412042 00:43:02.958 +41:20:42.54 0.3 84 0:21 0:02 0:84 0:20 0:75 0:25 0.16 p
r3-20 J004303.0+411015 00:43:03.089 +41:10:15.18 0.2 284 0:72 0:04 0:21 0:08 0:11 0:08 0.53 r
r2-3 J004303.1+411528 00:43:03.163 +41:15:28.00 0.04 1249 3:15 0:09 0:29 0:03 0:08 0:04 2.35 v,t
r3-19 J004303.2+412122 00:43:03.231 +41:21:22.42 0.1 632 1:59 0:06 0:33 0:05 0:08 0:06 1.19 g,r,v
r2-2 J004303.8+411805 00:43:03.812 +41:18:05.23 0.05 1174 2:96 0:09 0:41 0:04 0:11 0:04 2.21 r,v,sv
r2-1 J004304.1+411601 00:43:04.186 +41:16:01.62 0.1 249 0:63 0:04 0:29 0:08 0:11 0:09 0.47 v
r3-83 J004306.6+412243 00:43:06.618 +41:22:43.82 0.9 27 0:07 0:02 0:50 0:64 0:64 0:52 0.05 e
r3-67 J004306.7+411912 00:43:06.723 +41:19:12.26 0.6 32 0:08 0:02 0:02 0:23 0:24 0:27 0.06
r3-18 J004307.4+412020 00:43:07.472 +41:20:20.09 0.2 195 0:49 0:04 0:37 0:10 0:07 0:11 0.37 g,v
r3-95 J004307.7+412418 00:43:07.738 +41:24:18.16 1.2 33 0:08 0:02 0:11 0:39 0:08 0:37 0.06
r3-17 J004308.6+411248 00:43:08.609 +41:12:48.30 0.2 320 0:81 0:05 0:51 0:09 0:56 0:09 0.60 r
r3-16 J004309.7+411901 00:43:09.791 +41:19:01.22 0.2 396 1:00 0:05 0:35 0:06 0:08 0:07 0.74 r,v,t
r3-15 J004310.5+411451 00:43:10.587 +41:14:51.55 0.04 4432 11:17 0:17 0:39 0:02 0:26 0:02 8.32 g,r,v
r3-14 J004311.3+411809 00:43:11.313 +41:18:09.76 0.5 43 0:11 0:02 1:00 0:35 1:00 0:32 0.08
r3-13 J004313.1+411813 00:43:13.172 +41:18:13.96 0.3 103 0:26 0:03 0:32 0:13 0:02 0:16 0.19 v
r3-12 J004313.9+411712 00:43:13.908 +41:17:12.19 0.8 22 0:05 0:01 0:19 0:37 0:23 0:37 0.04
r3-112 J004314.2+410725 00:43:14.245 +41:07:25.42 0.8 1051 2:65 0:08 0:87 0:34 0:95 0:23 1.97 g,r
r3-11 J004314.2+411651 00:43:14.266 +41:16:51.31 0.6 42 0:11 0:02 0:38 0:21 0:23 0:32 0.08
r3-105 J004314.5+412513 00:43:14.507 +41:25:13.66 1.5 62 0:16 0:02 0:62 0:53 0:27 0:83 0.12 g
r3-10 J004315.4+411125 00:43:15.421 +41:11:25.00 0.6 77 0:19 0:02 0:28 0:16 0:13 0:21 0.14 g,r
r3-9 J004316.0+411841 00:43:16.087 +41:18:41.73 0.3 166 0:42 0:03 0:52 0:11 0:30 0:13 0.31 v
r3-8 J004318.7+412018 00:43:18.773 +41:20:18.52 0.6 85 0:21 0:03 0:96 0:19 1:00 0:19 0.16 v
r3-66 J004320.8+411852 00:43:20.844 +41:18:52.62 0.7 18 0:04 0:01 0:25 0:34 0:45 0:63 0.03
r3-7 J004321.0+411751 00:43:21.058 +41:17:51.19 0.4 194 0:49 0:04 0:35 0:10 0:01 0:12 0.37 p,r,v
r3-104 J004321.4+411558 00:43:21.432 +41:15:58.34 1.0 19 0:05 0:01 1:00 0:49 1:00 0:87 0.04
r3-82 J004322.2+411258 00:43:22.205 +41:12:58.67 1.0 16 0:04 0:01 0:78 0:63 0:70 0:83 0.03
r3-65 J004324.0+411314 00:43:24.070 +41:13:14.34 0.9 28 0:07 0:02 0:79 0:55 0:86 0:42 0.05
r3-6 J004324.8+411728 00:43:24.879 +41:17:28.33 0.5 101 0:25 0:03 0:52 0:17 0:53 0:17 0.19
r3-5 J004325.9+411935 00:43:25.948 +41:19:35.88 0.4 34 0:08 0:02 0:34 0:38 0:76 0:55 0.06
r3-4 J004326.2+411756 00:43:26.250 +41:17:56.87 0.8 22 0:05 0:01 0:11 0:40 0:61 0:79 0.04

