GRB Analysis

1. GRB Analysis David Band GSFC/UMBC

2. Goal • The data analysis questions are: • What is the burst spectrum (and its likely physical origin)? • Is there more than one spectral component? • How do the spectrum and its components evolve? • What is the time structure? • These questions should be answered regardless of the detector. • Therefore the analysis should involve all GLAST detectors (LAT, multiple GBM detectors of 2 types) as much as possible.

3. The Data • The LAT data will be the same type of photon list used for other data analysis. However: • Photons originate from a single point source • For a typical ~30 s burst we can assume: • Little change in inclination angle • Essentially NO background! • The GBM data result from the GBM’s burst mode. The burst data for DC2 consist of: • A count list for each detector • A response matrix for each detector • A background spectrum for each detector • Thus both the LAT and GBM data are event lists • The same bursts (time, location, spectra) will be in both the LAT and GBM data for DC2.

4. Binned Spectral Analysis Strategy: • The event list is binned in time and energy, resulting in a series of spectra. The count spectrum is Ci. • A detector response matrix (DRM) Dik is created; the DRM maps the input spectrum Fk (sampled at discrete energies) into the count spectrum. • The underlying background spectrum Bi is estimated. • A parameterized model is used for the input spectrum Fk. • A tool such as XSPEC is used to find the model parameters that best solve the equation Ci=DikFk+Bi • ‘Best’ means minimizing a statistic such as2, whose value quantifies whether the fit is good.

5. LAT Binned Analysis • Extract the photons from a region around the burst at the time of the burst. • Bin the photons with ‘gtbin’ • You choose the energy bins • The time bins can be based on the data (constant time bins, constant S/N, Bayesian blocks) or read in from a file • Output is a PHA file • Create the DRM with ‘gtrspgen’ • Output is a RSP file • Fit the resulting spectra with XSPEC • Input are the PHA and RSP files created above. Note, no background file!

6. GBM Binned Analysis • Bin the counts with ‘gtbin’ • The detectors have fixed energy bins • The time bins can be based on the data or read in from a file • Output is a PHA file • Fit the resulting spectra with XSPEC • Input are the PHA file created above and the RSP and background files provided for the burst.

7. Joint Binned Analysis • A major hurdle for joint fitting has always been getting spectra from different detectors with the same time bins. • But GLAST data are event lists, so we just bin the data with the same time bins. • gtbin can output the time bins used to bin an event list. Therefore: • Bin the data from one detector (for example using constant S/N binning) • Use the resulting time bins to bin data from other detectors • XSPEC can perform joint fits. A possible fit parameter is the relative normalization between detectors.

8. Unbinned LAT Spectral Analysis • For most bursts few LAT photons will be detected. For these bursts a likelihood analysis will be most appropriate. • A variant of the likelihood tool can do this analysis for LAT data. • Currently the separate binned GBM and unbinned LAT fits must be compared after the fitting. Eventually the GBM fit could be used as a ‘prior’ for the unbinned LAT fit.

9. Other GRB Analysis • To analyze emission that lingers for tens of minutes to hours after the prompt gamma-ray burst, standard point source likelihood analysis is required: • The background will not be insignificant • The burst source’s inclination angle will have changed • gtbin can bin data spatially and temporally. The resulting maps and lightcurves can then be inspected, e.g., with ds9 or fv. • A temporal analysis tool is being developed; the methods that will be included by DC2 are uncertain. It will have Bayesian Blocks and pulse fitting.