XMM-OM Serendipitous Ultraviolet Source Survey catalogue: SUSS6.0

XMM-OM-SUSS6.0 is a new 2023 release of the XMM OM Serendipitous Ultraviolet Source Survey catalogue, superseding the previous version, XMM-OM-SUSS5.0.

The catalogue was compiled for the Optical Monitor instrument on board the ESA XMM Newton observatory. This version, the 6th release of the catalogue, spans the period of observations from XMM-Newton revolution 34 (February 2000) to revolution 4207 (November 2022). A refereed paper on the first version can be found in Page M.J. et al. (2012).

In addition to covering a larger observation period, this sixth release reflects a change in philosophy with regard to the origin of the incorpoated data. In previous releases, the data were generated via a bespoke processing of the OM Observation Data Files (ODFs) while in this new release, we have been guided by the XMM user community and we have sought to harmonize the contents of the catalogue with those of the OM data in the XMM-Newton Science Archive (XSA), which derive from the standard XMM-Newton pipeline processing system. While the bespoke processing and pipeline systems are fundamentally very similar, they are not identical and the differences lead to some differences in the output.

All data have been fully processed at the European Space Astronomy Centre (ESAC, Madrid, Spain) using the XMM-Newton pipeline processing system (PPS) that invokes the XMM Science Analysis Software (SAS) - the vast majority of observations having been processed using SAS versions 18 and 19.

The number of observations (OBSIDs) included in the catalogue is 12057. The total number of entries (detections) in this release is 9,920,390. They correspond to 6,659,554 unique sources, of which 1,225,117 have multiple entries in the source table, corresponding to different observations.

The Catalogue is a FITS file consisting of two tables (please note that users wishing to use the TOPCAT tool to manipulate the catalogue, need to select the table format option, "FITS" to load the catalogue). The first table (SRCLIST) contains the source detections, with positional and photometric data (count rate, magnitude and flux) and quality flags for each measurement. The contents of the 114 columns are shown below:

