# Presentation: 124584

## Analyzing GRB Spectra

**David Band **
**(GLAST SSC—GSFC/UMBC)**** **

## Special Case—Gamma-Ray Bursts

**Bursts are (relatively) short—10’s of seconds—although emission up to an ~hour was seen in one burst**
**Within PSF length of the burst position no non-burst LAT photons are expected on a minute timescale**
**The pointing will not change significantly during the burst (~4o per minute)**
********** ****Treat all photons within 1-2 PSF lengths as burst photons**
**Many burst photons—bin in time and energy, fit spectra (e.g., with XSPEC)**
**Few burst photons—fit spectra using likelihoods; energy is the only observable**
**	****[Operationally, the GBM will detect and crudely localize bursts.  LAT GeV photons will localize bursts to ~0.1°.  GLAST may point at a burst’s location for ~5 hrs.]**

## Binned LAT Spectral Analysis

**Extract LAT photons from region around burst**
**Bin these LAT photons in energy (********count spectra) and time (********series of count spectra)**
**Time binning:**
**User selected**
**Constant ********t**
**Constant S/N**
**Bayesian Blocks**
**Binning tool will operate on LAT, GBM and Swift counts**
**Result is PHA-II readable by XSPEC ******** defining RMF-II format**
**Tool is called evtbin**

## Binned LAT Spectral Analysis, cont.

**Collapse instrument response (many observables) to DRM (one observable—apparent energy)**
**Format is RMF readable by XSPEC**
**May need series of DRMs for long burst**
**Tool is called rspgen**
**Fit resulting spectra with XSPEC**
**Provide scripts for fitting series of spectra**
**Working with XSPEC team in defining standard fitting output**
**Note that XSPEC can minimize either********2**** or the Cash statistic, based on the number of counts in a spectrum**

## Binned GBM Spectral Analysis

**The basic GBM burst data are ‘timed tagged events.’  Therefore GBM binned spectral analysis is nearly the same as for the LAT.**
**The counts are already from a region around the burst**
**Counts are binned in time and energy using evtbin**
**The GBM team will provide DRMs for the burst.  In addition, new DRMs can be calculated with DRMGen.**
**Backgrounds are required.  The GBM team will provide background spectra (PHA-II format).  A tool will be provided to calculate new backgrounds.**

## Joint Fits

**Because both the LAT and GBM data are both fit by XSPEC, joint fits are possible!**
**Because event lists are binned, the time binning can be the same.  **
**Joint fits with the data from other missions (e.g., Swift) are also possible.**

## Unbinned Spectral Analysis

**For a very small number of counts, a likelihood analysis of counts unbinned in energy may extract more information.**
**The Likelihood code discussed by Jim is flexible enough to fit data in only one dimension.**
**Note that XSPEC fits to the GBM data could be priors for a likelihood analysis.**

- Input value

- Likelihood maximum

**115 counts in an E****-2**** power law spectrum**

## Additional Burst Tools

**We will provide graphical tools to look at the burst data**
**We will provide a tool kit to analyze binned and unbinned lightcurves, e.g.,**
**Bayesian Blocks**
**FFTs**
**Wavelets**
**A temporal-spectral fitting tool is also being developed.  This tool will be able to fit physical models to the LAT data**

## What is NOT Included

**The SAE will not include:**
**Burst detection software**
**The instruments will have burst triggers**
**The instrument teams will look for additional bursts as part of the ground processing**
**Burst localization software**
**The GBM and LAT will localize bursts onboard; these localizations will be downlinked through TDRSS and distributed as GCN Notices.**
**A burst processor provided by the GBM team will localize bursts autonomously using data transmitted through TDRSS; the resulting localization will be distributed as a GCN Notice.**
**The instrument teams will localize (and analyze) bursts as part of the ground processing; the results will be distributed as GCN Circulars.**