{ 24 {
Table 2|Continued
ID IAU Name R.A. Dec. Positional Net Count Rate HR1 a HR2 b LX c Notes
(CXOM31) (h:m:s) ( ф : 0 : 00 ) Error ( 00 ) Counts (10 2 s 1 ) (10 37 )
r3-64 J004326.2+411910 00:43:26.298 +41:19:10.82 1.0 82 0:21 0:03 0:65 0:44 0:81 0:32 0.15
r3-63 J004327.7+411829 00:43:27.763 +41:18:29.83 0.6 258 0:65 0:04 0:45 0:12 1:00 0:18 0.48 r,s
r3-103 J004329.0+410749 00:43:29.038 +41:07:49.11 2.6 1387 3:49 0:10 0:76 0:23 0:84 0:19 2.60 r
r3-3 J004332.3+411041 00:43:32.382 +41:10:41.66 0.4 1419 3:58 0:10 0:80 0:12 0:92 0:08 2.66
r3-2 J004334.3+411323 00:43:34.332 +41:13:23.72 0.4 994 2:51 0:08 0:38 0:06 0:04 0:08 1.87 r,v
r3-1 J004337.1+411443 00:43:37.191 +41:14:43.07 0.4 2264 5:71 0:12 0:55 0:06 0:48 0:06 4.25 g,v
a HR1 = (M{S)/(M+S)
b HR2 = (H{S)/(H+S)
c Luminosity in 0.3{7 keV (erg s 1 )
Note. | e: Extragalactic objects; f: Foreground stars; g: GC; p: PN; r: ROSAT HRI sources; s: SNR;
v: Variables; sv: Spectral variables; t:transients
Table 3. Summary of Source IDs
Object Catalogs Searching Nm N acc N true
type radius ( 00 )
X-ray ROSAT HRI (PFJ93) 6 77 7.5 69
GC Ba87, Magnier (1993), and Barmby (2001) 3 22 1.75 21
SNR DO80, BW83, and Ma95 10 2 1.5 2
PN Ford78, Ci89 3 10 2.5 8
OB Assoc. Magnier et al. 1993 3 0 0.25 0
Nova IAUC 3 0 0.25 0
Extragalactic NED and SIMBAD 3 1 1 1
Stars Ha94 and SIMBAD 1 40 34.75 5
Note. | Nm : Number of all possible matches; N acc : Number of matches by accident; N final :
likely number of true matches.
References. | Ba87: Battistini et al. 1987; DO80: d'Odorico, Dopita, & Benvenuti (1980);
BW83: Braun & Walterbos (1993); Ma95: Magnier et al. (1995); Ford78: Ford & Jacoby
(1978); Ci89: Ciardullo et al. (1989); Ha94: Haiman et al. (1994)

{ 25 {
Table 4. Optical IDs
My Chandra Type Identication Oset
ref. Name ( 00 )
r3-59 CXOM31J004209.3+411745 GC mita140 1.9
r3-54 CXOM31J004212.0+411758 GC Bo78,mita153 2.9,1.8
r3-44 CXOM31J004218.5+411401 GC Bo86 1.9
r3-74 CXOM31J004219.5+412154 GC mita165,166 1.1
r2-36 CXOM31J004225.9+411915 GC Bo96,mita174 2.5,1.9
r2-33 CXOM31J004231.1+411939 GC Bo107,mita192 1.7,1.5
r2-42 CXOM31J004236.5+411350 Star Ha94(238140) 1.0
r2-54 CXOM31J004238.6+411526 Star Ha94(247501) 0.8
r1-15 CXOM31J004239.9+411547 PN Ford17 2.1
r3-34 CXOM31J004240.6+411032 GC mita212 1.0
r3-33 CXOM31J004240.8+412216 Star Ha94(278717) 0.7
r1-32 CXOM31J004241.3+411523 GC mita213 2.3
r1-24 CXOM31J004243.1+411640 PN Ford322 0.9
r1-21 CXOM31J004244.2+411605 PN Ford316 2.4
r1-33 CXOM31J004244.0+411604 2.4
r1-26 CXOM31J004244.9+411523 PN Ford21,74 1.6,2.9
r2-15 CXOM31J004245.9+411736 GC PB-in7 0.3
r1-2 CXOM31J004247.0+411628 PN Ford13 1.8
r2-56 CXOM31J004250.3+411556 PN Ford462 0.5
r3-71 CXOM31J004250.6+411033 GC mita222,PB-in2 1.2,1.8
r3-69 CXOM31J004253.5+412553 SNR DO80(13) 5.5
r2-9 CXOM31J004255.5+411835 GC Bo138 2.0
r2-6 CXOM31J004259.5+411919 GC Bo143 2.4
r2-5 CXOM31J004259.8+411606 GC Bo144 3.0
r3-96 CXOM31J004301.6+411052 Star SK98(237) 0.6
r2-46 CXOM31J004301.7+411726 Star Ha94(261262) 0.6
r2-4 CXOM31J004302.8+411522 GC Bo146 1.5
r3-21 CXOM31J004302.9+412042 PN Ford201 1.8
r3-19 CXOM31J004303.2+412122 GC mita240 1.3
r3-83 CXOM31J004306.6+412243 EO MLA93(686) 0.9
r3-18 CXOM31J004307.4+412020 GC mita246 1.0
r3-15 CXOM31J004310.5+411451 GC Bo153,mita251 2.0,1.1

{ 26 {
Table 4|Continued
My Chandra Type Identication Oset
ref. Name ( 00 )
r3-112 CXOM31J004314.2+410725 GC Bo158 2.9
r3-105 CXOM31J004314.5+412513 GC Bo159,mita258 2.3,1.8
r3-10 CXOM31J004315.4+411125 GC mita260 0.2
r3-7 CXOM31J004321.0+411751 PN Ford209 1.0
r3-63 CXOM31J004327.7+411829 SNR MA95(2-032),DO80(15) 6.0,4.0
r3-1 CXOM31J004337.1+411443 GC mita299 1.4

{ 27 {
Table 5. Cross-correlation of Chandra and ROSAT sources
My Chandra Name ROSAT HRI Oset
ref. Identication ( 00 )
r3-61 CXOM31J004207.6+411815 PF93(4) 0.7
r3-59 CXOM31J004209.3+411745 PF93(5) 2.6
r3-58 CXOM31J004210.1+411509 PF93(6) 2.4
r3-56 CXOM31J004211.6+411048 PF93(7) 1.9
r3-54 CXOM31J004212.0+411758 PF93(8) 2.9
r3-52 CXOM31J004213.0+411836 PF93(9) 2.4
r3-50 CXOM31J004215.0+411234 PF93(10) 3.5
r3-48 CXOM31J004215.3+412032 PF93(11) 1.7
r3-47 CXOM31J004215.5+411721 PF93(12) 2.4
r3-45 CXOM31J004218.2+411223 PF93(14) 1.8
r3-44 CXOM31J004218.5+411401 PF93(15) 2.5
r3-42 CXOM31J004221.3+411601 PF93(17) 2.2
r3-40 CXOM31J004222.3+411333 PF93(18) 2.3
r3-39 CXOM31J004222.8+411535 PF93(20) 2.0
r3-38 CXOM31J004223.0+411407 PF93(21) 2.2
r2-45 CXOM31J004225.0+411340 PF93(22) 2.0
r2-36 CXOM31J004225.9+411915 PF93(23) 2.5
r3-87 CXOM31J004226.0+412552 PF93(24) 1.9
r2-35 CXOM31J004228.1+411222 PF93(25) 3.1
r3-111 CXOM31J004228.7+410434 PF93(26) 2.8
r2-34 CXOM31J004231.0+411621 PF93(27) 1.4
r2-33 CXOM31J004231.1+411939 PF93(28) 1.7
r2-32 CXOM31J004231.9+411314 PF93(29) 2.3
r2-30 CXOM31J004233.8+411619 PF93(32) 5.8
r2-27 CXOM31J004235.1+412006 PF93(34) 1.9
r2-26 CXOM31J004238.5+411603 PF93(35) 1.8
r2-25 CXOM31J004239.4+411428 PF93(36) 2.3
r1-15 CXOM31J004239.9+411547 PF93(37) 1.4
r2-22 CXOM31J004240.5+411327 PF93(38) 1.8
r2-21 CXOM31J004242.2+411445 PF93(39) 0.4
r1-14 CXOM31J004242.3+411553 PF93(41) 1.4
r1-30 CXOM31J004242.4+411659 PF93(40) 1.2

