GBM Gamma-ray Burst Analysis

This section provides the information needed to obtain the GBM data and understand how the data is organized, as well as a step-by-step example of using the gtbin and gtbindef tools, the FTOOLS, and the X-Ray Spectral Fitting Package (Xspec) to analyze GBM Gamma-Ray Burst observations.

Prerequisites

• gtbin, gtbindef (available with the LAT Science Tools)
• XSPEC (see Xanadu Data Analysis for X-Ray Astronomy)
• FTOOLS (available from the HEASARC website )

Assumptions

• Basic familiarity with XSPEC is assumed.

Performing a GBM Gamma-Ray Burst Analysis

The following is an example of how to perform a GRB analysis using gtbin, gtbindef, and some FTOOLS.

1. Get the data. Go to the HEASARC Browse page.
   (For this example the analysis, GRB080730520 data was used.)
   Refer to Help: Browse Interface for GBM data.

2. Examine "tcat" file. Identify the tcat file, and examine its contents using fdump; for example:

fdump glg_tcat_all_bn080730520.fit

This will provide useful information such as which detectors triggered for this burst:

DET_MASK= '110000001100' / Triggered detectors: (0-11)

3. Create light curve. Run gtbin on the time-tagged event (tte) data for one of the triggered detectors and examine the burst light curve for that detector:

prompt> gtbin This is gtbin version v2r0p3 Type of output file (CCUBE|CMAP|LC|PHA1|PHA2) [LC] Event data file name[glg_tcat_all_bn080730520.fit] glg_tte_n9_bn080730520.fit Output file name[glg_tcat_all_bn080730520.lc] glg_tte_n9_bn080730520.lc Spacecraft data file name[NONE] Algorithm for defining time bins (FILE|LIN|SNR) [LIN] Start value for first time bin in MET[INDEF] Stop value for last time bin in MET[INDEF] Width of linearly uniform time bins in MET[0.25]

Notes:

• LC for 'Type of output file' indicates that you want to create a lightcurve.

• LIN for 'Algorithm for defining time bins' indicates that you want a linear time grid. The width of the time bins is the last parameter entered.

• Start and stop times are in Mission Elapsed Time; INDEF causes the program to use tstart and tstop of the original event.

4. Plot shifted light curve. To plot the light curve, shifted so that the trigger time is at t=0 fcal, first enter:

fcalc infile=glg_tte_n9_bn080730520.lc outfile=glg_tte_n9_bn080730520_trig_time.lc clname=TRIGTIME expr="Time-239113756.403240"

Then plot the light curve using fplot:

fplot infile=glg_tte_n9_bn080730520_trig_time.lc xparm=TRIG_TIME yparm=COUNTS device="/Xw"

Note: The interactive utility, Fv , is a more powerful alternative to fplot. (See Fv: The interactive FITS File Editor.)

For example, using fv, you can use the calculator function (in the 'tools' pull down menu of the 'summary' window of the extension with the quantities you want to plot) to define a new time variable equal to the time minus the trigger time. Then you can plot other variables vs. this new time variable.

And, when plotting quantities vs. time, you can subtract the trigger time from the time variable in the windows where the quantities to be plotted are specified.

5. Using gtbindef. In this case, the burst lasts from about 0-12 seconds. Therefore, we'll use that range as a time cut in order to construct a spectrum. (Note that we'll need to shift the 0-12-s time cut back to MET by adding TRIGTIME.)

1. Construct an ascii file, e.g. "myTimeBinFile.txt":

239113726.403240 239113746.403240 239113756.403240 239113768.403240

Note: The second line is the MET time rage for the burst. The first interval contains 20s of pre-burst background data, which can be used for spectral analysis. However, the archive will eventually contain more accurately constructed background spectral files (see Updated files).

2. Next run gtbindef

gtbindef bintype=T binfile=myTimeBinFile.txt outfile=myTimeBinFile.fits

3. Run gtbin , using the file you just created as input:

prompt> gtbin This is gtbin version v2r0p3 Type of output file (CCUBE|CMAP|LC|PHA1|PHA2) [PHA2] Event data file name[glg_tte_n9_bn080730520.fit] Output file name[glg_tte_n9_bn080730520.pha] Spacecraft data file name[NONE] Algorithm for defining time bins (FILE|LIN|SNR) [FILE] Name of the file containing the time bin definition[myTimeBinFile.fits]

Note: The type-II pha file, along with the corresponding respons matrix file, glg_cspec_n9_bn080730520.rsp, can be analyzed using XSPEC

Note: This represents the simplest example of fitting a spectrum with XSPEC.
(A working knowledge of the XSPEC package is assumed here. For additional information, refer to the XSPEC manual.)

