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The Astrophysical Journal, 601:831 - 844, 2004 February 1
# 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.

LONG-TERM X-RAY SPECTRAL VARIABILITY OF THE NUCLEUS OF M81
V. La Parola,1 G. Fabbiano,2 M. Elvis,2 F. Nicastro,2 D. W. Kim,2 and G. Peres
Received 2003 September 22; accepted 2003 October 15
1

ABSTRACT We have analyzed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX, and Chandra. The source flux is highly variable (sometimes dramatic: a factor of 4 in 20 days), showing variability at different timescales, from 2 days to 4 yr, and in particular a steady increase of the flux by a factor of k2 over 4 yr, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum ( from ROSAT /PSPC, BeppoSAX/LECS, and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained by adding an extra component, either a multicolor blackbody (MCBB) or an absorption feature. In the MCBB case, the inner accretion disk would be far smaller than the Schwartzchild radius for the 3 60 Â 106 M nucleus, requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole but consistent with that expected from a Kerr black hole. In the power law+absorption feature model, we have either high-velocity (0.3c) infalling C v clouds or neutral C i absorption at rest. In both cases the C : O overabundance is a factor of 10. Subject headings: galaxies: individual (M81) -- galaxies: nuclei -- X-rays: galaxies

1. INTRODUCTION M81 (NGC 3031) is a nearby galaxy (3.6 Mpc; Freedman et al. 1994), with well-defined spiral arms and a prominent bulge with a structure similar to that of M31. Its nucleus is the nearest example of a low-luminosity active galactic nucleus (AGN; Peimbert & Torres-Peimbert 1981; Elvis & van Speybroeck 1982) and has been studied at all frequencies; its emission properties make it both a LINER (low-ionization nuclear emission line region; Ho, Filippenko, Sargent 1996) and an LLAGN (low-luminosity AGN): it contains a broad component of the H emission line (Peimbert & TorresPeimbert 1981), a compact radio core (Bietenholz et al. 1996), and a variable pointlike X-ray source (LX $ 1040 ergs sÀ1), M81 X-5 (Elvis & van Speybroeck 1982; Fabbiano 1988), with a power-law continuum with photon index À $ 1:85 in the 2 - 10 keV energy band (Ishisaki et al. 1996; Pellegrini et al. 2000). Broad-line velocity (Ho, Filippenko, & Sargent 1996) and stellar dynamics (Bower et al. 2000) studies suggest a supermassive black hole of 3 Â 106 M < MBH < 6 Â 107 M. ASCA (0.5 - 10.0 keV; Ishisaki et al. 1996) and BeppoSAX (0.1 - 100 keV; Pellegrini et al. 2000) spectral analyses of the nuclear X-ray source show, in addition to the power-law component (À $ 1:85), a 6.7 keV He-like Fe resonance line and a thermal component with kT $ 0:86 keV (Ishisaki et al. 1996). A more recent study by Immler & Wang (2001), which uses all the co-added available PSPC observations and fixes the power-law photon index to the ASCA/BeppoSAX À ? 1:85, confirms this soft component and models it with a two-temperature optically thin plasma, ascribing it to the presence of optically thin diffuse gas (kT ? 0:15 keV) and to a population of X-ray binaries and supernova remnants

(kT ? 0:63 keV). However, this type of source would be harder than 0.63 keV. Significant variability in the flux of the nuclear X-ray source on a short timescale ($600 s) was reported by Barr et al. (1985) using EXOSAT data. Longer timescale ($2 days to few years) variability has also been reported (Pellegrini et al. 2000; Ishisaki et al. 1996; Immler & Wang 2001). No spectral variability has been reported; however, a full study of the spectral behavior using the wealth of data collected from the nucleus of M81 has not been done so far. In the present paper we revisit the soft X-ray emission of the nucleus of M81 through the observations of ROSAT instruments, concentrating on variability, and compare these data with those from ASCA, BeppoSAX, and Chandra. In particular, the ROSAT/Position Sensitive Proportional Counter (PSPC) data give us the opportunity to study any spectral variation of soft X-ray emission that occurred during the extensive ROSAT coverage of M81. This paper is structured as follows. Section 2 illustrates the data and their reduction. In x 3 we describe the source variability. Section 4 is devoted to spectral analysis, and our results are discussed in x 5. 2. OBSERVATIONS AND DATA REDUCTION A log of all the observations of M81 is given in Table 1. M81 was observed 22 times with ROSAT in a period of 7 yr from 1991 to 1998, 12 times with the PSPC (Pfeffermann et al. 1987) and 10 times with the HRI (High Resolution Imager).3 This frequent coverage is mainly due to the monitoring of the supernova SN 1993J (Zimmermann et al. 1994), 2A7 south of the nucleus of M81, whose evolution was followed with roughly one observation every 6 months. The large fields of view of the PSPC (2 diameter) and of the HRI (300 diameter) ensure that the nucleus is included in every observation of SN 1993J. The data have been processed using the IRAF
See L. P. David et al. 1996, in the ROSAT Users Handbook, at http:// heasarc.gsfc.nasa.gov/docs/rosat/ruh/handbook/rosathandbook.html.
3