{ 28 {
Table 5|Continued
My Chandra Name ROSAT HRI Oset
ref. Identication ( 00 )
r1-13 CXOM31 J004242.9+411543 PF93(42) 2.0
r1-12 CXOM31 J004243.6+411632 PF93(43) 1.2
r1-11 CXOM31 J004243.8+411629 3.2
r1-29 CXOM31 J004243.7+411629 3.7
r1-10 CXOM31 J004244.2+411608 PF93(44) 1.8
r1-9 CXOM31 J004244.2+411607 2.4
r1-21 CXOM31 J004244.2+411605 4.2
r1-22 CXOM31 J004244.2+411614 5.5
r1-8 CXOM31 J004244.5+411618 PF93(47) 2.1
r1-4 CXOM31 J004245.0+411621 4.5
r3-29 CXOM31 J004244.7+411137 PF93(45) 2.2
r2-17 CXOM31 J004245.0+411407 PF93(46) 2.6
r1-18 CXOM31 J004246.0+411543 PF93(48) 4.6
r1-3 CXOM31 J004246.8+411615 PF93(49) 2.7
r1-2 CXOM31 J004247.0+411628 PF93(50) 1.8
r1-6 CXOM31 J004247.8+411533 PF93(52) 2.5
r3-25 CXOM31 J004248.4+412523 PF93(53) 2.0
r1-1 CXOM31 J004248.4+411521 PF93(54) 2.4
r2-14 CXOM31 J004249.1+411816 PF93(55) 1.5
r2-12 CXOM31 J004252.4+411540 PF93(58) 1.6
r2-13 CXOM31 J004252.4+411854 PF93(57) 0.5
r3-69 CXOM31 J004253.5+412553 PF93(59) 2.7
r2-11 CXOM31 J004254.8+411603 PF93(60) 2.0
r3-23 CXOM31 J004255.3+412556 PF93(61) 1.1
r2-9 CXOM31 J004255.5+411835 PF93(62) 2.0
r2-10 CXOM31 J004255.1+411836 5.0
r3-22 CXOM31 J004257.8+411104 PF93(64) 3.3
r2-7 CXOM31 J004258.2+411529 PF93(65) 2.1
r2-6 CXOM31 J004259.5+411919 PF93(66) 2.4
r2-5 CXOM31 J004259.8+411606 PF93(67) 3.0
r2-37 CXOM31 J004301.0+411351 PF93(68) 3.3
r2-4 CXOM31 J004302.8+411522 PF93(70) 2.3

{ 29 {
Table 5|Continued
My Chandra Name ROSAT HRI Oset
ref. Identication ( 00 )
r2-3 CXOM31J004303.1+411528 3.9
r3-20 CXOM31J004303.0+411015 PF93(71) 2.4
r3-19 CXOM31J004303.2+412122 PF93(72) 3.4
r2-2 CXOM31J004303.8+411805 PF93(73) 3.3
r3-17 CXOM31J004308.6+411248 PF93(74) 4.5
r3-16 CXOM31J004309.7+411901 PF93(75) 1.6
r3-15 CXOM31J004310.5+411451 PF93(76) 1.4
r3-112 CXOM31J004314.2+410725 PF93(77) 3.0
r3-10 CXOM31J004315.4+411125 PF93(78) 4.2
r3-7 CXOM31J004321.0+411751 PF93(79) 2.4
r3-63 CXOM31J004327.7+411829 PF93(80) 2.4
r3-103 CXOM31J004329.0+410749 PF93(81) 2.8
r3-2 CXOM31J004334.3+411323 PF93(82) 5.6
Table 6. Average Hardness Ratios
Region HR1 HR2
Inner bulge 0:27 0:03 0:03 0:03
Outer bulge 0:30 0:05 0:09 0:04
Disk 0:45 0:03 0:27 0:04

{ 30 {
Table 7. Variable sources in M31
Region Detected Number of Fraction Number of Fraction Number of Fraction
sources variables spectral variables transients
Inner bulge 33 24 73% 3 9% 5 15%
Outer bulge 59 34 58% 7 12% 5 8.5%
Disk 106 41 39% 2 2% 3 2.8%
Note. | 6 sources are excluded in the disk region as they were in the eld in only one observation; the
actual total detected sources in this region is 112.

