The XMM-Newton Serendipitous Source Catalogue: 3XMM-DR8

User Guide to the Catalogue


Release 1.8 16th May 2018 Associated with Catalogue version 1.0

Prepared by the XMM-Newton Survey Science Centre Consortium

This User Guide refers directly to the full FITS and plain-text formats of the catalogue. Users interested in the details of changes to the data processing since the 3XMM-DR4 catalogue release, can refer directly to section 3. Information about the columns contained in the 3XMM-DR8 catalogue are presented in section 4. Brief summaries of some elements of the 3XMM-DR8 catalogue properties are provided in section 5 but a comprehensive evaluation of the catalogue is in Rosen, Webb, Watson et al., 2016, A&A, 590, 1.

Should you use the catalogue for your research and publish the results, please use the acknowledgement below and cite the paper describing 3XMM (Rosen, Webb, Watson et al., 2016, A&A, 590, 1).

This research has made use of data obtained from the 3XMM XMM-Newton serendipitous source catalogue compiled by the 10 institutes of the XMM-Newton Survey Science Centre selected by ESA.

Contents


Summary

3XMM-DR8 is the third generation catalogue of serendipitous X-ray sources from the European Space Agency's (ESA) XMM-Newton observatory, and has been created by the XMM-Newton Survey Science Centre (SSC) on behalf of ESA. It is an incremental release of the 3XMM catalogue and contains 532 more observations and 47363 more detections than the preceding 3XMM-DR7 catalogue, which was made public in June 2017.

The catalogue contains source detections drawn from a total of 10242 XMM-Newton EPIC observations made between 2000 February 3 and 2017 November 30; all datasets included were publicly available by 2017 December 31 but not all public observations are included in this catalogue. For net exposure time  ≥  1ksec, the total area of the catalogue fields is ~ 2010 deg2 but taking account of the substantial overlaps between observations, the net sky area covered independently is ~ 1089 deg2.

The catalogue contains 775153 X-ray source detections above the processing likelihood threshold of 6. These X-ray source detections relate to 531454 unique X-ray sources, that is, a significant fraction of sources (99580, 19%) have more than one detection in the catalogue (up to 59 repeat observations in the most extreme case).

The catalogue distinguishes between extended emission and point-like detections. Parameters of detections of extended sources are only reliable up to the maximum extent measure of 80 arcseconds. There are 70887 detections of extended emission, of which 16196 are 'clean' (in the sense that they were not manually flagged) and 12256 comprise the 'cleanest' set where no flags are set and they are not in fields with high background levels.

Due to intrinsic features of the instrumentation as well as some shortcomings of the source detection process, some detections are considered to be spurious or their parameters are considered to be unreliable. It is recommended to use a detection flag and an observation flag (and, possibly, a high background flag) as filters to obtain what can be considered a 'clean' sample. There are 633733 out of 775153 detections that are considered to be clean (i.e., summary flag < 3).

For 173208 detections, EPIC spectra and time series were automatically extracted during processing, and a χ2-variability test was applied to the time series. 5934 detections in the catalogue are considered variable, within the timespan of the specific observation, at a probability of 10-5 or less based on the null-hypothesis that the source is constant. Of these, 3603 have a summary flag  < 3.

The median flux (in the total photon-energy band 0.2 - 12 keV) of the catalogue detections is ~ 2.2 × 10-14 erg/cm2/s; in the soft energy band (0.2 - 2 keV) the median flux is ~ 5.2 × 10-15, and in the hard band (2 - 12 keV) it is ~ 1.2 × 10-14. About 23% have fluxes below 1 × 10-14 erg/cm2/s. The flux values from the three EPIC cameras are, overall, in agreement to ~ 10% for most energy bands. The meadian positional accuracy of the catalogue point source detections is generally < 1.6 arcseconds (with a standard deviation of 1.39).

1. Introduction

Pointed observations with the XMM-Newton Observatory detect significant numbers of previously unknown 'serendipitous' X-ray sources in addition to the proposed target. Combining the data from many observations thus yields a serendipitous source catalogue which, by virtue of the large field of view of XMM-Newton and its high sensitivity, represents a significant resource. The serendipitous source catalogue enhances our knowledge of the X-ray sky and has the potential for advancing our understanding of the nature of various Galactic and extragalactic source populations.

