grb analysis

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop

GRB Analysis

David Band

GSFC/UMBC

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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 as2, whose value quantifies whether the fit is good.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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!

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.

GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop—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.