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Catalogue Format

Introduction

The second OM Serendipituos Source Catalogue was produced by processing the XMM-Newton Optical Monitor (OM) data obtained from the beginning of the mission (1999) until the beginning of 2013. The proscessing was done by using XMM Science Archive Software (SAS) version 14.0. In addition to covering a larger observation period, the second release differs from the previous one by inclusion of all the OM observations (not only those containing UV filters) and by performing source detection on stacked images, thus facilitating the detection of fainter sources.

Description

The OM source catalogue is compiled in the form of a FITS-file containing two tables:

SRCLIST - the main table containing the sources

The SRCLIST table has the following columns which give the astrometric, photometric, source-parameterisation and source-flagging parameters.

IAUNAME
The column IAUNAME provides individual source identification strings following the convention of the International Astronomical Union. For each source, its name starts from the string XMMOM characterising the sources as obtained from the XMM-Newton mission with its OM-telescope. This string follows with the Right Ascension and Declination coordinates for the epoch J2000 in the form JHHMMSS.s±GGmmss.

N SUMMARY
This column provides the reference number of the observation summary given in the auxiliary table SUMMARY (i.e., this is the entry number to the SUMMARY table and links the two tables).

OBSID
Gives the unique observation identification number corresponding to the observation in which the source was detected.

SRCNUM
This column give a unique source number for each source in the XMM-SUSS2 catalogue (note that the number of entries in the SRCLIST table is larger that the total number of individual sources in the catalogue because some sources are observed more than once).

'filter'_SRCDIST
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. These are 6 columns corresponding to different OM filters giving the distance to the closest neighbour source in arcseconds.

RA and DEC
These columns contain the Right Ascension and Declination (J2000) of each source in degrees. All the entries in the table are in order of increasing RA value.

RA_HMS and DEC_DMS
These columns contain the Right Ascension and Declination (J2000) of each source in astronomical angular units (Hours, Minutes and Seconds of time for RA and Degrees, Minutes of arc and Arcseconds for Dec).

POSERR
This column contains the standard error of the merged source position – i.e. 

 (1/n) √(∑nk=1(errk)2) , where errk are the position errors of the n individual sources. Please, note that input errors from the individual source-list files are first converted from pixels to arcsecs.

LII and BII
These two columns contain the Galactic longitude and Galactic latitude computed from the RA and DEC columns.

N_OBSID
The number of individual observations (OBSIDs) in which the source was detected.

'filter'_SIGNIF
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the source significances (signal-to-noise). These values are the averaged values from the individual sources that were merged into a source.

'filter'_RATE and 'filter'_RATE_ERR
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the source count-rates (corrected for coincidence losses and detector time degradation) and for its standard error. These values are the averaged (without weighting) values from the individual sources that were merged into a source. The standard errors computed from (1/n) √(∑nk=1(errk)2) , where errk is the error for an individual source.

'filter'_FLUX and 'filter'_FLUX_ERR
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the source fluxes (and its standard errors). These values are computed assuming an AB spectrum from the computed averaged count-rates in the filter_RATE columns.

'filter'_AB_MAG and 'filter'_AB_MAG_ERR
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the source AB magnitudes and its standard error. These values are computed from the averaged count-rates in the filter RATE columns. No magnitudes are computed if the corrected count-rate exceeds 1000.

'filter'_VEGA_MAG and 'filter'_VEGA_MAG_ERR
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the source magnitudes in the Vega system, and its standard error. These values are computed from the averaged count-rates in the filter RATE columns. No magnitudes are computed if the corrected count-rate exceeds 1000.

'filter'_MAJOR_AXIS
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. This column gives the computed value for the mean full-width-half-maximum apparent size of the source along the major axis, using the values for each filter.

'filter'_MINOR_AXIS
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the computed full-width-half-maximum apparent size of the source along the minor axis.

'filter'_POSANG
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter, this column gives the computed value for the position angle of the major axis source (measured anti-clock wise from the Right-Ascension axis).

'filter'_QUALITY_FLAG
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the quality flag for each source, as determined from all the merged sources for that filter.

'filter'_EXTENDED_FLAG
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. For each filter there is a column for the extension flag for each source, which is set equal to the extended flag of the source with the maximum significance (the negative value if a significance value is negative) for the particular filter.

'filter'_QUALITY_FLAG_ST
where 'filter' can be UVW2, UVM2, UVW1, U, B, or V. This column gives the quality flag (equivalent to the 'filter'_QUALITY_FLAG column) but this time in the form of logical values (string F for false and T for true).

QUALITY FLAGS
Each source has an associated “quality flag”. The flag settings are shown here:

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 too bright (rate > 0.97 c/frame). 

 Table 1: Quality flag values for the XMM-SUSS2 catalogue

Examples of usage
Quality flag 1 - Source contains one or more bad pixels
Quality flag 3 - Source contains one or more bad pixels and lies on a read-out streak
Quality flag 7 - Source contains one or more bad pixels and lies on a read-out streak and lies within a “smoke-ring” region.
Notes on individual quality flags
For more information on what the quality flags mean please go to the Data Processing: quality flagging section.  

SUMMARY - the auxiliary table containing information about each observation

The SUMMARY table contains information about each observation included in the catalogue in the following columns:

N_SUMMARY
This column provides the link to the SRCLIST table. Each source can be linked to the observation details in the SUMMARY table via this number.

REVOLUTION
The XMM-Newton Revolution number when this observation was made.

OBSID
This gives the unique observation identification number.

TARGET
This is the name of the target given by the proposer of the observation.

FILTERS
This lists the filters used in the observation.

MJD_START and MJD_END
This is the date when the first exposures for this observation started to be taken and the last date.

RA_PNT and DEC_PNT
The Right Ascension for the nominal telescope pointing direction in decimal degrees and the Declination for the nominal telescope pointing direction in decimal degrees.

RA_PNT_RMS and DEC_PNT_DMS
The Right Ascension for the nominal telescope pointing direction in hours, minutes, seconds and the Declination for the nominal telescope pointing direction in degrees, arcminutes, arcseconds.

EXPOSURE_'filter'
The total exposure time in seconds for each filter summed together before detecting sources.

NSOURCES
The total number of sources detected in this observation.

NSOURCES_'filter'
where 'filter' can be UVW2, UVM2, UVW1, U, B or V. The total number of sources detected in this filter for this observation.

DETLIM_'filter'
where 'filter' can be UVW2, UVM2, UVW1, U, B or V. The three sigma detection limiting magnitude in this filter for this observation, in the Vega photometry system.

RAOFFSET and DECOFFSET
The offset between the requested (nominal) pointing and the actual pointing as determined by aspect correction using the USNO catalogue.

RAOFFSET_ERR and DECOFFSET_ERR
The error on RAOFFSET and DECOFFSET.

OFFSET_RMS 
The goodness of fit of the Aspect correction.

The link between these two tables is made throught the column N SUMMARY which for each source (each entry of the first table) gives the entry number in the SUMMARY table which characterises, in 35 columns, the observation corresponding to this source. There is also a column OBSID giving the observation identification number for each source.

In total, the table SOURCES has 108 columns with the source parameters for 5 034 450 source entries. The sources observed in different filters within each observation have been matched, so that a source identified in related exposures with different filters has a single catalogue entry. However, if the same source has also been detected in a different observation, it is linked by a unique source number. Thus, among the 5 034 450 catalogue entires there are in fact only 4 594 004 unique sources identified.

The catalogue gives the AB magnitudes and fluxes for each source and each filter (see Oke, J.B. 1974, ApJS, 27, 21) (in units of ergs sec−1cm−2Å−1)