This procedure provides a step-by-step example of using the X-Ray Spectral Fitting Package (XSPEC) to analyze LAT Gamma-Ray Burst observations.
Note: LAT GRB data can also be spectrally analyzed using the likelihood tools for source analysis.
It is assumed that:
Note: For the purposes of this thread, the relevant burst properties are:
We can perform further selections of the original event file with gtselect as desired. For example:
prompt> gtselect evclsmin=1 evclsmax=4
Input FT1 file LAT_090119205.fits
Output FT1 file GRB090119205_spec_ph.fits
RA for new search center (degrees) (0:360)  47.907
Dec for new search center (degrees) (-90:90)  -11.758
radius of new search region (degrees) (0:180)  10
start time (MET in s) (0:)  254033757
end time (MET in s) (0:)  254034457
lower energy limit (MeV) (0:)  100
upper energy limit (MeV) (0:) 
maximum zenith angle value (degrees) (0:180)  105
Notes: The photons are selected from:
Use gtbin to bin the photon data into spectra that XSPEC can analyze. The following is a transcript of running this tool.
This is gtbin version ScienceTools-v9r15p2-fssc-20090808
Type of output file (CCUBE|CMAP|LC|PHA1|PHA2) [PHA2] PHA1
Event data file name GRB090119205_spec_ph.fits
Output file name GRB090119205.pha
Spacecraft data file name[NONE] spacecraft_data_file.fits
Algorithm for defining energy bins (FILE|LIN|LOG) [LOG]
Start value for first energy bin in MeV 100
Stop value for last energy bin in MeV 300000
Number of logarithmically uniform energy bins 30
Here a logarithmic grid has been chosen with 30 bins between a start energy of 100 MeV and stop energy of 300,000 MeV.
A Detector Response Matrix, created as a single RSP file (i.e,. we do not split the DRM into separate RMF and ARF files), is required to analyze the .pha file with XSPEC.
The tool for the LAT is called gtrspgen, and the following is a transcript of running this tool.
This is gtrspgen version ScienceTools-v9r15p2-fssc-20090908
Response calculation method (GRB|PS) [GRB]
Spectrum file name GRB090119205.pha
Spacecraft data file name spacecraft_data_file.fits
Output file name GRB090119205.rsp
Time of GRB (s)[0.] 254033757
Response function to use, Handoff|DC2|DC2A|DC2FA|DC2BA|DC2FB etc[P6_V1_DIFFUSE] P6_V3_DIFFUSE
Algorithm for defining true energy bins (FILE|LIN|LOG) [LOG]
Start value for first energy bin in MeV[30.] 100
Stop value for last energy bin in MeV[200000.] 300000
Number of logarithmically uniform energy bins
This assumption is indicated by the choice of 'GRB' for 'Response calculation method' if our photons were selected over '700 s' because gtselect will not currently select photons over short durations. However, most of the photons are in the first ~6 s, justifying the 'GRB' choice.
The output file is GRB090119205.rsp (the .rsp extension indicates that this is a standard RSP file).
You now have the two files necessary to analyze the burst spectrum with XSPEC:
Note that there is no background file. All non-burst sources are expected to produce less than 1 photon in the extraction region during the burst! Here we provide the simplest example of fitting a spectrum with XSPEC; for further details you should consult the XSPEC manual.
Note: The default version is now release 12 (XSPEC12).
Note: Power law indices tend to be near 2 for gamma-ray sources.
Note that you may be prompted for the lower and upper energy boundaries, which you should provide in units of kev.
Note: While this is not the appropriate place to discuss spectral fitting with XSPEC, issues to consider are:
Last updated by: Analia Cillis 08/21/2009