UCL Department of Space and Climate Physics


Introduction to UVOTSSC

UVOTSSC was compiled from 23059 Swift datasets taken within the first 5 years of observations with the Swift UVOT (between 10th Jan 2005 and 1st Oct 2010). This is the first catalogue of serendipitous sources detected by UVOT through its six lenticular filters, imaging in V, B, U, UVW1, UVM2 and UVW2 bands. It is the result of a 5-year long project led by MSSL, on behalf of the UVOT team. 

To construct the catalogue, the UVOT data were processed through a purpose-built pipeline based around the Swift UVOT FTOOLS available in HEASOFT. The primary content of the catalogue is filter-dependent source positions and magnitudes. The catalogue consists of two tables. The first one contains the sources, with positional and photometric data (count rate, magnitude and flux) and quality flags for each measurement. In the second table we give a summary of the observations from which the sources have been detected and measured. The link between the two tables is made through the column N SUMMARY which, for each source in the first table, gives the entry number of the observation in the SUMMARY table in which this source was detected. For each of the sources, there there is also the observation identification number given in the column “OBSID” which coincides with the ObsID (see the Data Processing section) in the SUMMARY table.

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

• SOURCES – the main table containing UVOT sources

• SUMMARY – the auxiliary table containing information about each observation

In total, the table SOURCES has 82 columns with the source parameters. If more than one filter was used in the observation, any sources detected in more than one filter are matched, so that there is a single catalogue entry for each source for each observation. However, the same source could be detected in the exposures belonging to different observations, in which case it would have different entries in the catalogue table. Thus, among the 13 860 569 catalogue entries 6 200 016 unique sources are identified, whereas 2 027 265 sources have multiple entries.

A significant fraction of sources (ranging from 22% to 40%, depending on filter) are observed on more than one occasion in the same filter. Thus the scope for studies of variability is broad. Taking account of substantial overlaps between observations, the net sky area covered is 163 to 300 deg2, depending on filter.

The catalogue gives the AB and Vega magnitudes, as well as fluxes for each source and each filter (for flux conversion factors see Poole et al. (2008)) in units of ergs sec−1 cm−2 A ̊ −1 ). The S/N ratio for all sources exceeds 5 for at least one UVOT filter, the rest of the filters having a S/N greater than 3.  The mean AB magnitude of the catalogue detections in the different bands is: V 17.9, B 18.5, U 19.8, UVW1 20.7, UVM2 21.3, UVW2 21.5

The catalogue is also available in six sections, each one covering 4 hours in RA, to make them smaller and more manageable. The SUMMARY table is the same and complete for all 6 sections.