single- and two-component grb spectra in the fermi gbm-lat ... · overview 1 introduction 2...

Single- and Two-Component GRB Spectrain the Fermi GBM-LAT Energy Range

Peter Veres and Peter Meszaros

Dept. of Astronomy & Astrophysics,Dept. of Physics and Center for Particle Astrophysics

Pennsylvania State University

arXiv:1202.2821

5/7/2012

Peter Veres and Peter Meszaros (PSU) Multicomponent HE spectra GRB conf. 2012, Munchen 1 / 16

Overview

1 Introduction2 Magnetic dynamics3 Radiation components

Prompt emissionRadiation from rdec

4 Example spectraTheoreticalObserved

5 Conclusions


Introduction

Before Fermi - EGRET and projections for LAT

Extra component and/or cutoff

Hints of extra PL componentGonzalez et al. (2003)LAT GRBs predicted Band etal. (2009) -higher thanobservedSign of tentative cutoff inprompt (Kocevski et al.1201.3948)


Introduction




Number Of Photons Detected1 10 210 310

Nu

mb

er O

f G

RB

/yr

1

10

>1 GeV

>10 GeV

>100 MeV

>1 GeV

>10 GeV

All selected Burst

Only bursts with Beta<-2


Introduction





Introduction

Fermi GBM/LAT observations

GRB 080916C-smooth Band functionGRB 090902B-extra power law comp.GRB 090926A-extra PL comp.+ break

goals: reproduce simple PL high energy behaviour, extra PL and cutoff


Magnetic dynamics

Magnetically dominated jets

Magnetic dynamics: Γ ∝ r1/3 (baryonic Γ ∝ r)εB goes from ≈ 1 in the prompt to . 10−1 at rdecDissipative photosphere: reconnection, weak shocks, turbulencePhotosphere in accelerating phasePrevious studies: Band-like prompt spectrum naturally arises-Vurm et al. 2011, Thompson 1994, Giannios and Spruit 2007

Γphot.

R Rphot. saturation deceleration

magnetic model

R

η

barionic model

~R1/

3

Γ

Γ~

R


Radiation components

Radiation Sources- Two Zone Model

Prompt component from photosphere (e.g. Beloborodov 2010)FS/RS synchrotronPrompt up-scatters on FS/RS electrons (Murase et al., 2011)FS/RS SSC (Sari & Esin, 2001)BB, BB+FS, BB+RS (Ryde, 2005; Ando & Meszaros, 2008)p+ sync., FS+RS, RS+FS (Razzaque et al., 2009, He et al., 2011)Max synch./KN cutoffs (Guetta & Granot, 2003)

RphRdec

RS FS

RS−EICFS−EIC


Radiation components Prompt emission

Prompt Emission from Magnetic Dissipation

Magnetic energy dissipates at rph (e.g. semirelativistic shocks)(Giannios 2007, Lazzati & Begelman 2011, Meszaros & Rees,2011, McKinney & Uzdensky 2011)E.g. Alfvenic turbulence naturally produces the Band function withα = −1, β = −2 (Thompson, 1994)Prompt peak is synchrotron radiation from photosphere

rph = 6.5× 1012 cm L3/5t ,53 r2/5

0,7 η−3/5600 ∼ 0.5 AU

Γph = (rph/r0)1/3 = 87 L1/5

t ,53ζ1/5r r−1/5

0,7 η−1/5600

εobsph,syn = 310 keV ζ−1/2

r (1 − ζr)1/2r1/2

0,7 ε1/2B,0 Γ

3r(1+z

2

)−1

weak thermal componentT(rph) = 2.7 keV L−1/60

t ,53 ζ−4/15r η

4/15600 r−7/30

0,7 Γ−1/2r

(1+z2

)−1

Pairs at the photosphere? -consider both cases.