1 Dipartimento di Scienze Fisiche ed Astronomiche, Sezione di Astronomia, Piazza del Parlamento 1, 90134 Palermo, Italy; laparola@oapa.astropa.unipa.it. 2 Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138.

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TABLE 1 Log of X-Ray Obse rvations of M81 Angleb (arcmin) 0.3 36.9 41.7 36.9 0.3 0.3 0.3 2.8 16.0 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

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Referencea 1P ............. 2P ............. 3P ............. 4P ............. 5P ............. 6P ............. 7P ............. 8P ............. 9P ............. 10P ........... 11P ........... 12P ........... 1H............. 2H............. 3H............. 4H............. 5H............. 6H............. 7H............. 8H............. 9H............. 10H...........
a b

Observation ID rp600101a00 rp600110a00 rp600052n00 rp600110a01 rp600101a01 rp600382n00 rp180015n00 rp180015a01 wp600576n00 rp180035n00 rp180035a01 rp180050n00 rh600247n00 rh600247a01 rh600739n00 rh600740n00 rh600881n00 rh600882n00 rh600882a01 rh601001n00 rh601002n00 rh601095n00

Live Time 9296 12717 6588 12238 11085 27120 17938 8731 16412 17800 4234 1849 26320 21071 19902 18984 14826 18328 5091 19231 12590 12590

Start Date 1991 1991 1991 1991 1991 1992 1993 1993 1993 1993 1993 1994 1992 1993 1994 1995 1995 1996 1996 1997 1997 1998 Mar Mar Apr Oct Oct Sep Apr Apr Sep Nov Nov Mar Oct Apr Oct Apr Oct Nov Oct Mar Sep Mar 3 27 18 15 16 29 3 5 29 1 7 31 23 17 19 13 10 15 27 29 30 25

Referencea 1A............. 2A............. 3A............. 4A............. 5A............. 6A............. 7A............. 8A............. 9A............. 10A........... 11A ........... 12A........... 13A........... 14A........... 15A........... 16A........... 17A........... 18A........... 19A........... 20A........... 21A........... 1S ............. Ch .............

Observation ID 15000000 15000120 15000010 15000130 15000030 15000020 15000040 15000050 10018000 51005000 52009000 53008000 53008010 54008000 54008010 55018000 56020000 56020010 57048000 57006000 57006010 40732001 735

Live Time 6480 30144 6622 35680 154847 29440 28576 85376 45152 80016 98206 24128 101434 103616 39712 100640 102672 98224 79794 97104 99808 100000 50570

Start Date 1993 1993 1993 1993 1993 1993 1993 1993 1993 1994 1994 1995 1995 1996 1996 1997 1998 1998 1999 1999 1999 1998 2000 May 4 May 4 Apr 7 Apr 7 Apr 16 Apr 25 May 1 May 18 Oct 4 Jan 4 Oct 21 Jan 4 Oct 24 Apr 16 Oct 27 May 8 Apr 10 Oct 20 Apr 6 Apr 10 Oct 20 Jun 4 May 7

Angleb (arcmin) 2.9 2.9 10.1 6.7 6.6 6.5 6.6 5.9 4.0 6.6 7.5 7.2 7.6 7.0 7.7 6.5 6.9 7.6 7.4 6.7 7.6 0.0 2.8

Reference number for each observation. Instruments are coded as follows: P ? ROSAT /PSPC; H ? ROSAT /HRI; A ? ASCA; S ? SAX ; Ch ? Chandra. Nominal off-axis angle of the optical center of M81 on the detector

(version 2.11) / PROS (version 2.5) software system (Tody 1986; Worrall et al. 1992). The radius for source photon selection for the PSPC depends on the position of the source on the detector plane, because the radius of the point-spread function (PSF) increases with the distance from the center of the field of view (FOV; Boese 2000); for the on-axis observations we used a source radius of 30 , which includes 95% of the emission at all energies and an annular background region with inner and outer radii of 30 and 70 , respectively. For the off-axis observations, the source position and radius were evaluated with the galpipe processing (Damiani et al. 1997) applied to each observation and from the ROSAT/PSPC PSF description (Boese 2000): the radii are 50 and 60 for the M81 nucleus offaxis positions of 160 and 360 -420 , respectively. We have excluded any part of the selected region obscured by the PSPC window supporting ribs. The background was extracted from an annular region with outer and inner radii of 120 and 60 , respectively, after subtraction of the contribution of point sources. For convenience of reference, we designate the PSPC pointings P1 - P12 in time order. The 10 HRI observations pointed on M81 (H1 - H10) have a total exposure time of 177 ks. For each observation we used the position of the detected sources,4 excluding the nuclear source M81 X-5, to check for possible misalignments of the astrometric frames and to correct for small spacecraft errors; no further alignment was needed. The PSPC and HRI position determinations are consistent with each other, and from now on we will use the more accurate HRI centroid coordinates: R:A: ? 9h 57m 54F3 Æ 0F1 and decl: ? 69 03 0 46B4 Æ 0B5 (J2000.0). For each HRI observation, source counts were extracted from a circular region with a 1A3 radius, as found
4

with the ldetect IRAF/PROS algorithm (S. Dyson 1999, private communication). The 21 available ASCA observations of M81 (A1 - A21) were also used to estimate the flux of the nucleus (see Table 1; for a detailed analysis of these data, see Ishisaki et al. 1996 and Iyomoto & Makishima 2001). The ASCA/SIS data are less contaminated by the SN 1993J emission than the ASCA/GIS ones. We extracted ASCA/SIS spectra from a 40 circular region centered on the apparent centroid of the M81 X-5 emission, excluding a 20 circular region centered on the supernova (which lies 2A8 from M81 X-5). The background was extracted from the portion of the SIS chip not contained in the above regions. BeppoSAX observed M81 on 1998 June 4. This observation was studied in detail by Pellegrini et al. (2000). Here we compare the SAX/LECS (Low Energy Concentrator Spectrometer) spectrum and ROSAT/PSPC spectra in order to check for spectral variability. For the BeppoSAX data, we used the standard source and background spectra provided by the Narrow Field Instruments public archive,5 extracted from a 60 radius region centered on the source centroid position. Chandra /ACIS has observed M81 twice (Ho et al. 2001; Swartz et al. 2003). We used the observation with the longest exposure time (50 ks; Table 1). The high flux of the nucleus produces a pile-up fraction of greater than 80%, making the direct study of the properties of the nucleus unreliable. We therefore used the readout trailed image of the nucleus to extract a spectrum unaffected by pile-up, using a narrow box region (500 ) running along the readout direction of the chip containing the nuclear trailed image and excluding the direct image. The background was extracted from two similar regions adjacent to the source region, each 500 wide. In the
5

Using the detection algorithm in the IRAF/PROS task xexamine.

Available at http://www.asdc.asi.it/bepposax.


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following, this spectrum will be referred to as Chandra-T. The subarcsecond Chandra PSF allows the study of the circumnuclear region, and in particular of its contribution to the ROSAT and BeppoSAX spectra. To this aim we extracted separately the spectra of the pointlike sources (spectrum A) and of the unresolved emission (spectrum B), using the same extraction radius as for the on-axis ROSAT spectra (30 ). The coordinates of the pointlike sources are from Swartz et al. (2003). We excluded the pointlike nuclear source (a circular region with a 1000 radius) and the readout trailed image ( from a strip 500 wide running along through the chip readout direction). The background is extracted from the remaining portion of the chip. 3. ROSAT TEMPORAL ANALYSIS The ROSAT data form a long series of uniform observations, suitable for temporal analysis. Instead, the wide beam of ASCA and BeppoSAX data includes much Galactic emission, and we do not include them in the following analysis, except for the long-term light curve. In Figure 1 we show the 1991 - 1999 long-term light curve of the nucleus of M81 in the 0.5 - 2.4 keV band. The PSPC fluxes were calculated assuming a power law with À ? 1:79 plus a thermal component with kT $ 0:5 keV, obtained as final result of the spectral analysis (see x 4). The same model was used to calculate the fluxes from the ROSAT / HRI count rates, through the pimms tool.6 ASCA/SIS fluxes were evaluated using a power-law model, since the soft thermal component is negligible in the ASCA spectral band; the BeppoSAX flux was extrapolated from the value reported in Pellegrini et al. (2000) for the 0.1 - 2.0 keV band using their best-fit model. We note that the light curve in Figure 1 has been derived using data from four different detectors, and therefore one may expect some cross-calibration problems. In our case this is not a problem, since data taken
6 Portable Interactive Multi-Mission Simulator, at http://asc.harvard.edu/ toolkit /pimms.jsp.