IAUNAME Source IAU name
N_SUMMARY Index in the SUMMARY table for the source OBSID
OBSID Source OBSID number
SRCNUM Source number in this catalogue
UVW2_SRCDIST Dist. to the nearest source, UVW2 filter
UVM2_SRCDIST Dist. to the nearest source, UVM2 filter
UVW1_SRCDIST Dist. to the nearest source, UVW1 filter
U_SRCDIST Dist. to the nearest source, U filter
B_SRCDIST Dist. to the nearest source, B filter
V_SRCDIST Dist. to the nearest source, V filter
RA Source Right Ascension coordinate (deg)
DEC Source Declination coordinate (deg)
RA_HMS Source Right Ascension in hour units
DEC_DMS Source Declination coordinate in DMS
POSERR Source positional (coordinate) error
LII Source galactic longtude
BII Source galactic latitude
N_OBSID Number of observation IDs for this source
UVW2_SIGNIF Source significance for the filter UVW2
UVM2_SIGNIF Source significance for the filter UVM2
UVW1_SIGNIF Source significance for the filter UVW1
U_SIGNIF Source significance for the filter U
B_SIGNIF Source significance for the filter B
V_SIGNIF Source significance for the filter V
UVW2_RATE Source count rate for the filter UVW2
UVW2_RATE_ERR Source count rate error, filter UVW2
UVM2_RATE Source count rate for the filter UVM2
UVM2_RATE_ERR Source count rate error, filter UVM2
UVW1_RATE Source count rate for the filter UVW1
UVW1_RATE_ERR Source count rate error, filter UVW1
U_RATE Source count rate for the filter U
U_RATE_ERR Source count rate error, filter U
B_RATE Source count rate for the filter B
B_RATE_ERR Source count rate error, filter B
V_RATE Source count rate for the filter V
V_RATE_ERR Source count rate error, filter V
UVW2_AB_FLUX Source flux for the filter UVW2
UVW2_AB_FLUX_ERR Source flux error for the filter UVW2
UVM2_AB_FLUX Source flux for the filter UVM2
UVM2_AB_FLUX_ERR Source flux error for the filter UVM2
UVW1_AB_FLUX Source flux for the filter UVW1
UVW1_AB_FLUX_ERR Source flux error for the filter UVW1
U_AB_FLUX Source flux for the filter U
U_AB_FLUX_ERR Source flux error for the filter U
B_AB_FLUX Source flux for the filter B
B_AB_FLUX_ERR Source flux error for the filter B
V_AB_FLUX Source flux for the filter V
V_AB_FLUX_ERR Source flux error for the filter V
UVW2_AB_MAG Source AB magnitude for the filter UVW2
UVW2_AB_MAG_ERR Source AB magnitude error, filter UVW2
UVM2_AB_MAG Source AB magnitude for the filter UVM2
UVM2_AB_MAG_ERR Source AB magnitude error, filter UVM2
UVW1_AB_MAG Source AB magnitude for the filter UVW1
UVW1_AB_MAG_ERR Source AB magnitude error, filter UVW1
U_AB_MAG Source AB magnitude for the filter U
U_AB_MAG_ERR Source AB magnitude error, filter U
B_AB_MAG Source AB magnitude for the filter B
B_AB_MAG_ERR Source AB magnitude error, filter B
V_AB_MAG Source AB magnitude for the filter V
V_AB_MAG_ERR Source AB magnitude error, filter V
UVW2_VEGA_MAG Source Vega magnitude for the filter UVW2
UVW2_VEGA_MAG_ERR Source Vega magnitude error, filter UVW2
UVM2_VEGA_MAG Source Vega magnitude for the filter UVM2
UVM2_VEGA_MAG_ERR Source Vega magnitude error, filter UVM2
UVW1_VEGA_MAG Source Vega magnitude for the filter UVW1
UVW1_VEGA_MAG_ERR Source Vega magnitude error, filter UVW1
U_VEGA_MAG Source Vega magnitude for the filter U
U_VEGA_MAG_ERR Source Vega magnitude error, filter U
B_VEGA_MAG Source Vega magnitude for the filter B
B_VEGA_MAG_ERR Source Vega magnitude error, filter B
V_VEGA_MAG Source Vega magnitude for the filter V
V_VEGA_MAG_ERR Source Vega magnitude error, filter V
UVW2_MAJOR_AXIS Source major semi-axis for the filter UVW2
UVM2_MAJOR_AXIS Source major semi-axis for the filter UVM2
UVW1_MAJOR_AXIS Source major semi-axis for the filter UVW1
U_MAJOR_AXIS Source major semi-axis for the filter U
B_MAJOR_AXIS Source major semi-axis for the filter B
V_MAJOR_AXIS Source major semi-axis for the filter V
UVW2_MINOR_AXIS Source minor semi-axis for the filter UVW2
UVM2_MINOR_AXIS Source minor semi-axis for the filter UVM2
UVW1_MINOR_AXIS Source minor semi-axis for the filter UVW1
U_MINOR_AXIS Source minor semi-axis for the filter U
B_MINOR_AXIS Source minor semi-axis for the filter B
V_MINOR_AXIS Source minor semi-axis for the filter V
UVW2_POSANG Source position angle for the filter UVW2
UVM2_POSANG Source position angle for the filter UVM2
UVW1_POSANG Source position angle for the filter UVW1
U_POSANG Source position angle for the filter U
B_POSANG Source position angle for the filter B
V_POSANG Source position angle for the filter V
UVW2_QUALITY_FLAG Source quality flag for the filter UVW2
UVM2_QUALITY_FLAG Source quality flag for the filter UVM2
UVW1_QUALITY_FLAG Source quality flag for the filter UVW1
U_QUALITY_FLAG Source quality flag for the filter U
B_QUALITY_FLAG Source quality flag for the filter B
V_QUALITY_FLAG Source quality flag for the filter V
UVW2_QUALITY_FLAG_ST Source quality flag string, filter UVW2
UVM2_QUALITY_FLAG_ST Source quality flag string, filter UVM2
UVW1_QUALITY_FLAG_ST Source quality flag string, filter UVW1
U_QUALITY_FLAG_ST Source quality flag string, filter U
B_QUALITY_FLAG_ST Source quality flag string, filter B
V_QUALITY_FLAG_ST Source quality flag string, filter V
UVW2_EXTENDED_FLAG Source extended flag for the filter UVW2
UVM2_EXTENDED_FLAG Source extended flag for the filter UVM2
UVW1_EXTENDED_FLAG Source extended flag for the filter UVW1
U_EXTENDED_FLAG Source extended flag for the filter U
B_EXTENDED_FLAG Source extended flag for the filter B
V_EXTENDED_FLAG Source extended flag for the filter V
UVW2_SKY_IMAGE Source detected in stacked images filter UVW2
UVM2_SKY_IMAGE Source detected in stacked images filter UVM2
UVW1_SKY_IMAGE Source detected in stacked images filter UVW1
U_SKY_IMAGE Source detected in stacked images filter U
B_SKY_IMAGE Source detected in stacked images filter B
V_SKY_IMAGE Source detected in stacked images filter V

 

In the second table (SUMMARY) we give a summary of the observations from which the sources have been detected and measured. The contents of the 35 columns in this table are as follows:

