The XMM-Newton Serendipitous Source Catalogue: 4XMM-DR9

User Guide to the Catalogue

Release 1.9 18th December 2019 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-DR8 catalogue release, can refer directly to section 3. Information about the columns contained in the 4XMM-DR9 catalogue are presented in section 4. Brief summaries of some elements of the 4XMM-DR9 catalogue properties are provided in section 5 but a comprehensive evaluation of the catalogue is in Webb et al., submitted.

Should you use the catalogues 4XMM-DR9 or 4XMM-DR9s for your research and publish the results, please use the acknowledgement below and cite the paper describing 4XMM (Webb et al., submitted).

This research has made use of data obtained from the 4XMM XMM-Newton serendipitous source catalogue compiled by the 10 institutes of the XMM-Newton Survey Science Centre selected by ESA.



4XMM-DR9 is the fourth 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 a complete re-reduction of all of the data taken since the beginning of the mission using the best calibration and software at the time of reprocessing (March 2019). It contains just 962 more observations and 35642 more detections than the preceding 3XMM-DR8 catalogue, which was made public in May 2018, but vast improvements to the background model, as well as other calibration have led to an important improvement in source detection, so that the majority of detections in 4XMM-DR9 are clean detections and fewer spurious sources are registered. In addition, we provide spectra and lightcurves for more than 115074 more detections than in 3XMM-DR8 and the pn lightcurves are binned to a much higher resolution compared to 3XMM.

The catalogue contains source detections drawn from a total of 11204 XMM-Newton EPIC observations made between 2000 February 1 and 2019 February 26; all datasets included were publicly available by 2018 December 18 but not all public observations are included in this catalogue. For net exposure time  ≥  1ksec, the net area of the catalogue fields taking account of the substantial overlaps between observations is ~ 1152 deg2.

4XMM-DR9 contains 810795 X-ray detections above the processing likelihood threshold of 6. These X-ray detections relate to 550124 unique X-ray sources. A significant fraction of sources (104638, 19%) have more than one detection in the catalogue (up to 69 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 76999 detections of extended emission, of which 17295 are 'clean' (in the sense that they were not flagged).

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 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 288282 detections, EPIC time series and 288521 detections, EPIC lightcurves were automatically extracted during processing, and a χ2-variability test was applied to the time series. 6696 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, 4957 have a summary flag  < 3.

The median flux (in the total photon-energy band 0.2 - 12 keV) of the catalogue detections is ~ 2.3 × 10-14 erg/cm2/s; in the soft energy band (0.2 - 2 keV) the median flux is ~ 5.3 × 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 median positional accuracy of the catalogue point source detections is generally < 1.7 arcseconds (with a standard deviation of 1.4).

With 4XMM-DR9, we also release 4XMM-DR9s, a new version of the stacked catalogue built from 6604 4XMM-DR9 overlapping observations. 4XMM-DR9s contains 1329 stacks (or groups). Most of the stacks are composed of 2 observations and the largest has 352. The catalogue contains 288191 sources, of which 218283 have several contributing observations. Stacking observations allows yet fainter sources to be detected in sky regions observed more than once, increasing the number of detections and uncovering long-term variability on repeatedly observed objects.

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 4XMM-DR9 catalogue is the eleventh 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), 3XMM-DR7 (June 2017) and 3XMM-DR8 (May 2018) catalogues: 2XMMp was a preliminary version of 2XMM. 2XMMi and 2XMMi-DR3 are incremental versions of the 2XMM catalogue.

Whilst the 4XMM-DR9 catalogue is only about 5% larger than the 3XMM-DR8 catalogue, the source detection is markedly improved and we provide spectra and lightcurves for 67\% more detections than in 3XMM-DR8. In terms of the number of X-ray sources, combining the 4XMM-DR9 and 4XMM-DR9s catalogues gives a catalogue that is similar in size to the Chandra Source Catalogue. 4XMM-DR9 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 4XMM-DR9 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 4XMM-DR9 is a superb resource for exploring the variety of the X-ray source population and identifying rare source types.

The production of the 4XMM-DR9 and 4XMM-DR9s catalogues 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.

4XMM-DR9 and 4XMM-DR9s are based on the pipeline configurations 18. This pipeline version contains many changes with respect to the pipeline used to make the previous major version of the catalogue, 3XMM-DR5. Changes include source spectra and light curves created for pn Timing mode and small window data, source detection on pn small window data, energy dependent Charge Transfer Inefficiencies (CTI) and double event energy corrections applied, time and pattern dependent corrections of the spectral energy resolution of pn data, X-ray loading and rate dependent energy (PHA) and CTI corrections for EPIC pn Timing and Burst modes, binning of MOS spectra changed from 15 eV to 5 eV and filtering with XMMEA_EM, which is a bit-wise selection expression, automatically removing “bad events” such as bad rows, edge effects, spoiled frames, cosmic ray events (MIPs), diagonal events, event beyond threshold, etc, instead of XMMEA_SM (which removed all flagged events except those flagged only as CLOSE_TO_DEADPIX), background regions for EPIC spectra and light curves selected from the same EPIC chip where the source is found, observations of solar system objects processed such that X-ray images and spectra correctly refer to the moving target, pileup diagnostic numbers for EPIC sources included, and footprints for EPIC observations based on combined EPIC exposure maps provided as ds9 region files. Other changes carried out specifically for the production of 4XMM include a revised systematic position error, the modelling of the EPIC background and finer binning of EPIC lightcurves. More information on these changes can be found here and in the paper, Webb et al. submitted.

Users of the 4XMM 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 4XMM-DR9 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 4XMM-DR9 catalogue. However, a significant number of changes to the processing have been implemented for 4XMM and these are described in the Section 3. For convenience, Table 1, which gives the energy band definitions, is repeated here.

Table 1:  Energy bands used in 4XMM-DR9 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.   4XMM-DR9 -- key changes with respect to 3XMM

3.1 Data selection

XMM-Newton observations considered for inclusion in the 4XMM-DR9 catalogue were those with ODFs available for processing up to 2019 February 26 and which had public release dates up to 2019 December 18. After allowing for a small number of observations which failed in processing for a variety of reasons, 14041 observations were available to make the 4XMM-DR9 catalogue. Table 2.1 gives the list of the final 11204 observations which are included in the 4XMM-DR9 catalogue.

3.2 Naming convention for the DETID and the SRCID

Starting in 3XMM-DR5, the procedure for attributing the detection identification number (DETID) and the unique source identification number (SRCID), both being unique to each detection and each unique source respectively, has been modified. Previously, identification numbers were re-computed for each catalogue version leading to supplementary columns added to the catalogue with the DETID and SRCID from previous releases.

The DETID is now constructed from the OBS_ID, which always remains the same for an observation, coupled with the source number SRC_NUM (SRC_NUM is the source number in the individual source list for a given observation; Sources are numbered in decreasing order of count rate (i.e. the brightest source has SRC_NUM = 1)) as follow:


where the '+' sign indicates string concatenation and where SRC_NUM is zero-padded to form a 4 digit number. The SRCID of a unique source is then determined from the first DETID attributed to that source (i.e. in the observation where the source was first detected) and replacing the first digit '1' by '2'.

Despite the new naming convention that aims at preserving SRCID numbers across catalogue versions, a certain number of SRCID can disappear from one catalogue version to another. This is a normal consequence of the algorithm that groups detections together into unique sources (see section 6 of Rosen et al. 2016). When new data are added and statistics are improved, the algorithm might find a better association of detections into unique sources. As an example, a total of 134 SRCIDs listed in 3XMM-DR7 are absent in 3XMM-DR8.

3.3 Missing detections and DETID change

In addition, the pipeline reprocessing of the full public dataset for the 4XMM version of the source catalogue led to significant modifications of the detection list. There are 10 214 observations that are common between the 3XMM-DR8 and 4XMM-DR9 catalogues, resulting in 773 241 detections in 3XMM-DR8 and 726 279 detections in 4XMM-DR9. Of these, there are 608 071 point-like detections with a SUM_FLAG ≤ 1 in 3XMM-DR8 and 607 196 in 4XMM-DR9. However, amongst these observations, there are ~128 000 detections that appear in 3XMM-DR8 that are not matched with a detection in the same observation in the 4XMM-DR9 catalogue within a 99.73% confidence region (i.e 2.27 x POSERR). About 67 000 of these were classified as the cleanest (SUM_FLAG ≤ 1), point-like sources in 3XMM-DR8 -- these are referred to as missing 4XMM detections in what follows. It should be noted that in reverse, there are ~164 000 detections in the 4XMM-DR9 catalogue that are in common observations but not matched with a detection in 3XMM-DR8 within 99.73% confidence region, approximately 107 000 of which are classed as being clean and point-like.

This is an expected consequence of the reprocessing which was already encountered in the transition from 2XMM to 3XMM (see Section 8 and Appendix D in Rosen et al. 2016). The number of missing 4XMM detections is consistent with the number of missing 3XMM detections, where there were ~25700 good detections that appeared in 2XMMi-DR3 that were not matched with a detection in the same observation in the 3XMM-DR5 catalogue (Rosen et al. 2016). This amounts to ~4.5% which is of the same order as the number of missing sources in 4XMM (8.3%). The origin of these source discrepancies between the two catalogues are the improvements made to the pipeline and in particular the new background estimation. The majority of the detections present in 3XMM-DR8 that are not present in 4XMM-DR9 are from the lowest maximum likelihoods (see Webb et al. submitted). A small change in the parameters can cause a source with a maximum likelihood close to the cut-off of 6, but none the less slightly above, to have a value slightly below the cut-off and therefore be excluded from the catalogue. Conversely, the changes in the pipeline for sources just below the maximum likelihood cut-off of 6 and therefore not in 3XMM-DR8 can mean that they will then have a higher maximum likelihood and be present in 4XMM-DR9. Fewer obviously spurious detections are found in 4XMM-DR9 than in 3XMM-DR8, where the detections found in 4XMM-DR9 and not in 3XMM-DR8 are generally more reliable (higher maximum likelihood).

3.4 Systematic position error

The astrometry of the X-ray detections is improved by using the catcorr task to cross-correlate the X-ray detections with the USNO B1.0, 2MASS or SDSS (DR8) optical/IR catalogues. However, where catcorr fails to obtain a statistically reliable result (poscorrok=false), a systematic error of 1.5′′ was used to create the 3XMM catalogue. To check this value, we cross-matched SDSS quasars with the detection catalogue where poscorrok=false, out to r=30 arcsecs, filtering the latter with SUM_FLAG=0 and EP_EXTENT=0, to keep only the cleanest sample of secure point-like X-ray sources. For more information about what was done, see Webb et al. submitted. We define the combined positional error as σ=(Δ S2+Δ X2/2)0.5, where Delta; X = POSERR and Δ S is the radially-averaged uncertainty in the SDSS positions to which we had already added a systematic 0.1'' in quadrature, and x = r / σ. Our final filtering retained only the 157 QSO-X-ray pairs with x<5.

The expected probability density distribution of x should follow the Rayleigh distribution P(x) = x e-x2. Since this was not the case for the 157 pairs of sources found above, we added an additional positional uncertainty, σ, in quadrature, so that the total positional uncertainty is now σ = (σ2 + Σ2)0.5, looking for the value of Σ that minimizes the difference between the distribution of the x' = r/σ' and the Rayleigh distribution. We found Σ = 1.29 +/- 0.01 arcsec, where the uncertainty (1 σ) has been calculated by bootstrap with replacement. This value was then used to replace the 1.5 arcsec systematic error when poscorrok=false.

3.5 Modelling the EPIC background

For each input image to the source detection, the background is modelled by an adaptive smoothing technique. The method was initially applied to the proto-catalogue from overlapping XMM-Newton observations and described by Traulsen et al. 2019. Since the proto-catalogue was based on a selection of clean observations, the smoothing parameters were revised for the 4XMM catalogues, which cover observations of all qualities. The three parameters of the smoothing task are the cut-out radius to excise sources, the minimum kernel radius of the adaptive smoothing, and the requested signal-to-noise ratio in the map. Their best values were determined in a three-fold assessment which involved real observations, randomized images, and visual screening.

656 observations were chosen which cover positions of cluster candidates to involve a considerable number of extended and of point-like sources. Their background was modelled using different combinations of the smoothing parameters, and source detection was performed. The number of detections and recovered clusters, and the source parameters of the clusters and point-like detections were compared, opting for a reasonable compromise between total number of detections and potentially spurious content and for reliable fluxes and extent radius of the clusters. The source parameters of point-like detections were largely unaffected by the parameter choice in the tested parameter range.

The optimisation was then re-run on ninety observations, in which the background was replaced by a Poissonian randomisation. Finally, the two best combinations of smoothing parameters and the previously used spline fit were compared in a blind test. The detection images were inspected in randomised order, so the screeners could not know which source-detection results were based on which background model. The three parts of the assessment confirmed the preference for the adaptive smoothing approach over a spline fit and the final parameters: a brightness threshold for the source cut-out radius of 2 x 10-4 counts arcsec-2, a minimum smoothing radius of 10 pixels (40 arcsec in default image binning), and a signal-to-noise ratio of 12.

3.6 Hot areas in the detector plane

Warm pixels on a CCD (at a few counts per exposure) are too faint to be detected as such by the automatic processing, but can either push faint sources above detection level, or create spurious sources when combined with statistical fluctuations. This is an intrinsically random process, not visible over a short period of time, but which creates hot areas when projecting all sources detected over 18 years onto the detector plane.

We addressed this by projecting all sources onto CCD coordinates PN/M1/M2_RAWX/Y, keeping only sources above the detection threshold with the current instrument alone. In that way, we can distinguish hot areas coming from different instruments. We proceeded to detect hot pixels or columns in each CCD, using a similar method to the SAS task embadpixfind. For more information see Webb et al. submitted. Many of warm pixels were not present at the beginning of the mission, and some appear for a short amount of time. So we tested each hot area for variability using revolution number, and the same KS-based algorithm used to detect segments of bright columns, compared to the reference established over all sources on all CCDs and all instruments. This resulted in a revolution interval for each hot area.

Sources on a hot area for a particular instrument and within the corresponding revolution interval are flagged with flag 12. PN hot areas result in 16503 flagged sources, MOS1 in 6245 and MOS2 in 1382 flagged sources. The updated table for the 12 flags is given below.

Table 3.1:  Flag Keys
1   Low detector coverage ca_MASKFRAC <  0.5
2   Near other source R ≤ 65 * SQRT (EP_RATE); R(min) = 10", R(max) = 400"
3   Within extended emission R ≤ 3 * EP_EXTENT; R(max) = 200"
4   Possible spurious extended source near bright source Flag 2 is set and EP_CTS(min) = 1000 for the causing source
5   Possible spurious extended source within extended emission R ≤ 160" and fraction of rate wrt causing source is 0.4
6   Possible spurious extended source due to unusal large single-band DET_ML Fraction of ca_b_DET_ML wrt the sum of all ≥ 0.9
7   Possible spurious extended source At least one of the flags 4, 5, 6 is set
8   On bright MOS-1 corner or bright low gain PN column
9   Near bright MOS-1 corner R ≤ CUTRAD = 60" of a bright pixel the corner
10   Detection whose centre lies on any masked column or row due OoT and RGA features.
11   Within region where spurious detections occur Manual flag
12   Detection on hot area

The default value of every flag is F for False. When a flag was set it means it has been changed to T for True.

The task dpssflag sets all flags except the camera-specific flags (i.e., flags 2,3,4,5,6,7) on the summary row (EPIC band 8) which are then propagated backwards to the individual cameras and bands.

3.7 Lightcurve generation

Lightcurves are corrected using the SAS task, epiclccorr, to take into account events lost through inefficiencies due to vignetting, bad pixels, chip gaps, PSF and quantum efficiency, dead time, GTIs and exposure. epiclccorr also takes into account the background counts, using the background lightcurve, extracted over the identical duration as the source lightcurve. The time bin size for the pn lightcurves was previously a minimum of 10 s and could be as poorly sampled as tens of thousands of seconds for the faintest sources. To exploit the high time resolution and high throughput of the pn, for 4XMM we now extract the pn lightcurve such that each bin is 20 times the frame time, usually 1.46 s. The binning of the MOS data remains as it was for 3XMM.

3.8 Pile up information

As of 4XMM we provide three new columns (PN_PILEUP, M1_PILEUP and M2_PILEUP) quantifying whether each detection may be affected by pile-up in any instrument. A value below 1 corresponds to negligible pile-up (less than a few % flux loss) while values larger than 10 denote heavy pile-up. Pile-up is dependent on time for variable sources. We neglect that here, but note that a variable source is more piled-up than a constant one for the same average count rate, so our pile-up level can be viewed as a lower limit. We also neglect the slight dependence on the source spectrum due to the event grade dependence of pile-up.

Our pile-up levels are not based on a fit of the full images using a pile-up model (Ballet 1999). For point sources, they are based on the measured count rates reported in the catalogue over the full energy band, transformed into counts per frame. The thresholds (at which the pile-up level is set to 1) are set to 1.3 cts/frame for MOS and 0.15 cts/frame for PN.

For extended sources, the pile-up level is based on the measured count rate per CCD pixel at the source position, and therefore refers to the peak brightness, assuming this can be considered uniform at the pixel scale (4.1 arcsec for PN). The threshold is set for all instruments to 5 x 10-3 cts/frame/pixel, such that the flux loss is also a few % when the pile-up level is 1.

3.9 Extent maximum likelihood

All detections are tested for their potential spatial extent during the fitting process. The instrumental point-spread function (PSF) is convolved with a β extent model, fitted to the detection, and the extent likelihood EP_EXTENT_ML is calculated as described by Section 4.4.4 of Watson et al. 52009). A source is classified as extended if its core radius (of the β-model of the PSF), rc > 6 arcsec and if the extended model improved the likelihood with respect to the point source fit such that it exceeded a threshold of Lext,min = 4. In the 4XMM catalogues, EP_EXTENT_ML is included for all detections, while it was set to undefined for point-like detections in previous catalogues. Lext,min ≥ 4 indicates that a source is probably extended, whilst negative values indicate a clear preference of the point-like over the extended fit. As in the previous catalogue, a minimum likelihood difference of four has been chosen to mark a detection as extended. This threshold makes sure that the improvement of the extended over the point-like fit is not only due to statistical fluctuations but from a more precise description of the source profile.

3.10 The stacked catalogue

A second independent catalogue is compiled in parallel by the XMM-Newton SSC, called 4XMM-DR9s, where the letter 's' stands for stacked. This catalogue lists source detection results on overlapping XMM-Newton observations. The construction of the first version of such a catalogue, 3XMM-DR7s, is described in Traulsen et al. 2019. The construction of 4XMM-DR9s essentially follows the ideas and strategies described there with a few important changes that are described in full detail in the accompanying paper Traulsen et al. (sub.). The two main changes concern the choice of input observations and event-based astrometric corrections before source detection. Also it was found necessary to perform some visual screening of the detections, whose results are reported in the source catalogue.

Observations entering 3XMM-DR7s were filtered rather strictly. Only observations with OBS_CLASS$ < 2, with all three cameras in full-frame mode, and with an overlap area of at least 20% of the usable area were included. All those limitations were relaxed for the construction of 4XMM-DR9s which resulted in a much larger number of observations to be included and potentially much larger stacks (more contributing observations). Before performing simultaneous source detection on the overlapping observations, individual events were shifted in position using the results from the previous catcorr positional rectification of the whole image processed for 4XMM-DR9. This led to a clear improvement of the positional accuracy in stacked source detection.

All sources found by stacked source detection are listed in 4XMM-DR9s, including those from image areas where only one observation contributes. One may expect some differences between these same sources in 4XMM-DR9 and DR9s, because their input events were treated differently. More information is given in Traulsen et al. (sub.).

4XMM-DR9s is based on 1329 stacks (or groups) with 6604 contributing observations. Most of the stacks are composed of 2 observations, the largest has 352. The catalogue contains 288191 sources, of which 218283 have several contributing observations. Auxiliary data products comprise X-ray and optical images and long term X-ray light curves. Thanks to the stacking process, fainter objects can be detected and 4XMM-DR9s contains more sources compared to the same fields in 4XMM-DR9.

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 336 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 4XMM-DR9 catalogue was processed. It should be pointed out that the SAS used for the bulk reprocessing (for 4XMM) was from manifest pipeline version 18, which is based on SAS 18. 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: 228 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 45 columns in the 4XMM-DR9 'slimline' version of the catalogue, all of which are explained in Part 1 or Part 7 of the 4XMM-DR9 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 4XMM catalogue papers (Webb et al., submitted).

5.1 Overview

The catalogue contains source detections drawn from 11204 XMM-Newton EPIC observations made between 2000 February 1 and 2019 February 26 and which were publicly available by 2019 December 18. 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 sky area of the catalogue observations corrected for field overlaps with effective exposure > 1 ks ~1052 deg2.

The catalogue contains 810795 X-ray detections with total-band (0.2 -12 keV) likelihood values  ≥  6. These are detections of 550124 unique X-ray sources, that is, 104638 X-ray sources have multiple detections in separate observations (up to 69 detections). Of the 810795 X-ray detections, 76999 are classified as extended with 17295 of these being in regions considered to be 'clean' (SUM_FLAG  < 1).

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 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 4XMM catalogue paper (Webb et al., submitted).

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.

5.4 Astrometry

Comparing the astrometry between 3XMM-DR8 and 4XMM-DR9 shows very similar results. A more detailed analysis of these issues are presented in the 4XMM catalogue paper (Webb et al., submitted).

6. Known problems and other issues

Please read the Watchouts section of the 4XMM-DR9 catalogue page for the latest information on 4XMM-DR9 catalogue issues.

6.1 Problem cases


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 18 December 2019 First release for 4XMM


A.1 List of the observations used in the catalogue

List of observations ('fields').

A.2 Catalogue pipeline processing details

4XMM-DR9 was reduced with pipeline : version 18, with the Current Calibration Files (CCFs) of February 2019 pipeline release notes.