The 3XMM-DR8 catalogue is the tenth publicly released XMM-Newton X-ray source catalogue produced by the XMM-Newton Survey Science Centre (SSC) consortium. It follows the 1XMM (released in April 2003), 2XMMp (July 2006), 2XMM (August 2007), 2XMMi (August 2008), 2XMMi-DR3 (April 2010), 3XMM-DR4 (July 2013), 3XMM-DR5 (April 2015), 3XMM-DR6 (July 2016) and 3XMM-DR7 (June 2017) catalogues: 2XMMp was a preliminary version of 2XMM. 2XMMi and 2XMMi-DR3 are incremental versions of the 2XMM catalogue.

The 3XMM-DR8 catalogue is about 7% larger than the 3XMM-DR7 catalogue, which it supersedes. In terms of the number of X-ray sources, the 3XMM-DR8 catalogue is the largest ever produced. 3XMM-DR8 complements deeper Chandra and XMM-Newton small area surveys, probing a large sky area at the flux limit where the bulk of the objects that contribute to the X-ray background lie. The 3XMM-DR8 catalogue provides a rich resource for generating large, 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 active galaxies (AGN), clusters of galaxies, interacting compact binaries and active stellar coronae. The large sky area covered by the serendipitous survey, or equivalently the large size of the catalogue, also means that 3XMM-DR8 is a superb resource for exploring the variety of the X-ray source population and identifying rare source types.

The production of the 3XMM-DR8 catalogue has been undertaken by the XMM-Newton SSC consortium in fulfillment of one of its major responsibilities within the XMM-Newton project. The catalogue production process has been designed to fully exploit the capabilities of the XMM-Newton EPIC cameras and to ensure the integrity and quality of the resultant catalogue through rigorous screening of the data.

The incremental part of the 3XMM-DR8 catalogue uses slightly different pipeline versions to that which was used for 3XMM-DR7, 3XMM-DR6 and 3XMM-DR5. It is based on the pipeline configurations 15.23_20160428_1630 for observations made from May 2016 up to December 2016 and configurations 16.34_20170131_1230 for observations made from January 2017 up to June 2017 and 16.37_20170628_1250 for observations made from June 2017 up to December 2017. These pipeline versions contain minor changes to the processing with respect to the previous 3XMM versions. It makes use of the SAS version 15 and 16 and the latest calibration files available. The changes made to the pipeline may have a very small impact on some of the parameters provided for the extra data used for 3XMM-DR8. The changes to the pipeline that may cause a small effect on the some of the data in the catalogue are that a new algortithm is used to define the spatial region to estimate the PN background in EPIC source products More information on these changes can be found here .

Users of the 3XMM catalogue should be aware that the DETID and SRCID values bear no relation to those in the previous 2XMM series of catalogues. However, a cross-matching is provided in 3XMM-DR8 via the DR3DETID and DR3SRCID columns.

2. User Guide for 2XMM

The extensive User Guide (UG) for the 2XMM catalogue still describes many of the details of the data processing and compilation approach applicable to the 3XMM-DR8 catalogue. However, a significant number of changes to the processing have been implemented for 3XMM and these are described in the 3XMM-DR4 user guide. For convenience, Table 1, which gives the energy band definitions, is repeated here.

Table 1:  Energy bands used in 3XMM-DR8 processing
Basic energy bands: 1 = 0.2 -   0.5 keV  
2 = 0.5 -   1.0 keV    
3 = 1.0 -   2.0 keV    
4 = 2.0 -   4.5 keV    
5 = 4.5 - 12.0 keV    
Broad energy bands: 6 = 0.2 -   2.0 keV   soft band, no images made
7 = 2.0 - 12.0 keV   hard band, no images made
8 = 0.2 - 12.0 keV   total band
9 = 0.5 -   4.5 keV   XID band

3.   3XMM-DR8 -- key changes with respect to 3XMM-DR7

3.1 Data selection

XMM-Newton observations considered for inclusion in the 3XMM-DR8 catalogue were those with ODFs available for processing up to 2017 November 30 and which had public release dates up to 2017 December 31. After allowing for a small number of observations which failed in processing for a variety of reasons, 10242 observations were available to make the 3XMM-DR8 catalogue. Table 2.1 gives the list of the final 10242 observations which are included in the 3XMM-DR8 catalogue.

