Definition of the Contents of the LAT Source Catalog
The current draft definition of the contents of the source catalog is in the draft of the Science Data Products Interface Control Document, available in Word
and (in a document prepared for the review of the plans for the science tools in 2002) in
XTENSION 'BINTABLE' Extension type
BITPIX 8 Bits per pixel - assume single precision floating point
NAXIS 2 # of axes=2
NAXIS1 ### Number of bytes per row
NAXIS2 ### Number of point sources in file (~3e4)
PCOUNT ###
THEAP ###
GCOUNT 1 No multiplier
TFIELDS 22 Number of fields per row
CHECKSUM Checksum for entire HDU
DATASUM Checksum for data table
TELESCOP 'GLAST' Name of mission generating data
INSTRUME 'LAT'
EXTNAME 'LAT_Point_Source_Catalog' Name of the extension
TIMESYS 'TT' Time system used in time keywords
MJDREF 58300 MJD date of reference epoch
TIMEUNIT 's' Time unit used in TSTART, TSTOP and TZERO keywords
TSTART ### Time of start of observation offset from TZERO in units of TIMEUNIT
TSTOP ### Time of end of observation offset from TZERO in units of TIMEUNIT
TTYPE1 'Source_Name' Source name in standard format
TFORM1 '13A ' Character string
TUNIT1 'none' Units of field
TTYPE2 'RA'
TFORM2 '1E ' Floating point
TUNIT2 'deg'
TLMIN2 0.0 Minimum value
TLMAX2 360.0 Maximum value
TTYPE3 'DEC' DEC
TFORM3 'E ' Floating point
TUNIT3 'deg'
TLMIN3 -90.0 Minimum value
TLMAX3 90.0 Maximum value
TTYPE4 'Conf_68_Region' Semimajor, semiminor axes and position angle, 68% containment confidence region
TFORM4 '3E ' Floating point, 3 values
TUNIT4 'deg'
TLMIN4 0.0 Minimum value
TLMAX4 360.0 Maximum value
TTYPE5 'Conf_95_Region' Semimajor, semiminor axes and position angle, 95% containment confidence region
TFORM5 '3E ' Floating point, 3 values
TUNIT5 'deg'
TLMIN5 0.0 Minimum value
TLMAX5 360.0 Maximum value
TTYPE6 'Flux' Average flux >100 MeV
TFORM6 '1E ' Floating point
TUNIT6 'cm*(-2) s*(-1)'
TLMIN6 0.0 Minimum value
TLMAX6 ### Maximum value
TTYPE7 'Unc_Flux' Uncertainty (1-sigma) in average flux >100 MeV
TFORM7 '1E ' Floating point
TUNIT7 'cm*(-2) s*(-1)'
TLMIN7 0.0 Minimum value
TLMAX7 ### Maximum value
TTYPE8 'Spectral_Index' Photon spectral index
TFORM8 '1E ' Floating point
TUNIT8 'none' Dimensionless
TLMIN8 -10.0 Minimum value
TLMAX8 10.0 Maximum value
TTYPE9 'Variability_Index' Flux variability index
TFORM9 '1E ' Floating point
TUNIT9 'none' Dimensionless
TLMIN9 ### Minimum value
TLMAX9 ### Maximum value
TTYPE10 'Signif_Avg' Detection significance (whole time interval)
TFORM10 '1E ' Floating point
TUNIT10 'none' Dimensionless
TLMIN10 0.0 Minimum value
TLMAX10 ### Maximum value
TTYPE11 'Signif_Peak' Detection significance (peak)
TFORM11 '1E ' Floating point
TUNIT11 'none' Dimensionless
TLMIN11 0.0 Minimum value
TLMAX11 ### Maximum value
TTYPE12 'Flux_Peak' Peak flux (>100 MeV) for time interval above
TFORM12 '1E ' Floating point
TUNIT12 'cm*(-2) s*(-1)'
TLMIN12 0.0 Minimum value
TLMAX12 ### Maximum value
TTYPE13 'Unc_Peak_Flux' Uncertainty (1-sigma) in peak flux >100 MeV
TFORM13 '1E ' Floating point
TUNIT13 'cm*(-2) s*(-1)'
TLMIN13 0.0 Minimum value
TLMAX13 ### Maximum value
TTYPE14 'Time_Peak' Center of time interval of peak significance
TFORM14 '1D ' Double precision
TUNIT14 's'
TLMIN14 0 Minimum value
TLMAX14 ### Maximum value
TTYPE15 'Peak_Interval' Duration of time interval of peak significance
TFORM15 '1D ' Double precision
TUNIT15 's'
TLMIN15 0 Minimum value
TLMAX15 ### Maximum value
TTYPE16 'Flux_History' Flux (>100 MeV) history
TFORM16 'PE(100) ' Floating point variable-length array
TUNIT16 'cm*(-2) s*(-1)'
TLMIN16 0 Minimum value
TLMAX16 ### Maximum value
TTYPE17 'Flux_Unc_History' Flux uncertainty (1-sigma, >100 MeV) history
TFORM17 'PE(100) ' Floating point variable-length array
TUNIT17 'cm*(-2) s*(-1)'
TLMIN17 0 Minimum value
TLMAX17 ### Maximum value
TTYPE18 'Hist_Start' Start of time intervals of flux history
TFORM18 'PE(100) ' Floating point variable-length array
TUNIT18 's'
TLMIN18 0 Minimum value
TLMAX18 ### Maximum value
TTYPE19 'Hist_End' Ends of time intervals of flux history
TFORM19 'PE(100) ' Floating point variable-length array
TUNIT19 's'
TLMIN19 0 Minimum value
TLMAX19 ### Maximum value
TTYPE20 'ID_Counterpart' Source counterparts
TFORM20 'PA(20) ' Character variable-length array
TUNIT20 'none' Dimensionless
TTYPE21 'Conf_Counterpart' Confidence of association of counterpart with source
TFORM21 'PE(20) ' Floating point variable-length array
TUNIT21 'none' Dimensionless
TLMIN21 0.0 Minimum value
TLMAX21 1.0 Maximum value
TTYPE22 'Flags' Flags (TBD) for catalog entry
TFORM22 '1B '
