The XMM-Newton Serendipitous Source Catalogue: 2XMMi

User Guide to the Catalogue

Release 1.0 20 August 2008 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 the data processing can refer directly to section 2 and using the link from there to the 2XMM User Guide. Users interested in the main properties of the catalogue will find the summary and sections 1 & 5 of most immediate interest.



2XMMi is the incremental second 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. The catalogue has 626 more observations and about 17% more detections than the 2XMM catalogue, which was made public in August 2007. The processing used to generate the 2XMMi catalogue is the same as used for 2XMM and is based on the pipeline developed for the re-processing of all XMM observations conducted in 2006/7.

The catalogue contains source detections drawn from 4117 XMM-Newton EPIC observations made between 2000 February 3 and 2008 March 28; all datasets included were publicly available by 2008 May 01 but not all public observations are included in this catalogue. The total area of the catalogue fields is ~ 660 deg2, but taking account of the substantial overlaps between observations, the net sky area covered independently is ~ 420 deg2.

The catalogue contains 289083 X-ray source detections above the processing likelihood threshold of 6. The 289083 X-ray source detections relate to 221012 unique X-ray sources, that is, a significant fraction of sources (33302) have more than one detection in the catalogue.

The present 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 24533 detections of extended emission, of which 4684 are 'clean' (i.e., have not received a manual flag).

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 either a detection flag or an observation flag as a filter to obtain what can be considered a 'clean' sample. There are 233698 out of 289083 detections that are considered to be clean (i.e., summary flag < 3), and 2021 out of 4117 fields are considered to have no or at most a couple of spurious detections in them (observation class < 3).

For 44621 detections spectra and time series were automatically extracted during processing, and a χ2-variability test was applied to the time series. 2608 detections in the catalogue are considered variable at a probability of 10-5 or less based on the null-hypothesis that the source is constant.

The median flux (in the total photon-energy band 0.2 - 12 keV) of the catalogue detections is ~ 2.5 × 10-14 erg/cm2/s; in the soft energy band (0.2 - 2 keV) the median flux is ~ 5.8 × 10-15, and in the hard band (2 - 12 keV) it is ~ 1.4 × 10-14. About 20% have fluxes below 1 × 10-14 erg/cm2/s. The positional accuracy of the catalogue point source detections is generally < 5 arcseconds (99% confidence radius). The flux values from the three EPIC cameras are overall in agreement to ~ 10% for most energy bands.

Though the 2XMMi catalogue is a catalogue of serendipitous sources, the observations from which it has been compiled are pointed observations with one or more targets. An analysis to identify the target of each observation and to classify the contents of the field with regard to its serendipity has been carried out. In about 90% of the cases, the target could be identified via Simbad or NED and in 98% of the cases at least the object type could be determined.

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 2XMMi catalogue is the fourth publicly released XMM X-ray source catalogue produced by the XMM Survey Science Centre (SSC) consortium, following the 1XMM (released in April 2003), 2XMMp (July 2006) and 2XMM (August 2007) catalogues: 2XMMp was a preliminary version of 2XMM. 2XMMi is an incremental version of the 2XMM catalogue.

The 2XMMi catalogue is about 17% larger than the 2XMM catalogue, which it supercedes, due to the 1-year longer baseline of observations included (it is about 8 times larger than the 1XMM catalogue). As such, it is the largest X-ray source catalogue ever produced, containing more than twice as many discrete sources as either the ROSAT survey or pointed catalogues. 2XMMi 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 2XMMi 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 2XMMi is a superb resource for exploring the variety of the X-ray source population and identifying rare source types.

The production of the 2XMMi 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 exploit fully 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 predecessor 2XMM catalogue was made from a subset of public observations emerging from a re-processing (in 2006/7) of all XMM observations made prior to that point. The creation of the incremental 2XMMi catalogue has been driven by the desire to make public the additional data from that re-processing that were proprietary at the time of the 2XMM release but which subsequently became public before 01 May 2008. These have been augmented with a further 90 observations that have been processed more recently as part of the routine, day-to-day XMM data processing performed by the SSC, which were also public at 01 May 2008. Together, these amount to 626 additional observations (18%) with respect to 2XMM. With these new data, the sky area covered grows by 19% while the number of detections increases by 17% and the number of unique sources by 15%. A key factor in the decision to make the 2XMMi catalogue is the fact that, effectively, all the new data have been processed with the same science analysis software (SAS), pipeline and calibration as used in the aforementioned re-processing. As such, the products from 2XMM and from the new observations form a set of uniformly processed data. With software and calibration changes now being propogated into the SSC processing pipeline, it was deemed timely to make public the largest available uniform catalogue.

Users of the 2XMMi catalogue should note that every detection in 2XMM appears in 2XMMi with the same detection identifier, DETID. New detections have DETID values of 246898 or above. In general the 2XMM unique source identifiers, SRCID, have also been preserved but in a small number of cases, detections from 2XMM sources were regrouped and, in rare cases (essentially restricted to complex regions), 2XMM sources no longer appear in 2XMMi, being replaced by new sources comprised of different arrangements of detections (see Sec 3.2). The 2XMMi catalogue provides flags that reflect these changes.

2. User Guide for 2XMM

The User Guide (UG) for the 2XMM catalogue describes all the details of the data processing and compilation of the catalogue. The 2XMMi catalogue processing is practically unchanged. In the following section the differences or updates for the 2XMMi catalogue with respect to 2XMM are described. For convenience, Table 1, which gives the energy band definition, is repeated here.

Table 1:  Energy bands used in 2XMM 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. 2XMMi -- differences and additions with respect to the 2XMM catalogue

3.1 Data selection

The selection of XMM-Newton observations for inclusion in the 2XMMi catalogue follows the same guidelines as for the 2XMM catalogue but with a new public date: All observations that have a public release date prior to 2008 May 01 were eligible for inclusion. Table 2 gives the list of the final 4117 (3491 from 2XMM and 626 new) observations which are included in the 2XMMi catalogue.

3.2 Unique celestial sources

The details on the matching procedure can be found in the 2XMM UG, Sec. 3.2.3 a). The rest of this section describes how the procedure was updated to include the new detections in the 2XMMi catalogue and the possible consequences of it.

3.2.1 Re-matching and new flags

The process of finding the set of detections which appear to come from the same source and thus have a single SRCID number is simple and unambiguous in most of the sky. However, in a few areas, for example in ridges of extended emission or in shells around SNRs, the detection-matching process produces results which are occasionally unstable in the sense that minor changes to the parameters of the algorithm produce slightly different matches. Nevertheless, the algorithm used for 2XMM produced results which were satisfactory in all but a few small areas and was considered optimum for the task. As a result, the same matching algorithm was adopted for 2XMMi but here it was fed with the complete set of old and new detections (instead of just matching the new detections), with the aim that the new detections would improve the overall quality of the matching into unique sources. These emergent sources were compared with those from 2XMM, and for sources where at least half of 2XMM detections were still associated with that source the same SRCID number was used in 2XMMi as in 2XMM. Otherwise a new value of SRCID, from 191871 upwards, was allocated.

In addition, one correction to the matching algoritm was found to be necessary. As noted in the 2XMM documentation, the matching algorithm contained a condition designed to prevent two close but distinct detections in the same observation from being merged. However, during the creation of the 2XMM catalogue an error in the algorithm permitted such merging in rare cases. This affected only 50 detections in 2XMM. In 2XMMi, this error has been rectified.

To convey the changes that have taken place in the source assignments, two new flags have been introduced, SC_CHFLAG1 and SC_CHFLAG2: SC_CHFLAG1 indicates whether a source is (1) a 2XMM source with unchanged averaged parameters, (2) a 2XMM source with changed averaged parameters, or (3) a new source. SC_CHFLAG2 is set to reflect situations where the source was affected by the re-matching scheme. Note that the only changes in the catalogue parameters refer to the unique source identification (SRCID), source (SC_*) parameters and IAUNAME. All parameters refering to an observation or a detection remain unchanged.

A number of anticipated but not necessarily obvious consequences of running the matching algorithm over the entire 2XMMi catalogue are summarised below, specifying the relationship between unique sources in the 2XMM and 2XMMi catalogues, while Table 3 summarises how these cases relate to the two new flags.

i) A unique source present in 2XMM remains unchanged in 2XMMi when comprised only of the same detections as in 2XMM. In the 2XMMi catalogue the source ID, source parameters and IAU name are all unchanged from their 2XMM values. Note that the IAU name in 2XMMi retains the 2XMM prefix (Sec 3.2.3). These cases have SC_CHFLAG1 = 1 and SC_CHFLAG2 = 0, and represent 84% of sources in 2XMMi.
ii) One or more new detections in 2XMMi can, by virtue of their spatial separation, simply associate with the group of detections from a unique source in 2XMM. Here, the source ID and IAU name in 2XMMi are the same as in 2XMM but the merged source parameters will be modified. These cases have SC_CHFLAG = 2 and SC_CHFLAG2 = 0 and represent 3% of all 2XMMi sources.
iii) New detections in 2XMMi that do not associate with a unique source in 2XMM form new unique sources in 2XMMi with SRCID values  ≥ 191871. These acquire IAU names with a 2XMMi prefix. These cases have SC_CHFLAG1 = 3 and SC_CHFLAG2 = 0, and represent 13% of 2XMMi sources.

The following cases are rare and more complex; they are mainly identified by SC_CHFLAG1 = 2 and SC_CHFLAG2 = 1. Some of these cases are triggered by the presence of new detections. Forcing the exclusion of matches on the same observations (which was caused by the error in the matching algorithm mentioned above) are responsible for most other cases. A few cases, however, have no obvious explanation and are believed to be caused by a difference in the grouping mechanism that compares pairs of detections that have been ordered by rates.

iv) Detections that formed a source in 2XMM have been regrouped, that is, while a source in 2XMMi might map to one in 2XMM (having the same source ID and IAU name) it may have lost detections it had in 2XMM and it may have gained old detections from a different 2XMM source or new detections. The SC_* parameters will be modified. These cases have SC_CHFLAG1 = 2 and SC_CHFLAG2 = 1 and 87 2XMMi sources are affected. The lost detection(s), on the other hand, acquire a new SRCID of 191871 and above and are identified by SC_CHFLAG1 = 3 and SC_CHFLAG2 = 1.
v) Some 2XMM unique sources have been fragmented into two or even more sources. In this case, the fragment with the largest number of detections retains the same source ID and IAU name as the 2XMM source while the orphan fragment (sometimes joined with a new detection) acquires a new source ID of 191871 and above (identified by SC_CHFLAG1 = 3 and SC_CHFLAG2 = 1). SC_* values are modified for the existing source which has SC_CHFLAG1 = 2 and SC_CHFLAG2 = 1 and of which there are 57 2XMMi sources.
vi) Detections from two close 2XMM unique sources have coalesced to form a single source. The source identification and IAU name was retained from the original source with the higher number of detections (in case of equal number of detections the smaller source number was retained), while the source ID of the second source does not appear in 2XMMi anymore. This affects 10 2XMMi sources; they have SC_CHFLAG1 = 2 and SC_CHFLAG2 = 1.

A consequence of the re-grouping scenarios is that 158 sources in 2XMM do not appear in 2XMMi, but the detections are still present with different SRCID values (see cases (v) and (vi)). The SRCID_2XMM parameter (Sec. 3.2.2) can be used to find the original 2XMM source ID for all 347 detections affected by this phenomenon.

Note that no epoch information has been used for the matching and high proper motion stars may not been matched into a single unique source.

Table 3:  New flags identifying changed or new 2XMMi sources; F1 stands for SC_CHFLAG1 and F2 for SC_CHFLAG2
F1   Comment F2   Comment Case %age of sources in 2XMMi
1 Old source, SC_ values are unchanged 0 No detections gained or lost (i) 84%
2 Old source, SC_ values are updated 0 Only new detections added (ii) 3%
    1 Changes include rematching of old detections (iv), (v), (vi) 0.07%
3 New source 0 Only new detections (iii) 13%
    1 From new and old detections, or just old detections rematched (iv), (v) 0.07%

3.2.2 Cross-matching with 2XMM sources

Due to the change in the matching scheme for uniqe sources, some 'old' sources listed in the 2XMM catalogue are not present in the 2XMMi catalogue anymore -- instead, the constituent detection(s) have been re-matched with other (old or new) sources. To indicate whether a detection has been re-matched with a different source, a new column SRCID_2XMM has been added to the catalogue which gives the 2XMM source ID of each 'old' detection (note that the DETIDs of 2XMM detections are the same in the 2XMMi catalogue).