Radiation components Radiation from rdec

Forward- and Reverse Shock

At rdec = 4.8× 1016 cm ∼ 3000 AUεB goes from ≈ 1 to . 10−1

FFSmax(εc) = 0.15 Jyεc = 3.7 eV (UV) FAST coolingFRS

max(εm) = 92 JyεRS

m = 1.1× 10−5 keV (FIR) SLOW coolingCutoff at ε(γMAX)

Does a RS develop?- take both cases

RphRdec

RS FS

RS−EICFS−EIC


Radiation components Radiation from rdec

External inverse Compton

τRS = 6.4× 10−6

τFS = 1.1× 10−8

cutoff and peak at εKN(γ)

εFpeakε,RSEIC ≈ εbrNε,pτRSε

RSEICKN = 2.3× 10−8 erg cm−2 s−1

εFpeakε,FSEIC ≈ εbrNε,pτFSε

FSEICKN = 7.2× 10−10 erg cm−2 s−1

RphRdec

RS FS

RS−EICFS−EIC


Example spectra Theoretical

Example theoretical spectrum with pair cutoff

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 10610-9

10-8

10-7

10-6

¶ @MeVD

¶F¶@e

rgcm-

2s-

1D

prompt

RS-EICFS

RS

BB

RS-SSC

FS-SSC

pairs

FS-EIC

Lt = 1053 erg/s, ζr = 0.6,n = 10 cm−3,η = 600, εB,pr = 1, εB,FS = εB,RS =

10−2, εe,FS = εe,RS = 10−2, r0 = 107 cm, z = 1,β = 2.4,p = 2.4



Example spectra: model with no pair cutoff 1.

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 10610-9

10-8

10-7

10-6

¶ @MeVD

¶F¶@e

rgcm-

2s-

1D

prompt

RS-EIC

FS

RS BB

FS-SSC

FS-EIC

Lt = 1053 erg/s, ζr = 0.5,n = 30 cm−3,η = 400, εB,pr = 1, εB,FS = εB,RS =

2× 10−2, εe,FS = εe,RS = 5× 10−3, r0 = 107 cm, z = 1,β = 2.5,p = 2.4



Example spectra: model with no pair cutoff 2.

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 10610-9

10-8

10-7

10-6

¶ @MeVD

¶F¶@e

rgcm-

2s-

1D

prompt

RS-EIC

RS

RS-SSC

BB

FS-SSC

FS-EIC

FS

Lt = 5× 1052 erg/s, ζr = 0.6,n = 102 cm−3, η = 400, εB,pr = 0.9, εB,FS =

10−2, εe,FS = 2× 10−2, r0 = 107 cm, z = 1,β = 2.4,p = 2.4


Example spectra Observed

Example: fitting the model to Fermi measurements 1.

Data provided by Bin-Bin Zhang (Veres, Zhang and Meszaros in prep.)


Conclusions

Conclusions

Magnetic model fits well Fermi GBM/LAT dataGood for inetrpreting observationsExternal IC needed for additional PL comp.If RS is present, strong extra componentCan reproduce:-smooth Band spectrum-Band spectrum + extra PL component + cutoff

for more, please see:arXiv:1202.2821


Backup slides

Why we do not invoke internal shocks?Internal shocks are radiatively inefficient and also are typicallyexpected at radii ris = ctΓ2 ∼ 3× 1013tv,−3Γ

23 cm, too small for both

showing small variability and to allow GeV photons to escape(Rγγ > 1015 cm).Dissipative photospheres can be 50% radiatively efficient; theirspectrum reproduce the Band function, with a peak which isthermal (baryonic dyn) or synchrotron (mag.dyn), and where lowand high energy slopes are due to scattering (baryonic) and orSynch/IC (magn dyn) - see Thompson 94, Rees, Meszaros05,Pe’er, Meszaros, Rees 06 and Beloborodov 11.Magnetic dynamics: useful because (a) suspect mag. fieldsdominant in high Eiso bursts (since need Blandford-Znajek), (b)dissipation very efficient due to reconnection, (c) spectrum isBand-like (Giannios 07, Thompson 94 or Vurm 11).


Time resolved spectra, e.g. 080916C work in progress


single- and two-component grb spectra in the fermi gbm-lat ... · overview 1 introduction 2...

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