3.2 New naming convention for the DETID and the SRCID

To streamline the procedure for attributing the DETID number (which is unique to each detection) and the SRCID number (that is unique to each unique source) and to keep the same numbers from catalogue to catalogue, without providing supplementary columns in the catalogue with the DETID and SRCID from previous releases, starting in 3XMM-DR5 the numbering convention has been modified.

The OBSID which always remains the same for an observation is now coupled with the source number SRC_NUM to make the DETID. The SRCID attributed for a unique source is determined by using the first DETID attributed to that source (i.e. in the earliest observation that that source was detected).

Despite the new naming convention that aims at preserving SRCID numbers across catalogue versions, a certain number of SRCID numbers can disappear from one catalogue version to another. This is a consequence of the algorithm that groups detections together into unique sources. When new data is added and statistics is improved, the algorithm might find a better association of detections into unique sources. As a simple example, two detections initially considered as independent sources (therefore 2 distinct SRCID numbers) can be grouped together and considered as one unique source by the algorithm thanks to the inclusion of new detections in the area. Consequently, one SRCID number will disappear in the new version of the catalogue. A total of 134 SRCID listed in 3XMM-DR7 are absent in 3XMM-DR8. The list can be found here

4. Catalogue content and organisation

This section summarises the organisation of the catalogue and gives details of all the columns. Known problems with parameters presented in the catalogue or with products associated with it are listed in Sec. 6.

There are 332 columns in the catalogue; they are grouped together and explained in the links below.

For each observation there are up to three cameras with one or more exposures which were merged when the filter and submodes were the same (2XMM UG, Sec. 2.2). The data in each exposure are accumulated in several distinct energy bands (Table 1). Camera-level measurements can further be combined into observation-level parameters. Consequently, the source parameters can refer to some or all of these levels: on the observation level there are the final mean parameters of the source (prefix 'EP'); on the camera level the data for each of the three cameras (where available) are given (prefix 'PN', 'M1', or 'M2'), and on the energy band level the energy-dependent details of the source parameters are given (indicated by a 'b' in the column name where b = 1,2,3,4,5,8,9). Finally, on a meta-level, some parameters of sources that were detected more than once (prefix 'SC') were combined, see 2XMM UG, Sec. 3.2.4.

The column name is given in capital letters, the FITS data format in brackets and the unit in square brackets. If the column originates from a SAS task, the name of the task is given to the right hand side and a link is set to the SAS package documentation with which the data in the 3XMM-DR8 catalogue was processed. It should be pointed out that the SAS used for the bulk reprocessing (for 3XMM(DR4 and DR5)) was from manifest xmmsas_20121219_1645, which is based on SAS 12.0.1 but contains a number of SAS task upgrades that were required after the SAS 12.0.1 public release. The incremental part of the 3XMM-DR8 catalogue uses slightly different pipeline versions to that which was used for 3XMM-DR7, 3XMM-DR6 and 3XMM-DR5. It is based on the pipeline configurations 15.23_20160428_1630 for observations made from May 2016 up to December 2016 and configurations 16.34_20170131_1230 for observations made from January 2017 up to June 2017 and 16.37_20170628_1250 for observations made from June 2017 up to December 2017. These pipeline versions contain minor changes to the processing with respect to the previous 3XMM versions. A description of the column and possible cross-references follow.

Entries with NULL are given when no detection was made with the respective camera, that is, ca_MASKFRAC < 0.15 or NULL (i.e., a camera was not used in an observation).