1. To start XSPEC, enter (at the prompt): xspec

2. Load in the PHA2 file created by gtbin:

XSPEC12> data glg_tte_n9_bn080730520.pha{2}

Note: Ignore the XSPEC warning that there are no associated files.

3. Load in the response matrix:

XSPEC12> resp glg_cspec_n9_bn080730520.rsp Response successfully loaded.

4. Load in the background:

XSPEC12> back glg_tte_n9_bn080730520.pha{1} Net count rate (cts/s) for Spectrum:1 3.682e+02 +/- 1.419e+01 (22.4 % total)

5. Ignore the lowest and highest energy channels (see GBM Data Channels to be fit)

XSPEC12>ign 1-5,100-** 5 channels (1,5) ignored in spectrum # 1 29 channels (100,128) ignored in spectrum # 1

6. Load a model:

XSPEC12>mo grb 5 channels (1,5) ignored in spectrum # 1 29 channels (100,128) ignored in spectrum # 1 Input parameter value, delta, min, bot, top, and max values for ... -1 0.01 -10 -3 2 5 1:grbm:alpha> -2 0.01 -10 -5 2 10 2:grbm:beta> 300 10 10 50 1000 10000 3:grbm:tem> 1 0.01 0 0 1e+24 1e+24 4:grbm:norm> ======================================================================== Model grbm <1> Source No.: 1 Active/On Model Model Component Parameter Unit Value par comp 1 1 grbm alpha -1.00000 +/- 0.0 2 1 grbm beta -2.00000 +/- 0.0 3 1 grbm tem keV 300.000 +/- 0.0 4 1 grbm norm 1.00000 +/- 0.0 ________________________________________________________________________ Chi-Squared = 4.420590e+06 using 94 PHA bins. Reduced chi-squared = 49117.66 for 90 degrees of freedom Null hypothesis probability = 0.000000e+00 Current data and model not fit yet.

7. Fit the data:

XSPEC12>fit 1000 Chi-Squared Lvl Par # 1 2 3 4 93.1784 -1 -1.05918 -2.80717 248.084 0.0135866 91.0844 -1 -1.01679 -4.26303 214.978 0.0145607 89.76 -1 -0.979862 -8.85313 194.596 0.0155139 89.7599 3 -0.979862 -8.51717 194.596 0.0155139 ================================================== Variances and Principal Axes 1 2 3 4 3.33E-07 | -0.01 0.00 0.00 1.00 2.35E-03 | 1.00 0.00 0.00 0.01 3.15E+03 | -0.00 0.00 1.00 -0.00 4.24E+10 | 0.00 1.00 -0.00 0.00 -------------------------------------------------- ================================================ Covariance Matrix 1 2 3 4 2.800e-02 2.320e+04 -1.581e+01 7.088e-04 2.320e+04 4.241e+10 -1.721e+07 6.793e+02 -1.581e+01 -1.721e+07 1.014e+04 -4.348e-01 7.088e-04 6.793e+02 -4.348e-01 1.934e-05 ------------------------------------------------ ======================================================================== Model grbm<1> Source No.: 1 Active/On Model Model Component Parameter Unit Value par comp 1 1 grbm alpha -0.979862 +/- 0.167326 2 1 grbm beta -8.51717 +/- 2.05944E+05 3 1 grbm tem keV 194.596 +/- 100.681 4 1 grbm norm 1.55139E-02 +/- 4.39736E-03 ________________________________________________________________________ Chi-Squared = 89.76 using 94 PHA bins. Reduced chi-squared = 0.9973 for 90 degrees of freedom Null hypothesis probability = 4.873082e-01

8. Define the graphics interface before plotting the results of fitting the spectrum:

XSPEC12>cpd /Xw XSPEC12>plot data res XSPEC12>setplot energy XSPEC12>plot uf

Notes:

• cpd defines the graphics device; here xwindows.
• setplot changes the x-axis from channels to energy.
• plot indicates that the data and model should be plotted.
  

Spectra obtained for this example: