1. GRB Simulations in DC2
Valerie Connaughton with input from Nicola Omodei,
David Band, Jay Norris and Felix Ryde.
DC2 Workshop -- GSFC

2. Current GRB Activities
Generation of LAT GRB data –Nicola Omodei
Generation of GBM data – David Band using burst definition and spectral parameter history input from Nicola. Produces TTE, DRM, background data and CTIME and CSPEC in our Level 1 FITS format according to ICD.
Analysis of LAT data to extract GRB pha2 and rsp files – Nicola
Fitting of spectra with XSPEC – Nicola and David
Production of limited TRIGDAT – mostly to notify participants to presence of burst in LAT data – Valerie (+ Rob Preece)
Modelling of GRBs – Nicola, Jay Norris, Felix Ryde
Deciding what kind of bursts – models, numbers, realistic Log N-Log P, “interesting” events -- are included – ???
DC2 Workshop -- GSFC

3. GRBs at BATSE energies Bursts are varied in: Duration.
Intensity (Peak Flux).
Number, width, separation of peaks.
Fluence.
Spectral characteristics.
Over the large ensemble of BATSE GRBs (2704) these parameters allow us to characterize the GRB population at energies up to ~ MeV.

4. GRB characteristic distributions
Typical GRB spectrum characterized at BATSE energies by the Band parameterization α, β, A, Epeak or some similar function where the physical meaning of these parameters is not specified.

5. GRB characteristics at LAT energies
Band parameterization α, β, Epeak
Duration drawn from T90 (with ¼ short)
Peak flux drawn from BATSE distribution
Pulse width scaled with energy to LAT energies W(E)=W0 E-0.333
Number and spacing of pulses according to Norris et al 1996.

6. Spectra & Light Curve
DC2 Workshop -- GSFC

7. GRB physical model Shells emitted with relativistic Lorentz factors
Internal shocks (variability naturally explained)
Acceleration of electrons with a power law initial distribution, between min an max
Non-thermal emission (Synchrotron and Inverse Compton) from relativistic electrons
DC2 Workshop -- GSFC

8. Simulated GRB with c.o. at 4.5 GeV
Due to the finite value of the Lorentz factor of the accelerated electrons, the synchrotron spectrum presents a cut-off at LAT energies.
This cut-off is simulated by the GRB physical model.
Simulated GRB with c.o. at 4.5 GeV
Reconstructed c.o.
5.5±1.5 GeV
G=220±60
Xspec spectral fitting
Root fit
Reconstructed c.o.
± GeV
G=±
Simulated GRB with c.o. at 4 GeV
DC2 Workshop -- GSFC

9. GRB High-Energy Measurements
High-energy (> tens MeV) might be delayed, sometimes persistent beyond BATSE range, might fall above extrapolation of β inferred from lower-energy spectrum. s s s s s

10. Inverse Compton emission
The Inverse Compton emission (SSC) is parameterized by a parameter , which is the ration between the Synchrotron peak and the Inverse Compton peak of the vFv spectrum.
A simplified spectral shape is used in order to reduce the computational time (synchrotron spectrum shifted by m2 plus an exponential cut-off due to conservation of energy.
Synchrotron + Inverse Compton
Reconstructed photons!
Synchrotron Model
Band Function: “best” representation of the GRB flux between 20 keV and 1 MeV
High energy photons (>50 MeV)
DC2 Workshop -- GSFC

11. Hybrid Thermal/NonThermal Spectrum
Felix Ryde & Milan Battelino provide the N(e,t) ASCII file, describing the spectrum from their model.
GRBtemplate reads the ASCII file, and create a TH2D object
SpectrObj computes fluxes, fluences, and extracts photons for the LAT simulation.
DC2 Workshop -- GSFC

12. Simulation procedure

14. Extraction of photons
The energy is randomly extracted from the integrated spectrum E(t,e).
The “real” algorithm is more sophisticated!
(I.e. taking into account interpolation between bins)
DC2 Workshop -- GSFC

15. SpectrObj and the extraction of photons
GRBSim
Low Energy (GBM, BATSE)
Light Curves
High Energy (LAT)
Light Curves
Simulated GRB Fv Spectra
Band Fv Spectrum
Extracted HE Photons
DC2 Workshop -- GSFC

16. Simulated GRB from LAT Outputs events and S/C data for whole
Event and for GRB
Region. Also,
PHA and RSP files
For XSPEC.
Outputs GBM
Definition and
Spectral
Parameters file.

17. XSPEC result for simulated burst
Input spectrum, alpha=0.4, beta=2.25
XSPEC result from LAT data, beta= / with reduced chi2 0.5

18. DC2 preparation, new features
Burst can be generated inside a given FoV.
Burst are generated randomly in time (not as in DC1!)
Improvement of the GRB physical model: cut-off, IC.
GRB phenomenological model redesign.
New ‘class’ model implemented: GRBtemplate is able to accommodate any GRB model (it reads a file from someone else!)
GBM synchronization!!
DC2 Workshop -- GSFC

19. Production of GBM Data Products
Goal: Production of simulated GBM data products in the appropriate format (see data products talk Wednesday morning) consistent with simulated LAT bursts.
David Band created a system of IDL procedures to create these GBM data products.
This system does not worry about earth occultation, nor does it have any burst physics.
Input:
Burst name (for file names, headers, also source of date)
Burst time (assumed to be the same in the LAT data)
Burst location in spacecraft coordinates (to calculate response) and celestial coordinates (for file headers)
Time series of spectral parameters (every 16 ms). This is the input of the burst model.

20. Production of GBM Data Products, cont.
Output (a file for each burst-facing detector):
Time tagged events
Response matrices—direct component only
Background spectra
Rates for trigger  4000 s in two versions with different spectral and temporal resolution
Rates for day with burst
The spacecraft is assumed to maintain constant orientation during the burst (i.e., no autonomous repoint).
The GBM response and background models were provided by Marc Kippen a few years ago, and are therefore preliminary. The response does not include scattering off the Earth’s atmosphere.

21. Production of GBM Data Products—Methodology
The background and response matrices are calculated using Marc Kippen’s IDL procedures.
Source and background counts are created
Random source photons and background count arrival times are generated assuming Poisson statistics and the count rates
Energies are assigned to the generated photons and background counts by sampling from the source and background spectra
The source photons are ‘detected’ (i.e., some are kept and some are thrown out) using the response matrices
The counts are binned in time into count rates.
All data are written out in properly formatted FITS files.

22. GRB Analysis procedure in DC2
TRIGDAT
(RA/DEC)
(Time)
Extract LAT event data, pointing/livetime and IRF from
GRB position/time info
GRB spectral
catalog
GRBs in LAT
Sky map
Event binning, DRM gen, Likelihood
Feed GBM & LAT
data to XSPEC/RMFIT
for joint spectral fit.
Bin GBM TTE data to same resolution as LAT