Details of the columns:

Part 1: 14 columns: Identification of the source
This includes the basic static identifiers, IAU name, together with cross-references to the (spatially) nearest detection and source ID in the previous 3XMM-DR4 and 2XMMi-DR3 catalogues, where relevant, including information about the spatial displacements and the number of 'nearby' matches. Five new columns (introduced in 3XMM-DR7) are used to provide this information.
Part 2: 11 columns: Details of the observation and exposures
Part 3: 20 columns: Coordinates
The external equatorial and Galactic coordinates and the internal equatorial coordinates as derived from the SAS tasks catcorr and emldetect are given together with the error estimates. Two columns convey information about the absolute astrometric catalogue used for field rectification and whether the field was successfully rectified
Part 4: 225 columns: Source parameters
The parameters of the source detection as derived from the SAS tasks emldetect and srcmatch are given here.
Part 5: 8 columns: Detection flags
This part lists the flags to qualify the detections. The summary flag, which gives an overall assessment for the detection, is followed by particular flags for each camera. A flag each is given if there exists at least one time series or one spectrum for this source. One column is added at the end of the catalogue to provide information about detections arising in fields with high background levels
Part 6: 13 columns: Detection variability
This part gives variability information for those detections for which time series were extracted. This includes six columns which provide measures of the fractional variance of the timeseries
Part 7: 41 columns: Unique source parameters
This part lists the source parameters for the unique sources across all observations (using the prefix 'SC'); these are coordinates, fluxes, hardness ratios, likelihoods, extent information and a variability and a summary flag. The number of detections is given also. Of six columns introduced for 3XMM, two of these relate to a fractional variance measure (and error). The other four provide information about the maximum and minimum measured fluxes from constituent detections (and errors). Two further columns relating to the epochs of the first and last observations contributing to a unique source, replace two columns in 2XMMi-DR3

Columns in the slimline catalogue:

Table 6 lists the 44 columns in the 3XMM-DR8 'slimline' version of the catalogue, all of which are explained in Part 1 or Part 7 of the 3XMM-DR8 column description, except the WEBPAGE_URL column which is described at the end of the table. These are the same 44 columns as in 3XMM-DR7.

5. Catalogue Properties

This section summarises the main properties of the catalogue but does not provide a detailed analysis. A comprehensive evaluation of the catalogue is presented in the 3XMM catalogue paper (Rosen, Webb, Watson et al., 2016, A&A, 590, 1).

5.1 Overview

The catalogue contains source detections drawn from 10242 XMM-Newton EPIC observations made between 2000 February 3 and 2017 November 30 and which were publicly available by 2017 December 31. Net exposure times in these observations range from < 1000 up to ~ 130000 seconds (that is, a full orbit of the satellite). Figure 5.1 shows the distribution of fields on the sky.

The total sky area of the catalogue observations with effective exposure > 1 ks is ~ 2010 deg2 which translates to ~1089 deg2 when corrected for field overlaps.

The catalogue contains 775153 X-ray detections with total-band (0.2 -12 keV) likelihood values  ≥  6. These are detections of 531454 unique X-ray sources, that is, 99680 X-ray sources have multiple detections in separate observations (up to 59 detections). Of the 775153 X-ray detections, 70887 are classified as extended with 16196 of these being in regions considered to be 'clean' (SUM_FLAG  < 3).

5.2 Data quality

As part of extensive quality evaluation for the catalogue, each field has been visually screened. Regions where there were obvious deficiencies with the automatic source detection and parameterization process were identified and all detections within those regions were flagged (cf. 2XMM UG, Sec. 3.2.6 but importantly, note Section 3.11). Such flagged detections include clearly spurious detections (many of which are classified as extended) as well as detections where the source parameters may be unreliable. Each XMM-Newton field is also evaluated to assess the fractional area of the observation that is affected by flagged detections, as reflected by the OBS_CLASS parameter. For most uses of the catalogue it is recommended to use a detection flag (SUM_FLAG, EP_FLAG or SC_SUM_FLAG) and an observation flag (OBS_CLASS) as a filter to obtain what can be considered a 'clean' sample.

Note that no attempt is made to flag spurious detections arising from statistical fluctuations in the background. An updated analysis of the false detection rate are presented in the 3XMM catalogue paper (Rosen, Webb, Watson et al., 2016, A&A, 590, 1).

5.3 Sensitivity and Photometry

Figure 5.4 presents, for each of the three cameras, the distributions of flux for energy bands 1 to 5 and also for the combined (EPIC) data. These give an indication of the limiting flux available in the catalogues for each of the bands.

Comparison of the detection count rates and fluxes in the 3XMM and previous 2XMMi-DR3 version shows good agreement between the two catalogues. A more detailed analysis of photometric issues are presented in the 3XMM catalogue paper (Rosen, Webb, Watson et al., 2016, A&A, 590, 1).

5.4 Astrometry

As noted in section 3.4 of the 3XMM-DR4 user guide, the 3XMM catalogue benefits from a number of improvements to the astrometry, several of which resulted from effects only discovered in the process of compiling the catalogue. The net effect for 3XMM source positions is a small improvement in the statistical position errors, a reduction in the position error systematics and increased confidence in the reliability of the position errors. A more detailed analysis of these issues are presented in the 3XMM catalogue paper (Rosen, Webb, Watson et al., 2016, A&A, 590, 1).

6. Known problems and other issues

Please read the Watchouts section of the 3XMM-DR8 catalogue page for the latest information on 3XMM-DR8 catalogue issues.

6.1 Problem cases

6.1.1 Spurious sources arising from MOS low energy noise

A significant number of observations have shown clear evidence of low energy noise affecting specific CCDs in the MOS cameras. Generally but not exclusively, it is CCD4 or CCD5 in MOS1 and CCD2 and CCD5 in MOS2 that are affected and the effect predominantly affects energies below 1keV (bands 1 and 2). Affected CCDs often stand out in the MOS images as having notably higher count levels compared to the adjacent CCDs. Of itself, this increased noise primarily leads to reduced sensitivity in the relevant CCD sky area.

However, a further significant impact arises in source detection because the computation of the background map (see 2XMM UG, Sec. 3.1.2d) does not adequately cope with the step transition in the brightness level between the noisy CCD and adjacent CCDs. The result can be an over- or under- representation of the background map in the vicinity of the CCD boundary and this can then lead to the detection of spurious (often extended) sources near the edges of the noisy CCD where it borders another CCD. These sources generally receive a manual flag from the visual screening process (see 2XMM UG, Sec. 3.2.6, and the changes discussed in Section 3.11, user guide for 3XMM-DR4) but users should be aware of the issue.

6.1.2 Reduced sensitivity due to high background

The optimised flare filtering process generally results in greater sensitivity to sources. However, in some circumstances, the reverse can occur. About 595 fields that are present in 3XMM-DR8 show higher backgrounds in the EPIC images, and fewer detections, in the 3XMM-DR8 data. This arises because occasionally, one or more instruments can have a persistent high background while the other instruments have a lower background count rate. In previous processing the high background instruments were generally excluded from the source detection stage because, after applying the flare GTI filtering, less than 1ksec of data remained.

In the processing for 3XMM, the optimised flare filtering process determines an optimum background threshold, even if the count rate is persistently high. This may then leave significant apparently usable exposure with a high count rate. As such, instruments showing a persistently high background can still be included in the source detection stage and, even when combined with lower background data from the other instruments, can lead to reduced, rather than increased sensitivity.

6.1.3 Exposure correction failure for timeseries

In previous catalogues, a few cases have been noted where the detection shows a variability that is due to incorrect handling of the data. Two reasons have been considered responsible:

6.2 Other issues

6.2.1 New calibrations and fluxes

A number of improvements in the calibration of the MOS and pn have occurred which lead to slight changes in the Energy Conversion Factors (ECFs) (see here for information on the EPIC response files) that are used for converting EPIC band count rates to fluxes. Of note is the fact that MOS redistribution matrices are now provided for 14 epochs and for three areas of the detector that reflect the so-called 'patch', 'wings-of-patch' and 'off-patch' locations.

From 3XMM-DR5 a simple approach has been adopted. ECFs were computed following the prescription of Mateos et al. (2009), for energy bands 1-5 and band 9, for full-frame mode, for each EPIC camera, for each of the Open, Thin, Medium and Thick filters. For pn, the ECFs are calculated at the on-axis position. The pn response is sufficiently stable that no temporal resolution is needed.

