Definition of the Contents of the LAT Source Catalog
Note: This page was written in March 2005 and extensively re-arranged in October 2005
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 HTML.
The draft is in need of updating in several respects. Some of the issues were discussed during a splinter session
of the LAT collaboration meeting in September 2004. This presentation also includes the high-level requirements for producing the catalog.
This confluence page presents a summary of the issues and some suggested resolutions. Broadly speaking the issues are related either to what should be included in the catalog or how best to represent the included items in FITS. The issues and proposed resolutions, or any other aspect of the definition of the contents are open for comment (probably most effectively by editing this page rather than inserting Confluence comments).
Issues
Text in red shows what was adopted for the revised draft presented further below.
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.
1GL J123456-012345 naming scheme has been assumed.
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.
The fields have been split.
3. Spectral_Index
(Ballet) We should add an uncertainty (1 sigma) to it.
(Digel) I agree.
Done.
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.
I stuck with integral fluxes, for >100 MeV, >300 MeV, >1 GeV, and >3 GeV
energy ranges, with uncertainties included for each flux. This will still be useful for finding unusual spectra.
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.
The variability index can apply to whatever energy interval we choose, presumably specified along with the definition of the index
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.
The updated version has fixed one-month time intervals and includes integral fluxes and flux uncertainties (>100 MeV). As written, a fixed size array (12 elements) is specified. We may want to convert these back to variable length arrays so that the specification does not need to change when the time range gets longer.
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.
2-byte integer are now used.
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.
The catalog does not include an extendedness parameter. If we decide we need one, we can use some of the flag bits defined above.
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?
The updated definition includes space for just one counterpart source and for a flag value that defines the degree of confidence (1 for Figure-of-Merit above some threshold, 2 for correlated variability).
10. HDUCLASS Keywords (26 October 2005)
(Digel) I added placeholders for these. We need to figure out whether we are conforming enough to the HEASARC 'SRCLIST' definition to be able to use it.
11. Peak vs. Average (26 October 2005)
(Digel) It has gone unspecified until now, but the fluxes and flux history values described above are assumed to apply for the entire time range of the catalog (or of the time interval for a flux history evaluation). The catalog also includes Flux_Peak, Unc_peak_Flux, Signif_Peak, Time_Peak, and Peak_Interval to allow for specification of properties of flaring sources that we see only once for a short period of time. Defining significances for a specific time interval may be problematic, as the time interval has been selected to be when the detection significance is greatest.
Draft header (Last update: 26 October 2005)
Here is the FITS version of the first extension header of the FITS version of the catalog, with the changes specified above adopted. Actually, this is pseudo-FITS, as the comments are not properly set off, but all of the columns and keywords are present.
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 (~1e4) TFIELDS 27 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 51910 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 HDUCLASS 'OGIP ' Format conforms to OGIP standard HDUDOC '?' HDUVERS '1.0.0 ' HDUCLAS1 'SRCLIST' ? TTYPE1 'Source_Name' Source name in standard format, e.g., 1GL J123456-012345 TFORM1 '18A ' 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_SemiMajor' Semimajor axis, 68% containment confidence region TFORM4 '1E ' Floating point TUNIT4 'deg' TLMIN4 0.0 Minimum value TLMAX4 360.0 Maximum value TTYPE5 'Conf_68_SemiMinor' Semiminor, axis, 68% containment confidence region TFORM5 '1E ' Floating point TUNIT5 'deg' TLMIN5 0.0 Minimum value TLMAX5 360.0 Maximum value TTYPE6 'Conf_68_PosAng' Position angle, 68% containment confidence region, E of N TFORM6 '1E ' Floating point TUNIT6 'deg' TLMIN6 0.0 Minimum value TLMAX6 360.0 Maximum value TTYPE7 'Conf_95_SemiMajor' Semimajor axis, 95% containment confidence region TFORM7 '1E ' Floating point TUNIT7 'deg' TLMIN7 0.0 Minimum value TLMAX7 360.0 Maximum value TTYPE8 'Conf_95_SemiMinor' Semiminor, axis, 95% containment confidence region TFORM8 '1E ' Floating point TUNIT8 'deg' TLMIN8 0.0 Minimum value TLMAX8 360.0 Maximum value TTYPE9 'Conf_95_PosAng' Position angle, 95% containment confidence region, E of N TFORM9 '1E ' Floating point TUNIT9 'deg' TLMIN9 0.0 Minimum value TLMAX9 360.0 Maximum value TTYPE6 'Flux100' Average flux >100 MeV TFORM6 '1E ' Floating point TUNIT6 'cm**(-2) s**(-1)' TLMIN6 0.0 Minimum value TLMAX6 ### Maximum value TTYPE7 'Unc_Flux100' 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, >100 MeV TFORM8 '1E ' Floating point TUNIT8 'none' Dimensionless TLMIN8 -10.0 Minimum value TLMAX8 10.0 Maximum value TTYPE8 'Unc_Spectral_Index' 1-sigma uncertainty, photon spectral index TFORM8 '1E ' Floating point TUNIT8 'none' Dimensionless TLMIN8 0.0 Minimum value TLMAX8 10.0 Maximum value TTYPE9 'Flux300' Average flux >300 MeV TFORM9 '1E ' Floating point TUNIT9 'cm**(-2) s**(-1)' TLMIN9 0.0 Minimum value TLMAX9 1.0 Maximum value TTYPE10 'Unc_Flux300' Uncertainty (1-sigma) in average flux >300 MeV TFORM10 '1E ' Floating point TUNIT10 'cm**(-2) s**(-1)' TLMIN10 0.0 Minimum value TLMAX10 1.0 Maximum value TTYPE11 'Flux1000' Average flux >1000 MeV TFORM11 '1E ' Floating point TUNIT11 'cm**(-2) s**(-1)' TLMIN11 0.0 Minimum value TLMAX11 1.0 Maximum value TTYPE12 'Unc_Flux1000' Uncertainty (1-sigma) in average flux >1000 MeV TFORM12 '1E ' Floating point TUNIT12 'cm**(-2) s**(-1)' TLMIN12 0.0 Minimum value TLMAX12 1.0 Maximum value TTYPE13 'Flux3000' Average flux >3000 MeV TFORM13 '1E ' Floating point TUNIT13 'cm**(-2) s**(-1)' TLMIN13 0.0 Minimum value TLMAX13 1.0 Maximum value TTYPE14 'Unc_Flux3000' Uncertainty (1-sigma) in average flux >3000 MeV TFORM14 '1E ' Floating point TUNIT14 'cm**(-2) s**(-1)' TLMIN14 0.0 Minimum value TLMAX14 1.0 Maximum value TTYPE15 'Variability_Index' Flux variability index (TBD) TFORM15 '1E ' Floating point TUNIT15 'none' Dimensionless TLMIN15 ### Minimum value TLMAX15 ### Maximum value TTYPE16 'Signif_Avg' Detection significance (whole time interval) TFORM16 '1E ' Floating point TUNIT16 'none' Dimensionless TLMIN16 0.0 Minimum value TLMAX16 1.0E9 Maximum value TTYPE17 'Signif_Peak' Detection significance (peak) TFORM17 '1E ' Floating point TUNIT17 'none' Dimensionless TLMIN17 0.0 Minimum value TLMAX17 1.0E9 Maximum value TTYPE18 'Flux_Peak' Peak flux (>100 MeV) for time interval above TFORM18 '1E ' Floating point TUNIT18 'cm**(-2) s**(-1)' TLMIN18 0.0 Minimum value TLMAX18 1.0 Maximum value TTYPE19 'Unc_Peak_Flux' Uncertainty (1-sigma) in peak flux >100 MeV TFORM19 '1E ' Floating point TUNIT19 'cm**(-2) s**(-1)' TLMIN19 0.0 Minimum value TLMAX19 1.0 Maximum value TTYPE20 'Time_Peak' Center of time interval of peak significance TFORM20 '1D ' Double precision TUNIT20 's' TLMIN20 0.0 Minimum value TLMAX20 ### Maximum value TZERO20 ### TTYPE21 'Peak_Interval' Duration of time interval of peak significance TFORM21 '1D ' Double precision TUNIT21 's' TLMIN21 0 Minimum value TLMAX21 ### Maximum value TTYPE22 'Flux_History' Flux (>100 MeV) history (monthly) TFORM22 '12E ' Floating point array, 12 months TUNIT22 'cm**(-2) s**(-1)' TLMIN22 -1.0 Minimum value TLMAX22 1.0 Maximum value TTYPE23 'Flux_Unc_History' Flux uncertainty (1-sigma, >100 MeV) history TFORM23 '12E ' Floating point array, 12 months TUNIT23 'cm**(-2) s**(-1)' TLMIN23 0.0 Minimum value TLMAX23 1.0 Maximum value TTYPE24 'Hist_Start' Start of time intervals of flux history TFORM24 '12E ' Floating point array TUNIT24 's' TLMIN24 0.0 Minimum value TLMAX24 ### Maximum value TZERO24 ### TTYPE25 'ID_Counterpart' Source counterpart (if any) TFORM25 '20A ' Character string TUNIT25 'none' Dimensionless TTYPE26 'Conf_Counterpart' Confidence of association of counterpart with source TFORM26 '1I ' Index, 1 = Figure of Merit, 2 = Correlated variability TUNIT26 'none' Dimensionless TLMIN26 0 Minimum value TLMAX26 2 Maximum value TTYPE27 'Flags' Flags (TBD) for catalog entry TFORM27 '1I ' TUNIT27 'none' Dimensionless END