Simulations of Thermal GRBs as observed by

GLAST GBM+LAT

Milan BattelinoStockholm Observatory

DC2 Closeout Meeting 31 May - 2 June 2006

Outline

•Background•The Hybrid Model•Simulation flow•Results

GRB Standard Model

Excellent fit for majority of GRB spectra in BATSE energy window

Band, D. et al.:1993 ApJ 413, 281

, , Ec , Aband

Optically Thin Synchrotron Spectrum

Low energiesSelf absorption

÷2

High energiesCut off

Observed region

FC: s = -1.5

SC: s = -1.6s = -2.1s = -2/3

BATSE

Fermi-shock-accelerated

e– distribution: p ~ 2.2

-pNe

log

Characteristic synchrotron frequency Cooling frequency

Line of Death

Hard to explain with optically thin synchrotronmodel!

Crider et al: 1997, ApJ 479, L39+Preece et al: 1998, ApJ 506, L23

Time-resolved spectra from 57 bright BATSE burstsA substantial fraction of the time-resolved spectrafrom bright BATSE burstsshow hard sub-peakspectra!

-2/3

This has to be considered when implementing a model.

Also high energy componentsin the MeV – GeV band...

Other Models...Review paper by GRB Group, 2006

• Synchrotron and Inverse Compton– Electron IC Scattering– Synchrotron Self-Compton– Proton Synchrotron Emission– Thermal Components + Synchrotron

• Pion production and cascades – Hadronic Cascades– Neutron-proton decoupling

Hybrid ModelRyde, F: 2004, ApJ 614, 827

Wien spectrum : = 2 Planck spectrum : = 1

Blackbody + Powerlaw kT, AkT, s, Apow

GRB 911016

Bose-Einstein function+ Powerlaw(s)

LOD not a problem with

-2/3

LOD not a problem in Hybrid Model

-1.6

...also: a peak at s = -1.6, close to s = -1.5

How to simulate Hybrid Model?

• Model independent simulator software (C++) producing photon histogram files

• Extend gtobssim software (C++):– New celestialSource class:

GRBtemplateManager (by Nicola Omodei) that reads photon histogram files

• Extend GBM Tools package (IDL):– Read photon histogram files

Simple Burst Modeler

Model

N(E,t)

ComponentList

1

ParameterList

1

Parameter

P(t)

1..n

PlotDevice1

Hybrid

Blackbody Powerlaw

Lightcurve Breakpoint

Component

C(E,t)

1..n1

Hybrid Model Parameters

Blackbody Component• Lightcurve (Flux)• Temperature (kT)• Normalization

Powerlaw Component• Lightcurve (Flux) • Spectral index/indices• Breakpoints• Normalization

Hybrid Model Simulation

Blackbody component

Broken power-law : synchrotron emission spectrum

GRB 911016

BATSE window

~ 3 GeV based on results byde Jager et al, 1996, ApJ 457, 253

High energy cut-off

Competition between the acceleration (heating) of the electrons and the radiativecooling leads to a maximal energy that the electrons can be accelerated to.

File Header

GRBtemplateManagerNicola Omodei

• Photon Histogram:

• Number of energy bins (columns):

• Min Energy:

• Max Energy:

• Number of time bins (rows):

• Timebinwidth:

• Energy binning:

GBM Tools extensions

• GRBtemplate photon histogram file – populate energy bins from LAT photon

histogram file with internal energy binning defined by energy grid

• Energy grid photon histogram file1. GBM Simulator reads definition file,

determines energy grids for NaI and BGO detectors and saves energy grids as files

2. SBM reads energy grid files and produces histogram files

3. GBM Simulator reads SBM histogram files to produce, apportion photons and create TTE files.

Model Parameter Values + guesses

LAT lightcurve histogram file

GLAST Definition file

XSPEC

BATSE Trigger Data

1

LAT FITS and response files

gtobssimgtselectgtrspgen

3

SBM2

gtbin4

GBM Simulator

NaI/BGO Energy Grid

5

Separate NaI and BGO lightcurve histogram files

6

GBM Simulator 7

NaI/BGO TTE, background and response files

8

XSPEC 9

Simulation Flow

NaI #2 + NaI #9 + BGO #1 + LAT

= 27.1o, = 95.3o

s1 = -1.30 +/- 0.04

s2 = -1.71 +/- 0.02

2= 0.9

Resolution : 1 x 5.0 s

Joint Spectral AnalysisGRB 911016

s2 = -1.7

s1 = -1.3

SBM input XSPEC Model

Blackbody+

(Broken Powerlawx

High Energy Cutoff)

XSPEC Result

s1 = -1.33 +/- 0.02

s2 = -1.81 +/- 0.08

Resolution : 5 x 1.0 s

GRB 941026Time integrated: 0.0 - 5.0 s

S1 = -1.6

S2 = -2.1

SBM Input

S1 = -1.7 +/- 0.03

S2 = -2.1 +/- 0.2

XSPEC Result

ÿ2÷= 0.78

= 27.1o, = 95.3o

NaI #2 + NaI #9 + BGO #1 + LAT

Conclusion

• Hard to determine high energy cutoff in time-resolved spectra above 3 GeV

• Blackbody component easily detected if it is the cause of spectral hardness

• Position of powerlaw breakpoint hard to determine when strong blackbody component

The End

Thanks to: Nicola Omodei, Valerie Connaughton and Felix Ryde