{ 31 {
Table 8. Spectral ts to the 20 brightest sources
Power-law Raymond-Smith a
Source NH 2
=dof Flux b NH kTRS 2
=dof Flux b
Number (10 21 cm 2 ) (10 21 cm 2 ) (keV)
r1-1 2:3 +0:7
0:6 1:7 0:2 0.8/36 1.38 1:9 0:5 7:4 +3:3
1:9 0.8/36 1.32
r1-5 2:3 +0:7
0:6 1:6 0:2 0.9/31 1.25 1:9 0:6 10:9 +14:7
4:2 0.9/31 1.22
r2-3 2:7 +0:6
0:9 2:5 +0:4
0:3 1.2/31 0.49 1:0 0:5 3:3 +1:1
0:8 1.5/13 0.37
r2-5 0:5 +0:8
1 1:1 +0:2
0:3 0.9/11 0.58 0:8 +0:7
0:5 64:0 +1
46:5 1.0/11 0.52
r2-6 1:8 +1:0
0:8 1:9 +0:4
0:3 0.7/12 0.45 0:8 0:6 7:3 +10:5
2:7 0.9/12 0.44
r2-11 1:8 +0:7
0:6 1:8 0:2 1.4/30 0.96 1:0 0:5 7:5 +3:8
1:9 1.5/30 0.94
r2-13 1:8 +0:8
0:6 1:5 0:2 1.0/33 1.31 1:6 0:6 14:8 +28:3
6:2 1.0/33 1.29
r2-26 1:9 +0:7
0:5 1:0 0:1 1.0/62 3.42 3:2 0:5 64:0 +1
20:3 1.3/62 3.07
r2-26 d 3:2 +1:7
1:4 1:6 +0:3
0:2 0.8/22 3.07 2:6 +1:4
1:1 9:0 +13:0
3:5 0.8/22 3.00
r2-32 < 0:5 1:3 +0:2
0:1 0.6/12 0.49 0.0 c 48:6 +1
51:9 0.7/12 0.48
r2-34 1:5 +1:0
0:8 1:9 0:3 1.5/13 0.60 0:7 0:6 6:0 +4:7
1:8 1.5/13 0.57
r2-35 0:9 +0:7
0:6 1:4 0:2 1.0/22 0.89 0:7 0:5 24:4 +1
16:5 1.0/22 0.88
r3-15 0:9 0:5 1:4 0:2 0.9/40 1.62 0:8 +0:4
0:2 30:4 +1
18:3 0.9/40 1.60
r3-16 2:5 +0:2
0:1 1:9 +0:5
0:3 1.0/12 0.50 1:4 +0:8
0:6 6:2 +5:8
1:9 1.1/12 0.46
r3-22 2:6 0:1 2:0 0:3 0.7/13 0.51 1:4 +0:8
0:6 4:6 +2:5
1:3 0.7/13 0.45
r3-39 2:6 0:1 1:8 0:3 0.7/18 0.80 1:8 0:7 7:1 +6:4
2:3 0.8/18 0.76
r3-42 5:9 0:2 2:5 +0:5
0:4 0.5/11 0.65 3:5 +1:2
0:9 3:4 +1:5
0:9 0.6/11 0.46
r3-44 1:6 0:8 1:5 0:2 1.2/23 0.90 1:2 0:6 13:2 +43:2
5:9 1.1/23 0.87
r3-45 3:5 1:0 2:8 0:4 1.0/19 0.79 0:9 0:5 3:1 +0:9
0:6 1.6/19 0.51
r3-52 8:5 +0:2
0:1 3:0 0:3 0.8/26 2.05 5:4 +1:3
0:9 2:4 +0:5
0:4 1.1/26 1.05
r3-61 4:7 0:2 1:8 0:3 1.0/13 0.70 3:8 1:0 6:9 +7:8
2:3 0.9/13 0.67
Note. | All quoted uncertainties are 90% condence.
a Fixed at solar abundence
b Unabsorbed ux in 0.5{10 keV (10 12 erg cm 2 s 1 )
c NH hit the minimum value of 0 allowed by XSPEC
d Pile-up corrected