For MOS, to retain a direct connection between the ECFs and publicly available response files, the ECFs used are taken at epoch 14 and are for the 'off-patch' location. The latter choice was made because the large majority of detections in an XMM-Newton field lie outside the 'patch' and 'wings-of-patch' regions, which only relate to a region of radius  ≤  40 arcseconds, near the centre of the field. The use of a single epoch (epoch 14) was made to retain simplicity in the processing and because the response of the MOS cameras exhibits a step function change between epochs 5 and 6, with different but broadly constant values either side of the step. None of the 14 calibration epochs represent the average response and thus no response file exists to which average ECFs can be directly related. The step-function change in the responses for MOS is most marked in band 1 (0.2-0.5 keV) for the 'patch' location, where the maximum range in ECFs either side of the step amounts to 20%. Outside the 'patch' region, and for all other energy bands, the range of the ECF values with epoch is  ≤  5% and is  ≤  2.5% for the 'off-patch' region. Epoch 14 was chosen, somewhat arbitrarily, as being typical of epochs in the longer post-step time interval.

The ECFs, in units of 1011 cts cm2/erg, adopted for 3XMM-DR8, are provided here, for each camera, energy band and filter. The camera rate and flux are related via

ca_FLUX = (ca_RATE   / ECF )

Note that canned response matrices for basic XMM-Newton spectral analyses can be obtained from here.

6.2.2 High proper motion objects

No epoch information is used when matching detections to construct unique sources. As a consequence, detections of high proper motion stars from multiple observations spanning a significant period of time may not have been matched into a single unique source in the catalogue. A good example is 61 Cyg whose proper motion (~ 5 arcseconds/year) between observations from the earliest XMM-Newton revolution (539) in 3XMM-DR5 to the latest (2269) translates in to a shift in position of more than 45 arcseconds between the first and last observations. The detections of the stellar component at higher declination are mapped to two distinct unique sources due to its movement (i.e. 3XMM J210655.9+384516 and 3XMM J210657.4+384527) - the component at lower declination is grouped in to 3 unique sources. However, the more relaxed criteria for recognising potential confusion result in it being flagged as CONFUSED, the confusion arising from positional overlaps with other detections of itself.

References:

Budavari, T & Szalay, A 2009, Ap.J, 679, 301-309 Probabilistic Cross-Identification of Astronomical Sources

Edelson, R., et al. 2002, Ap.J, 568, 610-626 X-Ray Spectral Variability and Rapid Variability of the Soft X-Ray Spectrum Seyfert 1 Galaxies Arakelian 564 and Ton S180

Mateos, S., et al. 2009, A&A, 496, 879-889 Statistical evaluation of the flux cross-calibration of the XMM-Newton EPIC cameras

Pye, J., et al. 1995, MNRAS, 274, 1165-1193, The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources - II. The 2RE Source Catalogue

Read, A., et al. 2011, A&A, 534, A34 A new comprehensive 2D model of the point spread functions of the XMM-Newton EPIC telescopes: spurious source suppression and improved positional accuracy

Rosen, S.R., Webb, N.A., Watson, M.G., et al. 2016, A&A, 590, 1 The XMM-Newton serendipitous survey. VII. The third XMM-Newton serendipitous source catalogue

Vaughan, S., et al. 2003, MNRAS, 345, 1271-1284 On characterizing the variability properties of X-ray light curves from active galaxies

Watson, M., et al. 2009, A&A, 493, 339-373 The XMM-Newton serendipitous survey. V. The Second XMM-Newton Serendipitous Source Catalogue

Document revision history

Release No. Release Date      Comments
1.0 23 July 2013 First release
1.1 24 July 2013 Added section 3.12 and fixed minor typographic corrections
1.2 02 August 2013 Section 6.1.4 added. Reference added. Some SAS task links ammended. Links to CCF and SAS tasks provided in A.2
1.3 30 August 2013 Added link to watchouts in section 6.
1.4 15 September 2013 Added units and flux-rate conversion formula in section 6.2.1.
1.5 April 2015 Revised version for 3XMM-DR5
1.6 July 2016 Revised version for 3XMM-DR6
1.7 June 2017 Revised version for 3XMM-DR7
1.8 May 2018 Revised version for 3XMM-DR8

Appendices

A.1 List of the observations used in the catalogue

List of observations ('fields').