Next: The ISOCAM Parallel Mode Survey
Up: Sky Surveys
Previous: Sky Surveys
Table of Contents -
Subject Index -
Author Index -
Search -
PS reprint -
PDF reprint
Watson, M. G. 2003, in ASP Conf. Ser., Vol. 295 Astronomical Data Analysis Software and Systems XII, eds. H. E. Payne, R. I. Jedrzejewski, & R. N.
Hook (San Francisco: ASP), 107
The XMM-Newton Serendipitous Sky Survey
M. G. Watson
Department of Physics and Astronomy, University of Leicester,
Leicester LE1 7RH, UK
on behalf of the XMM-Newton Survey
Science Centre (SSC)
Abstract:
The main properties of the XMM-Newton serendipitous sky survey
are outlined. The XMM-Newton Survey Science Centre's role in
the survey is described with emphasis on its follow-up and
identification programme and the production of the XMM-Newton
catalogue.
Serendipitous X-ray sky surveys have been pursued with most X-ray
astronomy satellites since the Einstein Observatory. The
resultant serendipitous source catalogues have made a significant
contribution to our knowledge of the X-ray sky and our understanding
of the nature of the various Galactic and extragalactic source
populations. The
XMM-Newton Observatory
(Jansen et al. 2001) provides unrivaled capabilities for serendipitous X-ray surveys
by virtue of the large field of view of the X-ray telescopes with the
EPIC X-ray cameras1
(Turner et al. 2001; Strüder et
al. 2001), and the high throughput afforded by the heavily nested
telescope modules. This capability ensures that each XMM-Newton
observation provides significant numbers of previously unknown
serendipitous X-ray sources in addition to data on the original target
(Watson et al. 2001).
A follow-up and identification programme for the XMM-Newton sources
and the compilation of a high quality serendipitous source catalogue
from the XMM-Newton EPIC observations are major responsibilities
of the XMM-Newton Survey Science Centre
(SSC; Watson et
al. 2001). This paper describes these two aspects of the SSC's
activities, both of which are ongoing projects.
2. The Role of the XMM-Newton Survey Science Centre (SSC)
The SSC is an
international collaboration involving a consortium of ten institutions
in the UK, France, Germany, Italy and Spain. The SSC has several major roles in the project: the
development of the scientific processing and analysis software for
XMM-Newton, the routine `pipeline' processing of all the
observations, a follow-up and identification programme for the
XMM-Newton serendipitous survey and the compilation of the
XMM-Newton Serendipitous Source Catalogue. Here I briefly outline
the SSC's activities in the first two areas, the others being covered
in later sections.
Working closely with ESA's Science Operations Centre (SOC) staff, the
SSC has played a large role in the development of the scientific
analysis software for the XMM-Newton project: the
`SAS.'
SAS modules are used in a fixed configuration for the routine
processing of the XMM-Newton data, and in an interactive
configuration to carry out custom analysis of the data. The SSC also
carries out the routine `pipeline' processing of all the
XMM-Newton observations from each of the three science instrument
packages. The aim of this processing is to provide a set of data
products which will be of immediate value for the XMM-Newton
observer as well as for the science archive. The XMM-Newton data
products include calibrated, `cleaned' event lists which provide the
starting point for most interactive analysis of the data as well as a
number of secondary high-level products such as sky images, source
lists, cross-correlations with archival catalogues, source spectra and
time series. The data products also constitute the starting point for
the compilation of the serendipitous catalogue (see
section 5.).
The high throughput, large field of view and good imaging capabilities
of XMM-Newton mean that it detects significant numbers of
serendipitous X-ray sources in each pointing in addition to the main
target of the observation, as is illustrated in
Figure 1.
Figure 1:
Example XMM-Newton X-ray image.
Data shown are the combined images from all three EPIC cameras in the
0.5-4.5 keV band. The target of the observation is a relatively faint
quasar.
|
Typical observations yield serendipitous
sources per field.
As XMM-Newton makes of the order 700
observations per year, covering sq.deg. of the sky, the
number of serendipitous sources is thus growing at a
rate of sources per year, i.e., the annual rate is
comparable in size to the complete ROSAT All Sky Survey, but
reaches fluxes 2-3 orders of magnitude fainter. XMM-Newton data thus
provides a deep, large area sky survey which represents a major
resource for a wide range of programmes (see section 7.). The extended energy range of
XMM-Newton, compared with previous imaging X-ray missions such
as ROSAT and the Einstein Observatory, means that
XMM-Newton detects significant numbers of obscured and hard-spectrum
objects (e.g., obscured AGNs) which
are absent from earlier studies.
In comparison with Chandra, typical XMM-Newton observations
cover a significantly larger sky area but do not go as deep due to the superior imaging provided by the Chandra mirrors.
XMM-Newton has a very significant advantage at photon
energies above 4-5 keV: at low energies the ratio of
XMM-Newton/ Chandra effective areas is 3-4 but this increases to
at 5 keV and at 7 keV.
4. The `XID' Follow-up Programme
In order to exploit the full potential of the XMM-Newton
serendipitous survey in the context of a wide range of scientific
programmes, the key initial step is the `identification' of the X-ray
sources, i.e., a knowledge of the likely classification into different
object types. For XMM-Newton serendipitous sources, the X-ray
observations themselves will provide the basic parameters of each
object: the celestial position, X-ray flux and colours for all sources
and information on the X-ray spectrum, spatial extent and temporal
variability for the brighter objects detected. This information alone
will, in some cases, be sufficient to provide a clear indication of the
type of object, but for the vast majority of sources, additional
information will be required before a confident classification of the
object can be made. Some of this information can come from existing
astronomical catalogues, or from existing or planned large-scale
optical, IR and radio surveys (e.g., SDSS, 2MASS, Denis, FIRST/NVSS),
but full exploitation of the XMM-Newton serendipitous survey
requires a substantial programme of new observations, primarily in the
optical and IR using ground-based facilities. The SSC's
XID
follow-up
programme is designed to meet these aims.
The main new observational elements of the XID programme, the `Core
Programme' and the `Imaging Programme' are outlined below.
The aim of the Core Programme is to obtain the identifications for a
well-defined sample of X-ray sources drawn from selected
XMM-Newton fields, primarily using optical/IR imaging and
spectroscopy. Imaging is required both to locate potential candidates
accurately and reveal their morphology, whilst the optical
spectroscopy provides the diagnostics needed both for object
classification and for determining basic object parameters such as
redshift and spectral slope. The principal objective is to obtain a
completely identified sample which can be used to characterise the
XMM-Newton source population overall sufficiently well that we
can use the basic X-ray and optical parameters to assign a
`statistical' identification for a large fraction of all the
sources in the XMM-Newton serendipitous source catalogue.
The Core Programme has two main parts: one for the high and
one for the low galactic latitude sky. The high galactic latitude
part consists of three samples, each containing
X-ray sources in three broad flux ranges as summarised in Table 1. The
size of the subsamples is dictated by the need to identify enough
objects to reveal minority populations. Studying sources at a range of
X-ray fluxes is necessary because we already know that the importance
of different source populations changes with X-ray flux level.
Substantial progress has been made on the XID Core Programme over the
last two years, in particular in the medium, bright and Galactic samples
(Barcons et al. 2002a,b; Della Ceca et al. 2002; Motch et al. 2002). A large fraction of the ground-based observations needed to
support this programme have come from the
AXIS
programme
which gained
a substantial international time allocation on the Canary Islands
telescopes (Barcons et al. 2001).
Figure 2:
Distribution of X-ray and optical fluxes for serendipitous
sources identified in the XID programme. The symbols indicate object classifications as given in the key. The nominal
limit for the XID medium sample is indicated by the horizontal
line.
|
For example a significant number of
identifications have already been made in 19 medium sample fields
included in the XID programme. Of the 239 medium sample sources in
these fields around 2/3 of these have already been identified (see
Figure 2); the remaining sources are typically too faint for
4m-class telescopes and will be pursued with larger facilities over
the next year.
When complete, the
XID medium sample will bridge the gap between the deepest surveys
aimed at studying the most distant Universe, and
shallower surveys which aim to characterise the local and high
luminosity X-ray source populations.
The XID imaging programme aims to obtain optical/IR photometry and
colours for a large number of XMM-Newton fields. The rationale
is that a combination of X-ray flux & X-ray colours (from the
XMM-Newton data) and optical magnitude and optical colours (from
new ground-based observations) will provide the key parameters which
make possible an accurate `statistical' identification of the
XMM-Newton sources. This will be possible using the results from the
Core Programme to provide the `calibration' of the basic parameters
for different object types.
To date deep multi-colour optical imaging has been obtained for more
than 150 XMM-Newton fields using 2m-class telescopes in both
hemispheres. The prospects are good to expand this to 250-300 fields
over the next two years.
5. The XMM-Newton Catalogue
The first installment of the catalogue (Watson et al. 2002) will
include around 700 XMM-Newton observations (`fields') selected
purely on the public availability of the fields at the planned release
date (end 2002) and the observation being made in one of the
appropriate EPIC imaging modes. No other selection of fields, e.g., in
terms of sky location, exposure time etc., has been made.
Figure 3 shows the sky distribution of the catalogue fields.
The overall sky distribution is reasonably uniform, although there are
some biases such as the paucity of fields in the Cygnus region due to
XMM-Newton visibility constraints. The average exposure time per
catalogue field is ksec for the EPIC MOS cameras and ksec for the EPIC pn camera.
Figure 3:
Sky distribution of the XMM-Newton fields in the catalogue shown
in Galactic coordinates. Size of the symbols reflects the total number
of serendipitous sources detected: the average number is per
field.
|
5.2 Data Processing
Data processing for the production of the XMM-Newton catalogue
is based closely on the standard SSC pipeline (see section 2.)
used to process each
XMM-Newton dataset for distribution to the observer, and population
of the XMM-Newton Science Archive. Catalogue processing uses a
fixed software and calibration data configuration in order to
guarantee uniformity. The main processing stages for each
XMM-Newton observation are:
- production of calibrated events from the ODF science frames;
- generation of the appropriate low-background time intervals
using a threshold optimized for point source detection;
- generation of multi-energy-band X-ray images and exposure maps from the
calibrated events;
- a four-stage source detection and parameterization procedure:
- merging of the three camera-level source lists into an EPIC-level
source list with merging on the basis of positional coincidence alone;
and
- cross-correlation of the source list with a variety of archival
catalogues and other resources using the CDS facilities in Strasbourg.
The source search approach utilized involves simultaneous fitting of five
energy band images for each EPIC camera, thus producing a source list
for each camera which contains source and detection information in
each energy band (as well as the total band). The camera lists
combined in the penultimate stage described above thus produce a
merged source list which forms the reference source list for that
observation.
5.3 Quality Control
Although the source detection algorithm described above is now mature,
typically producing reliable source lists from most XMM-Newton
observations, the approach is not perfect and is known to have
problems in producing reliable results in a number of (rare)
circumstances.
Each XMM-Newton field included in the catalogue is therefore
visually screened to locate such defects. Where problems are noted the
sources affected are `flagged' and the flag values transferred to the
source lists. In rare cases (% of the total) the entire field
has significant problems, e.g., very high background or very high
surface brightness diffuse sources, which mean that it is of marginal
value for detecting serendipitous sources. Such fields will be
excluded from the final catalogue. Apart from these rare cases, the
median fraction of sources flagged as being spurious amounts to only
4% overall, reflecting the maturity of the source detection approach.
6. Catalogue Properties
The working catalogue contains a total of
source detections in any EPIC camera (i.e., in one or more cameras) and
a total of sources detected in all three EPIC cameras. These
numbers refer to a broad-band (0.2-12 keV) detection above a likelihood of 10,
corresponding to
. At this significance the a
priori probability of spurious detections is low,
corresponding to spurious source per field, although simulations
are underway to verify the calibration of the likelihood
parameterisation. The total sky area
covered for detections in any camera is sq.deg., whilst for
detection in all three cameras the area is sq.deg.
For the released version of the catalogue these numbers will be
reduced by the fraction flagged as spurious (and the small number of
fields totally excluded). We also anticipate setting a somewhat higher
likelihood threshold once we have completed our investigation of the
reliability of detections as a function of likelihood, being pursued
by simulations.
6.2 Source Count Distribution
Figure 4 shows the
distribution for all EPIC
pn sources in the working catalogue. The distribution is not
corrected for sky coverage, i.e., how the actual sky area covered varies with
X-ray flux.
Comparing the uncorrected
with the expected source counts demonstrates that the
catalogue is essentially complete down to an X-ray flux
(0.2-12 keV) (equivalent to
in the 0.5-2 keV band and
in the 2-10 keV band). This
limit is
in line with
expectations given the exposure time distribution of catalogue fields.
Around 30% of the catalogue sky area is covered to
(0.2-12 keV). Work to establish the
definitive coverage corrections is underway and will be part of the
ancillary information included with the released version of the
catalogue.
Figure:
Uncorrected
distribution for all EPIC pn detections
in the working catalogue. The solid curve is for all sources, the
dashed curve is for high latitude sources ()
and the dashed-dot curve for low
latitude sources ().
|
6.3 Astrometry
For each XMM-Newton field, the catalogue processing
(section 5.2.) attempts to correct the astrometric reference frame using
cross-correlation of the XMM-Newton source list with the USNO
A2.0 catalogue (Monet et al. 1998). The technique employed
involves finding the maximum likelihood in a grid of trial astrometric
shifts and rotations with a likelihood function depending on the
angular separation between each potential XMM-Newton-USNO
object match. If an acceptable solution is found (% of fields)
the resultant astrometric correction is applied to the
XMM-Newton source list for that field. (The cases where an acceptable
solution is not found are primarily fields with low numbers of X-ray
sources and/or fields with high optical object density).
The results of applying this technique to the catalogue fields can be
employed to quantify the initial accuracy of the astrometry of each
XMM-Newton field (i.e., before correction).
Fits to the distribution of shifts in RA, Dec and field rotation imply
that the intrinsic accuracy of the XMM-Newton field astrometry
(as determined solely from the in-orbit attitude solution) can be
characterized by a Gaussian with
arcsec. After correction using this technique, the residual
field astrometric errors are of the order 0.5-1 arcsec, close to the
nominal 1 arcsec astrometric accuracy of the USNO catalogue itself.
As the typical statistical error-circle for a faint
XMM-Newton source has
arcsec, the
size of the field systematic component determined justifies the
arcsec positional accuracy which has been assumed to date as
the effective % confidence radius of uncorrected positions,
e.g., for the identification of XMM-Newton source counterparts
(see section 4.).
7. Scientific Potential of the Catalogue
The XMM-Newton catalogue represents a significant resource that
can be used for a variety of astrophysical projects. Although deep
Chandra and XMM-Newton pencil-beam surveys (e.g.,
Mushotzky et al. 2000; Hasinger et al. 2001; Giacconi
et al. 2001; Brandt et al. 2001) have probed the faintest
parts of the extragalactic source population, the XMM-Newton
catalogue also can make a major contribution. The XMM-Newton
catalogue reaches modest depths (
) at high coverage
(tens of square degrees). As this flux limit is where
the bulk of the objects that contribute to the X-ray background lie
(due to the fact the
distribution breaks to a flatter
slope at around this flux), the large samples of medium-deep flux
sources that the XMM-Newton catalogue provides will thus be a
significant resource
for X-ray background studies.
Figure 5:
X-ray colour-colour plot showing the distribution of HR2-HR3 values
for EPIC pn catalogue sources (
only). The gray-scale
and contours indicate the logarithmic density of points down to % of the peak. The dashed-dot lines indicate the approximate HR2,
HR3 values for the X-ray background. HR2 is the ratio (B-A)/(A+B) and
HR3 is the ratio (C-B)/(B+C) where A= 0.5-2 keV count rate, B=2-4.5
keV count rate and C=4.5-7.5 keV count rate.
|
The XMM-Newton catalogue also provides a rich resource for
generating well-defined samples for specific studies, utilizing the
fact that X-ray selection is a highly efficient (arguably the most
efficient) way of selecting certain types of object, notably AGN,
clusters of galaxies, interacting compact binaries and very
active stellar coronae. AGN samples have obvious value in
evolution studies and cluster samples can provide, potentially, key
measurements of cosmological parameters. Selecting `clean' samples
requires knowledge of the likely parameter ranges of different types
of object: some of this is already known but further `calibration' of
this concept is one of the key aims of the SSC's XID programme
(section 4.; Watson
et al. 2001; Barcons et al. 2002a,b). The inclusion of
matches with archival catalogues for each XMM-Newton catalogue
source also provides a valuable starting point for investigation of
well-defined samples.
To illustrate some of the potential described above, Figure 5
shows the overall distribution of X-ray colours in the catalogue. This
figure provides a glimpse of the power of the XMM-Newton
catalogue for providing interesting samples.
The spectrum of the X-ray
background corresponds to HR2 , HR3 , but
evidently the bulk of the catalogue sources have spectra softer than
this. Thus merely by extracting a subset of XMM-Newton sources
with X-ray colours harder than these limits, one automatically selects
those objects that must be an important constituent of the background.
Optical and near-IR observing programmes to investigate the nature of
the hard source samples constructed in this manner are already
underway.
As well as the potential for building up large samples, the brighter
sources in the XMM-Newton catalogue also provide the prospect of
obtaining high quality X-ray spectra and time series data (and
morphology). Amongst the catalogue sources more than 15% have more
than 200 EPIC pn counts, enough for a reasonable spectral
characterization and crude variability indications, whilst 3% have
more than 1000 counts, sufficient for a very good X-ray spectral
measurement or variability analysis.
Acknowledgments
The SSC's activities relating to the XMM-Newton serendipitous
sky survey are a consortium effort involving all the SSC institutions.
I gratefully acknowledge the efforts of many colleagues which continue
to contribute to the success of these projects.
References
Barcons, X. et al. 2001, New Century of X-ray Astronomy,
eds. H. Inoue & H. Kunieda (San Francisco: ASP), 160
Barcons, X. et al. 2002a, A&A, 382, 522
Barcons, X. et al. 2002b, Astron Nachr., in press
Brandt, W. N. et al. 2001, AJ, 122, 1
Della Ceca, R. et al. 2002, New visions of the X-ray
Universe in the
XMM-Newton and Chandra era, in press, astro-ph/0202150
Giacconi, R. et al. 2001, ApJ, 551, 624
Hasinger, G. et al. 2001, A&A, 365, L45
Jansen, F. et al. 2001, A&A, 365, L1
Monet, D. et al. 1998, The USNO-A2.0 Catalogue, (U.S. Naval
Observatory, Washington DC; VizieR Online Data Catalog, 1252)
Motch, C. et al. 2002, New visions of the X-ray Universe in the
XMM-Newton and Chandra era, in press, astro-ph/0203025
Strüder, L. et al. 2001, A&A, 365, L18
Turner, M. J. L. et al. 2001, A&A, 365, L110
Watson, M. G. et al. 2001, A&A, 365, L51
Watson, M. G. et al. 2002, Astron Nachr., in press
Footnotes
- ... cameras1
- There are three EPIC cameras
on-board XMM-Newton, one in the focal plane of each of the
co-aligned X-ray telescope
modules: two EPIC MOS cameras and one EPIC pn camera. In normal
operations observations are made with all three cameras simultaneously.
© Copyright 2003 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: The ISOCAM Parallel Mode Survey
Up: Sky Surveys
Previous: Sky Surveys
Table of Contents -
Subject Index -
Author Index -
Search -
PS reprint -
PDF reprint