3.2.3 IAU identification

An IAU identification, IAUNAME, has been assigned to each unique source (SRCID) based upon the IAU registered classifications 2XMM and 2XMMi: a source that is included in the 2XMM catalogue has kept its 2XMM IAU identification, while all new sources have received a 2XMMi prefix.

The form of the IAU names is "2XMM Jhhmmss.sSddmmss" or "2XMMi Jhhmmss.sSddmmss" where hhmmss.s is taken from the averaged and eposcorr corrected right ascension coordinate given in the column SC_RA and Sddmmss is the averaged and eposcorr corrected declination taken from the column SC_DEC of the respective catalogue. Note that the averaged coordinates may have changed with respect to 2XMM but the IAU name with the 2XMM prefix will be that same as in the 2XMM catalogue.

The correct nomenclature for references to detections in the catalogue is the IAUNAME followed by a colon and the detection identification number DETID (with six digits), that is: "2XMM Jhhmmss.sSddmmss:detid" and "2XMMi Jhhmmss.sSddmmss:detid".

3.3 Updated summary html pages

The summary html pages are either new (for all new detections), unchanged (SC_CHFLAG1 = 1), or updated to account for the changes to the unique source parameters. They can be accessed through LEDAS. The slimline catalogue lists a column with the LEDAS URL which can be activated from within applications such as topcat.

3.4 Updated visual screening

The distribution of the six observation classes has slightly changed with respect to 2XMM due to an increased occurence of 'noisy' CCDs (CCD 4 of MOS1 and CCD 5 of MOS 2) in the more recent observations that were added in 2XMMi. Table 4 lists the observation class, the fractional area of exclusion with respect to the total detection area, and the percentage of observations affected for the two catalogues as well as for the new observations.

Table 4:  Distribution of observation class for the 626 new observations, for 2XMMi and for 2XMM observations
Obs class 'bad' area fraction    New observations    2XMM    2XMMi
0 0% area 33% 38% 37%
1 0% < area < 0.1% 10% 12% 12%
2 0.1% <= area < 1% 5% 10% 9%
3 1% <= area < 10% 35% 25% 26%
4 10% <= area < 100% 14% 10% 11%
5 100% 3% 5% 5%
Total number of observations 626 3491 4117

3.5 Updated target classification

The 2XMMi fits catalogue includes an extension with a target identification and classification table; Table 5 is the ascii version of it. The columns in the table and the options are explained at the end of the table. A more detailed description of this work is given in together with updated statistics of the distribution of target types for 2XMMi.

4. Catalogue content and organisation

This section summarises the organization 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 300 columns in the catalogue (3 more than for 2XMM); 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). 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 online SAS 6.9 package documentation (see 2XMM UG, App. A.3 for more details). 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).

Every detection in 2XMM appears in 2XMMi with the same detection identifier, DETID. New detections have DETID values of 246 898 or above. In general the 2XMM unique source identifiers, SRCID, have also been preserved but there are a few cases in which this was not possible, see Sec. 3.2.

Details of the columns:

Part 1: 10 columns: Identification of the source
This includes cross matches with the 1XMM and 2XMMp catalogues as well as a new 2XMMi column with the 2XMM source ID.
Part 2: 11 columns: Details of the observation and exposures
Part 3: 9 columns: Coordinates
The external equatorial and Galactic coordinates and the internal equatorial coordinates as derived from the SAS tasks eposcorr and emldetect are given together with the error estimates.
Part 4: 223 columns: Source parameters
The parameters of the source detection as derived from the SAS tasks emldetect and srcmatch are given here.
Part 5: 7 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.
Part 6: 7 columns: Source variability
This part gives variability information for those detections for which time series were extracted.
Part 7: 33 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, a variability and a summary flag. The number of detections is given also. In addition, two new 2XMMi columns are flags indicating specifics in the possible change of these unique source parameters with respect to the 2XMM catalogue.

Columns in the slimline catalogue:

Table 6 lists the 37 columns in the 2XMMi 'slimline' version of the catalogue, most of which are explained in Part 7 of the 2XMMi column description.

5. Catalogue Properties

Some of the more important properties of the 2XMM Catalogue have been discussed in Sec. 5 of the User Guide for the 2XMM catalogue and, in more detail, in the 2XMM catalogue paper (Watson et al. 2008). In the following only a short update for the 2XMMi catalogue is given where relevant.

5.1 Overview

The catalogue contains source detections drawn from 4117 XMM-Newton EPIC observations made between 2000 February 3 and 2008 March 28 and which were publicly available by 2008 May 01. 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 with net exposure time, Fig. 5.2 shows the distribution of fields on the sky, and Fig. 5.3 shows the distribution of fields with Galactic latitude.

The total sky area of the 4117 XMM-Newton observations is ~ 660 deg2 which translates to ~ 420 deg2 when corrected for field overlaps. Figure 5.4 shows the sky area as a function of net exposure time (based on vignetted exposure maps, cf. 2XMM UG, Sec. 3.1.2 a)) including the maximum coverage of the 1XMM and 2XMM catalogues.

The catalogue contains 289083 X-ray detections with total-band (0.2 -12 keV) likelihood values ≥ 6. Of these 221012 are unique X-ray sources (Sec. 3.2), that is, 33302 X-ray sources were observed more than once and up to 34 times in total. Of the 289083 X-ray detections 24533 are classified as extended.

Sources with extended emission vary considerably in size and form. The fitting task emldetect (cf. 2XMM UG, Sec. 3.1.2 f)) allows the fitting of a circular shape with 6" < r < 80", where r is the extent parameter EP_EXTENT. The fractional frequency distribution of the extent values, where the area under the curve is normalised to 1, is shown in Fig. 5.5 for all detections as well as for 'good' detections (with SUM_FLAG < 3). The figure shows clearly that real extended sources are on average smaller than spurious detections. In particular, the fractional detections in the tail at 80", which represent detections that have reached the extent limit and are in fact larger, is 0.066 for the total sample but considerably smaller, that is, 0.022, for the good detections.

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 process were identified and all detections within those regions were flagged (cf. 2XMM UG, Sec. 3.2.6). Such flagged detections include spurious detections (many of which are classified as extended) as well as detections where the source parameters may be unreliable. It is recommended to use either a detection flag (SUM_FLAG, EP_FLAG or SC_SUM_FLAG) or an observation flag (OBS_CLASS) as a filter to obtain what can be considered a 'clean' sample. There are 204110 out of 289083 detections that have not received any flag designating a possible spurious detection or affected paremeters, while 233698 out of 289083 detections (81%) can be considered to be 'clean', i.e., are unlikely to be spurious due to insufficient background determination in the source detection process (summary flag < 3). On the other hand, 2021 out of 4117 fields are considered to have no or at most a couple of spurious detections in them (observation class < 3). Note that no attempt has been made to flag spurious detections caused by background noise as chracterised by their likelihood (but see Sec. 5.2 for a discussion of these).

5.2 False detection rates

In an attempt to investigate the expected false detection rate, realistic Monte-Carlo simulations of the 2XMM catalogue source detection and parameterisation process were carried out. The simulations represent typical high-latitude fields without bright sources or extended X-ray emission apart from the unresolved cosmic X-ray background. The distribution of X-ray point sources, with uniform spectral shape, was drawn from a representative extragalactic  log N - log S  relationship (e.g., Hasinger et al., 2001). The source spectrum was assumed to be the same as used in the determination of the ECFs (2XMM UG, Sec. 3.1.2 f)), i.e., a power law characterised by Gamma = 1.7  with a Galactic column density Nh = 3.0 × 1020 cm2. Finally, a particle background component was added to the images. The simulation creates images in the five standard energy bands using the appropriate calibration information (i.e., energy- and position-dependent PSFs, vignetting, detection efficiency, etc.). The simulated images were then processed in the same way as the observed images (2XMM UG, Sec. 3.1.2) and the detections were compared against the list of known input sources using a statistical matching procedure from which numbers of false detections in each simulated field could be ascertained.

The simulations have been carried out for three different exposure times: a nominal exposure time (12 ks for MOS and 8 ks for PN), corresponding to around 70% of the median exposure time, as well as three and ten times higher exposure values. The resulting numbers of false detections per field as a function of the minimum detection likelihood are shown in Fig. 5.6. In addition, the expected false detection number for the number of 'beams' (i.e., independent detection cells) per field is shown. The exact determination of the number of 'beams' depends on the search box size (2XMM UG, Sec. 3.1.2 c)) and the degradation and change of shape of the PSF with the offaxis angle and is not straight forward to calculate. For the purpose of comparison with the simulations, it has been estimated to be 5000.