N_SUMMARY Reference index for the source OBSID data
REVOLUTION XMM revolution number
OBSID Observation ID number
TARGET Observation target name
FILTERS XMM OM filter names used in this observation
MJD_START Initial observation date/time
MJD_END End of observation date/time
RA_PNT Right Ascension of the telescope pointing direc
DEC_PNT Declination of the telescope pointing direction
RA_PNT_HMS Right Ascension of the telescope pointing in HM
DEC_PNT_DMS Declination of the telescope pointing in DMS
EXPOSURE_UVW2 Total exposure time for the filter UVW2
EXPOSURE_UVM2 Total exposure time for the filter UVM2
EXPOSURE_UVW1 Total exposure time for the filter UVW1
EXPOSURE_U Total exposure time for the filter U
EXPOSURE_B Total exposure time for the filter B
EXPOSURE_V Total exposure time for the filter V
N_SOURCES Total number of sources
N_SOURCES_UVW2 Number of sources for the filter UVW2
N_SOURCES_UVM2 Number of sources for the filter UVM2
N_SOURCES_UVW1 Number of sources for the filter UVW1
N_SOURCES_U Number of sources for the filter U
N_SOURCES_B Number of sources for the filter B
N_SOURCES_V Number of sources for the filter V
DETLIM_UVW2 Vega magnitude limit for the filter UVW2
DETLIM_UVM2 Vega magnitude limit for the filter UVM2
DETLIM_UVW1 Vega magnitude limit for the filter UVW1
DETLIM_U Vega magnitude limit for the filter U
DETLIM_B Vega magnitude limit for the filter B
DETLIM_V Vega magnitude limit for the filter V
RAOFFSET Corrected offset in Right Ascension
DECOFFSET Corrected offset in Declination
RAOFFSET_ERR Standard error of the RA offset
DECOFFSET_ERR Standard error of the DEC offset
OFFSET_RMS Offset RMS

 

The meaning of the quality flags (columns filter_QUALITY_FLAG and filter_QUALITY_FLAG_ST) in the SRCLIST table are as follows:

bit 0 (value 1) source on a bad pixel
bit 1 (value 2) source on a readout streak
bit 2 (value 4) source on a smoke-ring
bit 3 (value 8) source on a diffraction spike
bit 4 (value 16) source affected by Mod-8 pattern
bit 5 (value 32) source within the central enhancement
bit 6 (value 64) source near a bright source
bit 7 (value 128) source near the edge
bit 8 (value 256) point source within an extended source
bit 9 (value 512) weird source (bright pixel)
bit 10 (value 1024) multiple exposure values within photometry aperture
bit 11 (value 2048) the source is affected by the reduced sensitivity patch*
bit 12 (value 4096) the source is too bright (rate > 0.97 c/frame)

 

* - see known issues

The filter_SKY_IMAGE columns indicate if a source has been detected in stacked (sky) images obtained with that filter. The catalogue contains 1,422,957 such detections.

A cross-match of the whole catalogue with the GAIA EDR3 data set, using a search radius of 3 arcsecs, gives 80.4% matches (73.2% within a matching radius of 1 acrsecs). This increases to 81.4% within 3 arcsecs if we consider only SUSS6 detections with the cleanest (0) quality flag. The mean angular offset of all matched OM-GAIA sources is 0.50 arcsecs. 90% of the matches lie within 0.96 arcsecs and 95% have separations < 1.21 arcsecs. Detections in the V filter have 88.9% matches to GAIA within 3 arcsecs while corresponding values for the UVW1 and UVM2 filters are 81.9% and 73.7%, respectively.

Comparing the observations from the new SUSS6 catalogue that are common to the previous SUSS5 catalogue, we find, 92.8% of the SUSS6 detections have a match in SUSS5, within 1 arcsec (94.0% within 2 arcsecs). Considering V band detections, this becomes 94.4%, and 90% of observations have > 83% of SUSS6 detections with a SUSS5 match within 1 arcsec. The mean systematic difference in measured count rates between matched detection, in all filters, is less than 1% while in all filters, around 90% of matched detections have count rates in agreement to < 5% - one must bear in mind that there are differences in calibration (principally the time-dependent sensitivity degradation) between data used in SUSS6 and in SUSS5.

The XSA (https://www.cosmos.esa.int/web/xmm-newton/xsa) provides access to the OM Catalogue as a complete download of the file (~1.48 Gb, compressed fits format) as well as through a search interface whose results can be stored by the user in several formats. It should be noted that this interface allows the user to access only part of the information contained in the catalogue: source coordinates, magnitude and flux and their corresponding errors and quality flags. A reduced version of the complete catalogue (708 Mb, compressed fits format) can be downloaded as well.

The ESA-Sky utility allows the user to display the sources from the catalogue on top of sky images from different instruments at all wavelengths. This includes also images obtained with OM in two color combination, Visible (U, B and V OM filters) and UV (UVW1, UVM2 and UVW2 OM filters). It should be noted that OM images have a low dynamic range.

 

Known Issues and watchouts

 

A number of low-level issues, outlined here, affect the XMM-OM-SUSS6 catalogue. Fixes and improvements to these will be made in future catalogue releases, where possible.

  • Flagging of detections is not foolproof. Problems can arise in crowded regions where sources may be contaminated by nearby objects but not always be flagged (observations containing more than 10000 detections are excluded from the SUSS6 catalogue). Another cause is fields that contain read-out streaks caused by bright stars that lie outside the window - in these cases, sources detected along a read-out streak may have compromised parameters or even be spurious but not be flagged accordingly. Similarly, sources detected on smoke rings (out-of-focus ghost images of bright stars), are often flagged but where the bright star causing the smoke ring lies outside the window where the smoke ring falls, they are generally not flagged. Likewise issues can occur in scattered light features caused by reflections, within the detector, of light from bright stars outside the OM field of view, which can form complex, extended, generally low-intensity features in the images where spurious detections may be found.
  • In the case of very bright sources, the photometric data is nullified because the applied corrections for coincidence loss are high and not reliable. However, other quantities remain in the table, such as the major and minor axes, which may appear confusing to users.
  • A small number of detections are erroneously associated with another detection from the same observation, and are asigned the same unique source number. These arise when detections of the same source in multiple exposures of the same observation, have slightly different positions. This can happen, for example, because of systematics in the rectification process (when a mean adjustment in RA and DEC is made to all sky positions in an exposure so as to minimise the spatial offsets when matched against an astrometric reference catalogue) or where centroiding is biased due to source counts in the source wings being outside the window, for sources very near window edges. These multiple detections can be divided into more than one unique source within the observation but be erroneously assigned the same unique source number in the catalogue. Although, in principle, the separate unique sources may, in reality, belong to the same object on the sky, if identified as separate unique sources (correctly or not), they should not acquire the same unique source number within an observation, but as noted, this does happen in a small number of cases.
  • Following the accidental observation of Jupiter by the OM in July 2017, a small region of the photocathode, near the centre of the OM field of view, has reduced sensitivity - this is referred to as the Jupiter patch (JP). Much of this region is currently identified via 'bad pixels' in the bad pixel calibration table, and sources that are detected within the bad pixel region are flagged as such in the catalogue. However, the region of reduced sensitivity is larger than that currently translated into the bad pixel map and sources in the shallow wings of the JP, beyond the mapped bad pixel area, are not flagged. In fact, the wings of the JP include the location of the pn boresight in the OM image (i.e. where sources (generally, XMM-Newton targets), placed at the pn boresight, appear within the OM field of view). The impact of the sensitivity loss at the pn boresight is filter-dependent but ranges from about 8% in the V filter, to around 3% in the UV filters (see CAL-TN-0019.pdf) at the epoch of the Jupiter observation. The degradation at the pn boresight has since continued to decline, by about 1% per year in the V band.
  • Flag 11 is used to indicate sources that are affected by pixels of reduced sensitivity, as defined in the OM bad pixel calibration table - note that, previously, this flag was erroneously combined with flagging of very bright sources (these bright sources are now indicated by flag 12). The bad pixel table includes pixels in the Jupiter patch but also pixels in a set of small patches of reduced sensitivity elsewhere in the OM detector. As such, currently, no distinction is made between flagging of sources affected by these other patches and those affected by the Jupiter patch.
  • In around 1% of entries for detections, the seconds part of the RA component of the IAUNAME is erroneously rounded up to 60.0, or the arcseconds part of the Declination component is rounded up to 60.
  • Cross analysis with the previous SUSS5 catalogue, for the common set of observations, shows that some 92.8% of SUSS6 detections have a match in the SUSS5 catalogue within 1 arcsec. Equivalently, 7% are unmatched to a SUSS5 detection within 1 arcsec. In reverse, about 5% of SUSS5 detections are not matched with detections in SUSS6. One reason for this is a difference, unrecognized at the time of SUSS6 creation, in the way the bespoke pipeline (used to create SUSS5) and the routine XMM pipeline (used to generate this new SUSS6 catalogue), process OM data for a given filter when part of the field is only covered by one exposure in that filter. Both pipelines source-search individual exposure-level detector images, and also stacked sky-aligned images in a given filter when multiple overlapping exposures are present, to find fainter sources than in the per-exposure data - these fainter sources are added to the list of sources detected in the individual exposures. However, OM processing in the routine XMM pipeline also source-searches a sky image made from a single exposure in a given filter (erroneously, given that it is the same data, just rotated to sky coordinates), ensuring there are always the same number, or more detections than from the individual exposures. Additionally, differences may arise due to (i) differences in calibration (the routine pipleine processing has used different SAS versions and calibration files while the bespoke processing uses a single SAS version and a fixed set of calibration files) and (2) randomisation effects at the pixel scale, involved in the image creation.