with different instruments very close in time give consistent results. This happens in most HRI/ASCA pairs of data (see, e.g., the groups of observations at 5:00 Â 104 , 5:02 Â 104 , 5:04 Â 104 days); see also the group of observation at 5:09 Â 104 days that consists of HRI, ASCA, and BeppoSAX points. We observe a factor of 2 - 4 flux difference between the two high count rate PSPC observations 1P and 3P and all the other PSPC pointings (all of which yield a similar, lower, count rates). We can confidently exclude an unlikely variability in the instrumental calibration, because no flux enhancement is seen in any of the sources in the field (see, e.g., X-9; La Parola et al. 2001) and other fainter sources (Immler & Wang 2001). We can also rule out transient sources near the nucleus, because there is no significant spectral variation between 1P and 3P and the immediately following pointings (x 4). The above considerations suggest that this variation should be ascribed entirely to the nuclear source. To search for short-term variations, we examined the light curves from individual ROSAT observations. Figure 2 presents these light curves in order of their observation. Each bin corresponds to one good time interval (GTI),7 and the time is given in days starting from the ROSAT launch (1990 June). The data show variability on three different timescales: Slow variability.--A slow regular ascending trend can be observed, starting on 1993 November and ending on 1996 April, with a difference of a factor of $ 2 in the flux between the first and the last points (Fig. 1). This trend was reported by Pellegrini et al. (2000) and Iyomoto & Makishima (2001) for the 2.0 - 10.0 keV band from BeppoSAX and ASCA data. Medium variability.--Both before and after the period of regularly increasing flux, there are two periods of irregular
7 Most targets are occulted by Earth for part of the orbit, and the observations are broken into smaller segments that may be interleaved with observations of other targets. Hence, the resulting data stream for a particular target will in general show large data gaps, with effective observing intervals lasting a few hundreds of seconds; each of these continuous observation is a ``good time interval.''

Fig. 1.--Observed flux (not corrected for cold absorption) in the 0.5 - 2.4 keV band (left Y-axis). The details of models used for flux calculation and spectral analysis are given in xx 3 and 4. The luminosity (right Y-axis) is calculated assuming a distance of 3.6 Mpc (Freedman et al. 1994). The points from other instruments are derived from the literature and are marked as follows: E ? Einstein/IPC (Fabbiano 1988); EX ? EXOSAT , B ? Goddards Broad Band X-Ray Telescope (Petre et al. 1993); and C ? Chandra /ACIS (Swartz et al. 2003); X ? XMM (Page et al. 2003). The numbers near the symbols of the 12 PSPC observations show the time sequence of the relevant data points and clarify the rapid and large transients in the first part of the light curve.


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Fig. 2.--Light curves of PSPC and HRI observations (identified with a P and a H on the right of the graph, respectively). Time is in days since the satellite launch (1990 June); the number on the right of each curve identifies the observations sequence as in Table 1.

variability (Fig. 1). This is more marked before 1993 November, where two very bright flarelike episodes are seen, with variations of up to 4 times in flux. It is weaker (but nevertheless extremely significant) after 1996 April. This kind of variability was not previously reported. Fast variability.--The light curves of many observations (Fig. 2) show variability of up to 30% on timescales of the order of 1 day (e.g., observations 9P, 4H, 6H, and 9H), as found with BeppoSAX (Pellegrini et al. 2000), or even a few hours (e.g., observations 5P, 9P, and 10H), confirming the early EXOSAT report (Barr et al. 1985). A Kolmogorov-Smirnov (K-S) variability test (see, e.g., Conover 1971) applied to the unbinned data and a 2 test applied to the light curves binned into GTIs show that 11 of 22 observations are variable in both tests with more than 99% probability of rejecting the hypothesis of a constant rate