For the nominal exposure times (assumed to be typical for the observations included in the catalogue) the number of false detections is ~ [1, 0.3, 0.1] per EPIC field at detection likelihood thresholds (EP_DET_ML) of [6, 8, 10] respectively. These values increase to ~ [4, 2, 1.5] for the longest exposure time.

To summarise, one can say that

A more detailed analysis can be found in the catalogue paper (Watson et al. 2008) as well as in a dedicated paper about the simulations (Sakano et al., in preparation).

6. Known problems and other issues

6.1 Problem cases

6.1.1 Incorrect variability measurement

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 tentatively identified:

6.1.2 Dark columns in PN images are not represented in the exposure maps

The presence of a broad dark column in some of the PN images (and which is not represented in the exposure maps) is related to the onboard MIP rejection and to bad pixels, see the Appendix of CAL-TN-0067-0-0.pdf. As a consequence, the parameters of sources near these columns (with distance < 60") are underestimated.

6.1.3 PN low energy noise can cause spurious detections

Most of the PN low energy noise is suppressed by the use of PATTERN 0 events. However, in some cases bright columns or clumps of pixels appear in the images and may cause spurious detections. In most cases detections on bright columns are automatically and manually flagged (2XMM UG, Table 3.3a) but detections on the bright clumps are not flagged at all. In most cases these sources only appear in band 1 where they could, for example, be misinterpreted as super-soft sources.

6.1.4 EPIC band 4 fluxes

New problem: In early 2010 it was discovered that approximately 17% of the EPIC band 4 fluxes (EP_4_FLUX column) are incorrect in version 1.0 of the 2XMMi catalogue. The corresponding SC_EP_4_FLUX column for unique sources is consequently also affected. It should be stressed that these are the only columns affected. The problem and access to correct data is described in more detail here. Replacements of the full and slim 2XMMi catalogues are now available.

6.2 Other issues

6.2.1 New calibrations and improved fluxes

Since early 2008, an improved calibration of the MOS and PN leads to slight changes in the ECFs, particular at the soft energy bands. These have not been used in the pipeline processing for the 2XMMi catalogue. Therefore, Table 8 gives correction factors to be applied to the ECFs and fluxes that were used in the catalogue (see 2XMM UG, Table 3.2 for the original ECFs) in the sense

improved flux = correction factor * 2XMMi flux.

Based on these corrections, fluxes between cameras (cf. 2XMM UG, Sec. 5.3.2) are in better agreement, see for details.

Table 8: Correction factors to the ECFs and fluxes used in 2XMM and 2XMMi processing
Camera Band Open Thin Medium Thick
PN 1 1.02176 1.01139 1.00767 0.997455
2 1.02128 1.02017 1.02007 1.01950
3 0.999321 0.999288 0.999274 0.998995
4 1.01190 1.01152 1.01136 1.01107
5 1.00063 1.00000 1.00000 1.00064
9 1.01128 1.00974 1.00996 1.00902
MOS-1 1 0.981613   0.956452   0.945457   0.925115  
2 0.961381 0.961301 0.960208 0.960476
3 0.988680 0.988948 0.988765 0.989250
4 1.00000 1.00074 1.00000 1.00000
5 1.00194 1.00171 1.00170 1.00173
9 0.977969 0.978943 0.979105 0.98017
MOS-2 1 0.989302 0.959068 0.946630 0.922199
2 0.961461 0.960747 0.960899 0.959776
3 0.991415 0.991824 0.991676 0.992057
4 1.00073 1.00000 1.00075 1.00000
5 1.00300 1.00300 1.00301 1.00305
9 0.979524 0.980446 0.980451 0.981477

6.2.2 High proper motion objects

Since no epoch information has been used for the matching of unique sources (Sec. 3.2), high proper motion stars may not always have been matched into a single unique source. Note though that for three known high proper motion targets the correct DETID has been identified in the target identification table.


Hasinger, G., Altieri, B., Arnaud, M., et al. 2001, A&A 365, L45

Watson, M., et al. 2008, The XMM-Newton serendipitous survey. V. The Second XMM-Newton Serendipitous Source Catalogue, A&A, submitted

Document revision history

Release No. Release Date      Comments
1.0 20 August 2008 First release


A.1 Lists of the extensions in the fits catalogue

List of observations ('fields').

List of target identifications.