{ 32 {
Table 9. Luminosity functions of M31
Region Cumulative a Dierential b
1
c 2
d Break 1
c 2
d
(10 37 erg s 1 )
Inner bulge 0:14 0:03 0:70 0:08 0:15 0:04 0:88 +0:45
0:34 0:73 0:25
Outer bulge 0:22 0:03 0:93 0:13 0:57 +0:18
0:12 0:55 +0:17
0:15 1:00 +0:45
0:34
Bulge 0:29 0:02 1:12 0:15 0:89 +0:21
0:17 0:50 0:10 1:15 +0:43
0:33
Disk 0:43 0:02 2:0 +0:46
0:40 1:77 +0:37
0:27 0:51 +0:15
0:12 2:17 +0:91
0:72
Integrated 0:38 0:01 1:63 0:20 1:69 0:24 0:47 +0:07
0:05 1:63 +0:56
0:45
a N (> L) = K 1 L where K 1 is normalization
b dN
dL = K 2 L (+1) where K 2 is normalization
c Power-law slope below the break
d Power-law slope above the break

{ 33 {
Fig. 1.| Stacked \true color" Chandra ACIS-I image (39.7 ks) of the central 17 0 17 0 region
of M31. This image was constructed from the soft (red; 0.3{1 keV), medium (green; 1{2 keV) and
hard (blue; 2{7 keV) energy bands. The pixel size is 1:96 00 and the image has been smoothed with
a 1 Gaissian function.

{ 34 {
Fig. 2.| Stacked \true color" Chandra ACIS-I image of the central 2 0 2 0 region (\region 1") of
M31. The color representation is the same as Figure 1 and the pixel size is 0:123 00 . M31 candidate
is marked.

{ 35 {
1 0.5 0 0.5 1
1
0.5
0
0.5
1
HR1 [(MS)/(M+S)]
Fig. 3.| Color-color diagram for all sources with more than 20 counts. Color symbols represent
dierent regions of the eld: region 1 (blue); region 2 (red) and region 3. Also plotted are the
estimated hardness ratios estimated from dierent spectral models. From top to bottom: power-
law model with of 1, 1.7, 2 and 3, and blackbody model with kT of 0.2 keV and 0.1 keV. For
each model, NH varies from the left from 5 10 20 , 10 21 , 5 10 21 and 10 22 .

{ 36 {
1
0.5
0
0.5
1
1
0.5
0
0.5
1
Fig. 4.| Hardness-intensity diagram for all sources with more than 20 counts. Color symbols are
the same as Fig. 3.

{ 37 {
Fig. 5.| Light curves of 3 bright transients and globular cluster Bo 86 as seen in the past two
years with Chandra. Observations from HRC-I, ACIS-S and XMM-MOS are included. The inset of
r1-5 shows that the energy spectrum during the whole outburst is consistent with a power-law with
slope of 1:5 (see also Trudolyubov, Borozdin, & Priedhorsky 2001). It is worth noting that the
three HRC-I data points during the early stage of the outburst indicate that the source underwent
arings or state transitions.

{ 38 {
1 2 3 4 5 6
0.001
0.01
0.1
Energy (keV)
Fig. 6.| The softest (r3-52; triangles) and the hardest (r2-5; circles) energy spectra from the 20
brightest X-ray sources in the rst observation.

{ 39 {
1 2 3 4 5 6
0.01
0.1
Energy (keV)
1 2 3 4 5 6 7
0.001
0.01
0.1
Energy (keV)
Fig. 7.| Spectral changes of two of the sources. They change from lower luminosity at softer state
to higher luminosity with harder state.

{ 40 {
35 36 37 38
0
0.5
1
1.5
2
2.5
Integrated (Region 1+2+3)
Region 1+2
35 36 37 38
Region 3
Region 2
Region 1
Fig. 8.| Left: Luminosity functions for all sources (Regions 1+2+3) and bulge (Regions 1+2).
Right: Luminosity functions for inner bulge (Region 1), outer bulge (Region 2) and disk (Region
3).