UVOTSSC Format

There are two sections to the UVOTSSC, the first SOURCES table lists the individual sources and the second SUMMARY table summarises each observation. The link between these 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 in the SUMMARY table.

Sources Table Contents

The UVOT SOURCES table contains the following columns which give the astrometric, photometric parameters for the sources, plus information about quality flagging.

IAUNAME

The column IAUNAME provides individual source identification strings in the conventions of the International Astronomical Union. For each source, its name starts from the string UVOTSSC1 characterising the sources as obtained from the NASA SWIFT mission with the UVOT telescope, version 1. This string is followed with the Right Ascension and Declination coordinates for the epoch J2000 in the form JHHMMSS.s±GGmmss. N.B. Because of problems with the rounding of position coordinates a single IAU name can refer to more than one SRCNUM; there are 149 known cases. They have been distinguished by having a letter a,b,c,..etc. appended to their IAU name.

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 of the SUMMARY table).

OBSID

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

NFILT

This column gives the number of the UVOT filters included into this source observation.

SRCNUM

This column gives a unique source number throughout the UVOTSSC source catalogue (please note that the number of entries in the SOURCES table is larger that the total number of individual sources).

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 ERR and DEC ERR

These columns contain the standard errors (in arcseconds) of the source RA and DEC positions – i.e. (1/n) √(∑nk=1(errk)2) , where errk  are the errors of the n source positions for different filters. Please note that input errors from the individual source-list files are first converted from pixels to arcsecs.

filter SRCDIST

These are 6 columns corresponding to different UVOT filters (UVW2, UVM2, UVW1, U, B and V) and giving the distance (in arcseconds) to the closest neighbouring detected in that filter (the search for the neighbouring sources was limited to 30 arcseconds radius).

N OBSID

The number of individual observations (ObsIDs) in which the source was detected. This should correspond to the number of entries for that particular source in the SOURCES table.

filter SIGNIF

For each filter there is a column for the source significances (signal-to-noise).

filter VEGAMAG

For each filter there is a column for the source Vega-system magnitudes.

filter ABMAG

For each filter there is a column for the source AB magnitudes. These values are computed from the Vega magnitudes.

filter MAG ERR

For each filter this column gives the error on the magnitude, a single error value being appropriate for both Vega magnitudes and AB magnitudes.

filter FLUX and filter FLUX ERR

For each filter there is a column for the source fluxes and their standard errors in units of erg/s/cm2/Angstrom. The conversion factors to flux were calculated using GRB model spectra folded through the effective area curves. Please note that these factors may not be appropriate for all spectral types.

filter MAJOR

This column gives the computed value for the mean full-width-half-maximum of the source along the major-axis (in arcseconds), using the values for each filter.

filter MINOR

For each filter there is a column for the computed full-width-half-maximum of the source along the minor-axis (in arcseconds).

filter POSANG

For each filter, this column gives the computed value for the position-angle of the major-axis source (measured in degrees anti-clock wise from the Right-Ascension axis).

filter EXTENDED

For each filter there is a column to show if the source is considered extended, which was determined by comparing the source major axis with the size of the UVOT point spread function.

filter QUALITY FLAG

For each filter there is a column for the quality for each source, as determined from all the merged sources for that filter.

Quality flag

Each source has an associated “Quality flag” which is given in the final columns of the SOURCES table. The flag bit values are shown in the table below. Multiple flags are summed to give the final quality flag value.

Examples of usage

Quality value = 1 - Source contains one or more bad pixels 

Quality value =  3 - Source contains one or more bad pixels and lies on a read-out streak

Quality value = 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

Bad Pixels: A point-source will have this flag set if any pixel within the photometry aperture, or any within the background annulus, has a corresponding pixel in the quality image with a non-zero value. Similarly, an extended source will have this flag set if any corresponding pixel in the quality image has a non-zero value.

Count Rate: If the count per frame exceeds 0.96 counts/frame, it cannot be corrected for coincidence loss and the magnitude will be suspect. These magnitudes (and other associated columns) have been removed from the catalogue. If no valid measurements are available for any filter the source has been removed entirely from the catalogue.

Bit numberReasonInteger Value
 0Cosmetic defects (BAD PIXELS) within the source region 1
 1Source on a READOUT STREAK 2
 2Source on a “SMOKE RING” 4
 3Source on a DIFFRACTION SPIKE 8
 4Source affected by MOD-8 noise pattern 16
 5Source within a “HALO RING” 32
 6Source near to a BRIGHT source 64
 7MULTIPLE EXPOSURE values within photometry aperture 128
 8Source within an EXTENDED FEATURE 256
   

Table 1: Quality flag values for the UVOTSSC source catalogue

Summary Table Contents

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

N_SUMMARY

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

OBSID

This gives the unique observation identification number.

TARGET_ID

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

FILTERS

This lists the filters used in the observation.

DATE_MIN

This is the date when the first exposures for this observation started to be taken. Most observations are completed within a few days of this date.

RA_PNT

The Right Ascension for the telescope pointing direction in decimal degrees.

DEC_PNT

The Declination for the telescope pointing direction in decimal degrees.

PA_PNT

Position Angle for the telecope pointing in degrees.

EXP_filter

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

NSOURCES_filter

The total number of sources detected in this filter for this observation.

VEGAMAG_LIM_filter

The three sigma detection limiting magnitude in this filter for this observation, in the Vega photometry system.

ABMAG_LIM_filter

The three sigma detection limiting magnitude in this filter for this observation, in the AB photometry system.

Swift UVOT Serendipitous Source Catalogue (UVOTSSC):

Key Links:

Associated PDFs:

uvotssc_brief_description_v1_1.pdf