TUNIT22 'none' Dimensionless
END
Issues
1. Source_Name
(Ballet) I do not believe it is appropriate to define TLMIN/MAX for a string column (or is it ?). In any case, most likely we will number the versions of our catalog as everybody does, so that will be 1GL, 2GL, ... Are those registered at the IAU already ?
{Digel} I agree. The LAT catalog designation is not registered yet. I bet that the SSAC or SWG would like to have the responsibility of picking the letters to use, although there are not very many obvious choices. GL looks good to me. I imagine that GBM sources mostly will be given GRB ######-type names.
2. Conf_68_Region and Conf_95_Region
(Ballet) I think we had better split those fields into individual scalar columns. This is much easier to use in searches with standard tools.
3. Spectral_Index
(Ballet) We should add an uncertainty (1 sigma) to it.
(Digel) I agree.
4. Energy bands
(Ballet) I suggest we add the source count rates or fluxes (plus uncertainty) in several broad energy bands. This can be very useful when looking for unusual spectra (very different from a power law). I imagine the way to get that would be to run likelihood after event selection on energy in each energy band, then converting the prefactor value to a flux in that band for the spectral index of the source. Running likelihood directly on the image accumulated in that band would be another option, but this unfortunately requires an assumption on the spectral index as well (to build the PSF).
A simple choice of bands would be logarithmically spaced, with boundaries at log(EMeV) = 1.5, 2, 2.5, 3, 3.5 and maximum.
(Digel) This sounds sensible to me, although I'd prefer to use integral fluxes.
5. Flux_History
(Ballet) Did anybody study what was the best energy interval (in particular the lower boundary) to look for variability? Is 100 MeV indeed the best trade-off between number of source photons and number of background photons plus contamination by other sources ?
(Digel) This has not been studied, to my knowledge. Certainly 100 MeV is not the best in terms of being able to distinguish sources. Quoting fluxes for the range >100 MeV, though, is common.
6. Hist_Start/End
(Ballet) If I understood correctly the ideas developed in the September 2002 review of how to obtain a light curve, this will be done by running likelihood in each time bin (fixing the diffuse emission and the spectral indices). That document implied that this would be done in smaller time bins for brighter sources.
It would be much more homogeneous to do that in the same time intervals for all sources. It would ensure that the likelihood results for closeby sources would be obtained consistently, and would also be easier to use for systematic studies.
I would like to propose that we do that in large time intervals (like one month) for all sources. So the time interval (not the number of bins) would be fixed. In addition, we could add specific files (one per source) for bright sources where much more detailed information can be obtained (including spectral variability for example, or going beyond 100 bins).
(Digel) I like the idea of homogeneous time intervals in the catalog, but I am not sure what is best for sources that are variable on other time ranges. We'll certainly have ancillary information for the catalog (like images of the confidence contours, I think) that probably won't go in the FITS file for the catalog. I have to be careful about going too far down this path because then it starts to sound like an additional data product that needs defining.
7. Flags
(Ballet) 1 Byte (8 binary flags) is not much. Let's use I (2 Bytes) instead. This is a negligible size increase anyway.
8. Extended Sources
(Digel) Should we expect the catalog to include extended sources in addition to point sources? If so, we should defined an 'extendedness' parameter and possibly also list an angular extent. This can be kind of a slippery slope, getting into semimajor and semiminor axes and position angles. I'd at least include a flag indicating whether a source is resolved.
9. Source Identification
{Digel) For some sources, e.g., bright pulsars, identifications will be possible with high confidence. For most of the sources, though, the best that will be possible is a list of candidate sources. Is it realistic to assume that we will be able to assign confidence levels for assocations with counterpart sources? How about a Sowards-Emmerd-type "figure of merit"? If not, should we at least include angular offsets of the prospective counterparts from the maximum likelihood position of the source?