(Table 2). Despite the abrupt drop in flux by a factor of $3 in 1 day between observations 1P and 2P and the factor of $2 rise 20 days later between 2P and 3P (Figs. 1 and 2), the individual light curves of these observations do not show any sharp change of the count rate. We investigated the spectral variability in the PSPC observations by calculating a hardness ratio of each observation in two pairs of bands (Table 2). The first, HR1 (0.11 - 0.42/0.52 - 2.02 keV), covers the whole spectral range of the PSPC and can give information on the absorbing column. The second, HR2 (0.52-0.91 keV/ 0.92-2.02 keV), covers only the hardest part of the spectrum and is more sensitive to spectral index variations. We performed a 2 test against the hypothesis of constant distribution centered on the average value: HR1 does not show any variability (2 = ? 13:6=11, where is the number of degrees of freedom), while HR2


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Fig. 2.--Continued

shows evidence of variability (2 = ? 65=11). We found no evidence of correlation of the hardness ratio with the total flux (Fig. 3). The spectral analysis revealed that the spectrum is indeed variable, as discussed in x 4. 4. SPECTRAL ANALYSIS The spectra were analyzed using XSPEC version 10.0/11.1 (Arnaud, George, & Tennant 1992).

4.1. The Galactic Emission The large-beam spectra from BeppoSAX and ROSAT include a significant contribution from the circumnuclear region of the galaxy, within the 30 extraction radius around the nucleus. The high-resolution Chandra observation allows us to measure the galaxy emission (both diffuse and from individual point sources) included in the wider beam ROSAT and BeppoSAX spectra. As described in x 2, we used the same


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TABLE 2 Prop erties of Individual ROSAT Observations of M81 Reference 1P ........................................ 2P ........................................ 3P ........................................ 4P ........................................ 5P ........................................ 6P ........................................ 1H........................................ 7P ........................................ 2H........................................ 8P ........................................ 9P ........................................ 10P ...................................... 11P ...................................... 12P ...................................... 3H........................................ 4H........................................ 5H........................................ 6H........................................ 7H........................................ 8H........................................ 9H........................................ 10H......................................
a b

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À 2.20 1.92 2.10 2.1 2.22 2.30 2.32 2.25 2.18 2.41 2.47 2:

a

Fluxb 15.12 7.67 19.37 4.51 7.34 7.41 6.29 6.94 7.22 7.37 6.67 9.00 6.71 6.91 9.64 9.81 11.32 13.79 12.46 8.34 12.02 16.56

P

c

KS

P

2

d

HR1e 0.765 0.773 0.755 0.832 0.751 0.755 0.756 0.757 0.785 0.751 0.749 0.768 Æ Æ Æ Æ Æ Æ .. . Æ .. . Æ Æ Æ Æ Æ .. . .. . .. . .. . .. . .. . .. . .. . 0.011 0.018 0.015 0.026 0.014 0.009 0.011 0.016 0.014 0.010 0.023 0.037

HR2e 0.227 0.237 0.201 0.220 0.229 0.196 0.186 0.195 0.234 0.177 0.162 0.224 Æ Æ Æ Æ Æ Æ . .. Æ . .. Æ Æ Æ Æ Æ . .. . .. . .. . .. . .. . .. . .. . .. 0.007 0.012 0.010 0.016 0.010 0.006 0.008 0.011 0.009 0.007 0.016 0.025

Æ 0.08 Æ 0.15 Æ 0.12 Æ 0.2 Æ 0.10 Æ 0.07 .. . Æ 0.09 .. . Æ 0.12 Æ 0.10 Æ 0.06 Æ 0.20 2þ0::4 À0 3 .. . .. . .. . .. . .. . .. . .. . .. .

0.21 <0.01 0.49 <0.01 0.16 <0.01 <0.01 <0.01 0.53 <0.01 <0.01 0.55 0.02 0.06 0.16 <0.01 <0.01 <0.01 <0.01 0.02 <0.01 <0.01

0.26 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.71 <0.01 <0.01 0.50 0.03 0.14 0.40 <0.01 <0.01 <0.01 <0.01 0.03 <0.01 0.09

Best-fit power-law photon index. Average flux in units of 10À12 ergs sÀ1 cmÀ2. The model used to calculate fluxes are described in x 3. c Probability of a constant count rate within the observation according to a K-S test. d Probability of a constant count rate equal to the average count rate according to a 2 test. e Average PSPC hardness ratios defined as HR ? H À S =H þ S ,where HHR1 ? C0:52 2:02 , SHR1 ? C0:11 0:42 , H and SHR2 ? C0:52À0:91 are the photon counts in the subscripted energy bands (in keV).

HR2

?C

0:92

2:02

,

extraction radius as for the ROSAT data (30 ) but excluded the region affected by the nuclear point source and the strip containing the trailed image of the nucleus. We then extracted two spectra: the individual point-source spectrum, A, obtained by summing the spectra of the bright sources falling within the 30 circle, as listed in Swartz et al. (2003), and the diffuse spectrum, B, i.e., the emission within the same region excluding the point sources. These have roughly equal fluxes. We then formed the total spectrum, C, summing the diffuse and the point-source spectra. We fitted simultaneously the spectra A, B, and C, imposing the best-fit model of spectrum C to be the sum of the best-fitting model of spectra A and B. The best-fit parameters are in Table 3. We find that spectrum A can be described with power law with À $ 1:7 plus a multicolor blackbody (MCBB; model diskbb in XSPEC) at $0.08 keV and a Gaussian at $0.8 keV. To describe spectrum B, we need a Raymond-Smith spectrum at $ 0.25 keV and, again, a power law with À $ 1:7. Spectra A and B are not consistent with each other. However, in both cases the NH is consistent with the Galactic line-of-sight value and the two power-law slopes are consistent with each other. In merging the models to fit spectrum C we used only a single power-law component. The [0.5 - 2.4] keV flux corrected for absorption by the best-fit is 8:1 Â 10À13 ergs sÀ1 cmÀ2 for the integrated point-source emission and 6:9 Â 10À13 ergs sÀ1 cmÀ2 for the diffuse emission, for a total of 1:5 Â 10À12 ergs sÀ1 cmÀ2. 4.2. The Nuclear Spectra We first analyzed the ROSAT spectra, extracted with the IRAF routine qpspec and processed with proscon, using the response matrices publicly available through the HEASARC/ROSAT

Web page.8 As a preliminary step, we fitted the spectra of each of the 12 observations with a simple absorbed power-law model, in order to search for spectral variability. A comparison of residuals of each observation to the relevant best-fit model (Fig. 4) shows that some observations (6P, 7P, 8P, and 10P [hereafter ``dip'' observations]) show a large dip in the residuals between 0.2 and 0.7 keV, while the remaining seven observations (hereafter ``flat'') are well described by a power law. We improved the statistics of the spectral fit by summing the spectra by type. We then fitted the two resulting spectra with a simple power law. The results show that in the summed dip spectrum the absorption feature is clearly visible between 0.3 and 0.5 keV (Fig. 5); however, in the summed flat spectrum there is also a broad, weak absorption-like structure, at the same energy, suggesting that the feature is in fact present in all the observations, albeit with varying intensity. We can confidently rule out the hypothesis that the feature is an instrumental effect, as there is no hint of it in the spectra of other sources of the same field (see, e.g., X-9 in La Parola et al. 2001). In order to minimize the contribution from other bright sources in the vicinity of the nucleus, and because the effective area is poorly calibrated for off-axis PSPC sources, the subsequent analysis was carried on a spectrum selected from the sum of the eight observations where the nucleus was on-axis. We also checked for the presence of the absorption feature in observations by other instruments. ASCA has too little response below 0.5 keV. We can, however, use SAX/LECS and Chandra-T (readout trail) spectra (see x 2 for details on the extraction of these spectra). We decided to fit the whole
8

See http://heasarc.gsfc.nasa.gov/docs/rosat /rosgof.html.


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Fig. 3.--Hardness ratios (as defined in Table 2) vs. count rate. Triangles and stars, respectively, indicate the observations with and without the strong absorption feature (see x 4.2 for a discussion).

energy range (0.1 - 4.0 for LECS and 0.3 - 8.0 for ACIS, compared with 0.1 - 2.4 for PSPC), in order to have a better estimate of the continuum. We fitted each of these spectra with a simple power-law model (Table 4). An absorption feature at $0.3 keV is seen in these spectra as well. This is made more evident in Figure 5, where we plot the residuals obtained by fitting with a power law only the energy range above 1.0, which shows also how the feature is at slightly different energy in the three spectra.

We then made a more careful analysis (see Table 4) of the ROSAT/ PSPC and BeppoSAX/LECS data by adding a fixed component to model the extended galaxy contribution, set to the best-fit model derived from the Chandra data of the 30 radius extraction region around the nucleus (see x 4.1) in addition to the variable components. A power law plus this extended component gives a good fit to energies higher than 1 keV but still overpredicts the emission between 0.3 and 0.6 keV. To investigate the nature of this ``dip,'' we tested four models in

TABLE 3 Be st-F it Model t o t he Circumnuclear Galactic Emission fr om Cha ndr a Point-Source Dif fuse Emission Component Parameters NH (1020 cmÀ2) À Flux (ergs sÀ1 cmÀ kT (keV) Flux (ergs sÀ1 cmÀ E (keV) (keV) Flux (ergs sÀ1 cmÀ kT (keV) Flux (ergs sÀ1 cmÀ 2/ Spectrum A 4.1+0.4 1:74þ0::05 À0 04 6.5 Â 10À13 0:083þ0::003 À0 006 8.9 Â 10À14 0:86þ0::03 À0 04 0.15 Æ 0.03 7.0 Â 10À14 .. . .. . 256/197 Spectrum B 4.1+0.4 1:74þ0::05 À0 04 4.6 Â 10À13 .. . .. . .. . .. . .. . 0.273 Æ 0.011 2.3 Â 10À13 169/126
20

Power-law ...................... MCBB ............................ Gaussian .........................

2) 2)

2) 2)

Raymond-Smith .............

Notes.--NH is constrained to be not less than the Galactic line-of-sight value (4:1 Â 10 fluxes are calculated in the 0.5 - 2.5 keV band and corrected for best-fit NH.

cmÀ2). All


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Fig. 4.--Residuals in units of for all the PSPC spectra best-fitted with a power-law model

which a single component was added to the power law: an optically thin Raymond-Smith plasma, an MCBB, an absorption edge, and a Gaussian line in absorption (Table 4 and Figs. 6 and 7). In all cases the improvement over a simple power-law model is highly significant, with F-test probabilities lower than 1%. The BeppoSAX and ROSAT data give similar results for both the MCBB and the Raymond-Smith components, with a better 2 for the disk model. In both cases, the edge model has a 2 comparable with that of the disk model. The energy of the edge found for BeppoSAX data (0:22þ0::03 keV) is lower than À0 06 for ROSAT data (0:41þ0::08 keV). In the Chandra data we found À0 13 that the fits require a lower temperature Raymond-Smith or MCBB component, while the edge energy is 0:29 Æ 0:02 keV.

The Gaussian model significantly improves the fit of the Chandra data (2 = ? 104=127), while for the PSPC data it is equivalent to the other models and for BeppoSAX it is slightly worse. The best-fit energies of the Gaussian absorption line are consistent with each other in the three instruments. The power-law index in the ROSAT data is steeper than in the BeppoSAX data by ÁÀ $ 0:4 to 1.0, depending on the spectral model. An offset of ÁÀ $ 0:4 has been observed in other objects too (e.g., NGC 5548; Iwasawa, Fabian, & Nandra 1999) and is thought to be a systematic calibration offset. In this case the difference is sometimes larger than this offset (in NGC 5548), suggesting that at least part of the steepening is real, not an instrumental effect.


No. 2, 2004

SPECTRAL VARIABILITY OF NUCLEUS OF M81

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Fig. 5.--Spectra of the nucleus of M81: residuals with respect to a power-law model. Top to bottom: ROSAT/PSPC spectrum (obtained from the sum of the onaxis observations), BeppoSAX/LEX spectrum, and Chandra-T spectrum. These residuals have been obtained by fitting with a power-law energy range not affected by the absorption feature: 0.1 - 0.3 and 1.0 - 2.4 keV for the PSPC and greater than 1.0 keV for LECS and Chandra-T.

The luminosity of the power-law source in the 0.5 - 2.4 keV range is 2:3 Â 1040 ergs sÀ1 (corresponding to 1:6 Â 10À11 ergs sÀ1 cmÀ2) for BeppoSAX and 1:4 Â 1040 ergs sÀ1 (1:0 Â 10À11 ergs sÀ1 cmÀ2) for the PSPC data used in the spectral analysis. Since the contribution of circumnuclear emission from the Chandra analysis was included in the fit models for the ROSAT/PSPC and BeppoSAX/LECS data, these luminosities are truly representative of the nuclear source. The Chandra-T spectrum cannot be used directly to obtain a normalization and thus a flux. We use here the estimates of Swartz et al. (2003; see