glast science support center june 29, 2005data challenge ii software workshop grb analysis david...
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![Page 1: GLAST Science Support Center June 29, 2005Data Challenge II Software Workshop GRB Analysis David Band GSFC/UMBC](https://reader035.vdocuments.mx/reader035/viewer/2022072010/56649dd95503460f94acf6fd/html5/thumbnails/1.jpg)
GLAST Science Support CenterJune 29, 2005 Data Challenge II Software Workshop
GRB Analysis
David Band
GSFC/UMBC
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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.
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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.
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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.
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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!
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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.
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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.
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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.
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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.