ESA Science & Technology - XMM-Newton Slew Survey and Catalogue

Typical XMM-Newton scientific operations are performed by observing pre-defined targets for a fixed period of time. Open- or closed-loop slew manoeuvres are used to reorient the spacecraft between targets. The open-loop slew is performed at a steady rate of about 90 degrees per hour making it suitable to use for a uniform survey of the X-ray sky.

Figure 1: All-sky image of XMM-Newton slew observations (A. Read (University of Leicester) & ESA)

There is already a strong tradition in X-ray astronomy of using data taken during slewing manoeuvres to perform shallow surveys of the sky. Slew surveys from missions such as Einstein, Exosat and RXTE have proven to be useful complements to the variety of surveys produced by other X-ray missions such as ROSAT, ASCA, XMM-Newton and Chandra.

XMM-Newton is characterised by a number of factors which contribute to making it the most powerful X-ray observatory ever launched. The large collecting area of its mirrors, the high quantum efficiency of the CCDs, the wide energy band (0.2 - 12 keV) and the tight point spread function all contribute to making it a remarkably sensitive X-ray observatory.

Even before operations began it was recognised that the short exposures during a slew - typically 15 seconds on a source - would be sufficient to construct a hard (2-12 keV) X-ray survey that would be significantly deeper than the current all-sky X-ray catalogues.

Since late August 2001 a concerted effort has been made to record slew data for scientific purposes. Prior to that date all instruments were switched off during the slews. Since then the EPIC-MOS and EPIC-pn cameras have been left on as the spacecraft moves from one target to the next.

In practice only the EPIC-pn data are used for the slew survey. The short CCD frame time of the EPIC-pn camera allows slew sources to be imaged essentially as point sources whereas the relatively slow readout time of the EPIC-MOS camera results in sources that are smeared across the CCD frame making them less suitable for the survey. Up until mid-December 2004 only science data from slews which were longer than 30 minutes were downloaded; since then the criteria have been relaxed to include slews lasting longer than 15 minutes.

Data that have been recorded during the slew are made available in the XMM-Newton Science Archive, following preliminary processing, typically about 10 days after the slew has been completed. The data are processed separately in three bands (see table) to give three independent surveys.  These are subsequently combined to form the XMM-Newton Slew Survey Catalogue (see below).

A large variety of sources have been detected in the slew survey including AGN, individual galaxies, clusters and groups of galaxies, active stars, supernova remnants, low and high-mass X-ray binaries and white dwarfs.

Strengths of the slew survey

The XMM-Newton slew survey is an important scientific product of the XMM-Newton mission.

The hard band (2-12 keV) flux limit represents an order of magnitude improvement over previous missions resulting in the deepest hard-band all-sky survey to date.

The soft band (0.2-2 keV) flux limit is sufficiently deep to provide a useful comparison with the ROSAT All-Sky Survey (RASS). This is a valuable asset for studies of long-term variability in objects.

The excellent sensitivity, good spatial resolution, and low background of XMM-Newton mean that the slew survey can usefully image bright extended sources. This can reveal interesting spatial structure in, for example, supernova remnants and clusters of galaxies.

For objects that are detected in both the hard and soft band a hardness ratio can be derived - this is a useful diagnostic measure for many objects.

A number of very bright objects have been detected in the slew survey with count rates that are sufficient to allow spectra to be extracted and analysed.

An examination of the distribution of slews shows that some areas have been covered more than once (see Figure 1). In this case variability studies can be performed within the survey.

The XMM-Newton Slew Survey Catalogue

 (A comprehensive guide to the Catalogue is available at the XMM-Newton Science Operations Centre web site. This text provides a brief overview.)

The three slew surveys (0.2-2 keV, 2-12 keV, and 0.2-12 keV) have been combined to form one catalogue: the XMM-Newton Slew Survey Catalogue. The catalogue is constructed by members of the XMM-Newton Science Operations Centre and the EPIC consortium on behalf of ESA, and is available to download or to query online via the XMM-Newton Science Archive.

Sources within the catalogue are assigned a name beginning with XMMSL1 followed by the J2000 sky coordinates. This name is maintained even when a counterpart has been identified by cross-correlation with another catalogue.

Constructing the catalogue

The slew data files for EPIC-pn are processed using the XMM Science Analysis Software which is available from the XMM-Newton Science Operations Centre web site. The data are pre-selected to include only data which can contribute meaningful results: for example, slews with a high background are discarded and only some observation modes are accepted. Coordinates are transformed from detector coordinates to sky coordinates using the satellite attitude history. The quoted positions have a 1 sigma error of 8 arcseconds (which is sufficient to allow for optical follow up of detected objects). Spurious detections are identified and flagged as such.

Figure 2: Distribution of sources in the first XMM-Newton Slew Survey Catalogue (ESA/XMM-Newton/EPIC/R.D. Saxton et al.)

The slew survey catalogue is constructed in two forms: the full catalogue contains all detected sources with a detection likelihood greater than 8. This is known to include a number of spurious detections. Using stricter selection criteria such as removal of known bad sources, including only sources with likelihood greater than 10 in most cases, and greater than 14 for regions with high background, a clean catalogue is constructed.

With the release of version 1.5 of the catalogue approximately 52.5 per cent of the sky has been covered, at least once. A final sky coverage of in excess of 70 per cent is anticipated.

A complete description of the catalogue construction process can be found in the User Guide of the slew survey.

Identifying slew survey objects

Objects that have been detected in the slew survey are correlated with a number of astronomical catalogues in order to match the slew survey sources with previously identified objects.

The SIMBAD and NED database are the first source for cross-correlation and provide about 90 per cent of the identifications. If a match cannot be found in these databases a search is then made for a correlation with clusters of galaxies listed in the catalogues of Abell and Zwicky. If this does not result in a successful match then the following catalogues, listed in order of priority, are searched for a close match: the All-sky Optical Catalog of Radio/X-Ray Sources, the Catalog of PSPC WGA sources, Einstein IPC Sources Catalog, EXOSAT CMA Images and Lightcurves, the ROSAT All-Sky Survey Catalogue, ROSAT Results Archive Sources for the PSPC, ROSAT Results Archive Sources for the HRI, RXTE Master Catalog, XMM-Newton Serendipitous Source Catalog and the INTEGRAL Bright Source Catalog.

About 70% of the sources in the first XMM-Newton Slew Survey have an identified counterpart.

Further details of this process are given in the User Guide to the catalogue.