Ehud Nakar California Institute of Technology

Gamma-Ray Bursts and GLAST

GLAST at UCLA May 22

Outline• GRBs: observations and model –very very brief overview

• Sources of GeV emission in GRBs

• Some physics probed by GLAST •The Lorentz factor during the prompt emission•The magnetic field strength •The jet structure

• Predictions based on EGRET observations

• Summary

Observationsprompt emission

Time

Flu

x

F

• Fluence ~ 10-7- 10-4 erg/cm2 Isotropic Energy ~ 1050-1054 erg

• Duration 0.01- 1000 s

• Non-thermal spectrum (peaking at ~0.1-1Mev)

• Highly variable temporal structure

AfterglowRadio – optical – X-rays

Following soft -rays we observe:X-rays (minutes-weeks), optical emission (hours-months) radio emission (weeks-years)

Fox et. al. ‘05

Longs & shorts

Kouveliotou et al. 1993

LongsCollapsar (Woosley et al., …)

(Review by Piran 05, Meszaros 06)

ShortsA merger of compact binary ??? (Eichler et al 1989; …)

(Review by Nakar 07)

?

Collimated

Baryonic flow

emEk

Goodman 86’ Paczynski 86’Shemi & Piran 90’, …

The Fireball ModelPrompt emission

Poynting flux

dominated flowE

em >>Ek

Thompson 94’, Usov 94’, Katz 97’, Meszaros & Rees 97’, …

CompactSource

Internal Shocks1013-1015cm

synchrotron-rays

(Rees & Meszaros 94, …)

EM instabilities

Particle acceleration(~1016 cm)

synchrotron-rays

Lyutikov & Blandford 02,Thompson 06

Baryonic flow

Externalmedium

Relativisticejecta

Afterglow (in the fireball model)

Reverse shock††

(~1017 cm)

X-raysOpticalRadio

Poynting flux dominated flow Magnetic†††

bubble

X-raysOpticalRadio

Forward shock†

(1017-1018 cm)

† Meszaros & Rees 92… †† Meszaros & Rees 92; Katz 94; Sari & Piran 95…†††Luytikov & Blandford 02

GeV-TeV photons

Gev-TeV photons are expected to result from

Inverse compton: • Comptonization of the self synchrotron emission (SSC) in the internal, external and reverse shocks (Meszaros et al 94, Waxman 97, Wei & Lu 99, Dermer et al, …)

• IC of photons produced in one shock by electrons that are accelerated in another shock (e.g., Pe’er & Waxman 04, Beloborodov 05. Wang et al. 2006, Fan & Piran 2006)

decay, proton synchrotron: Expected to be fainter than IC component (e.g., Bottcher & Dermer 98, Totani 98, Bahcall & Meszaros 00, Zhang & Meszaros 01)

GeV spectrum of the prompt emission Constraining the Lorentz factor

High opacity to MeV photons is avoided by high Lorentz factor

Long GRBs - assumption of high energy power-law spectrum up to Gev (supported by EGRET) implies >~100-300 (e.g., Lithwick & Sari 01)

Short GRBs – Observatoins hint on a spectral cutoff (indication of particle acceleration cutoff???) around 300 keV implying >~15 (Nakar 07)

Detection of opacity spectral cutoff will provide a measurement of

Synchrotron Self-Comptonconstraining the magnetic field strength

1 if

1 if ~

2/1

B

e

B

e

B

e

B

e

syn

IC

L

L

SSC emission is predicted to dominate at GeV

Afterglow observation indicate e~0.1 and B~10-3-10-2

In the prompt emission e>0.1, B is not well constrained

e – fractional electron energyB – fractional magetic field energy

A collimated relativistic jets predict:

Off-axisorphan afterglow(Rhoads ‘97)

On-axisorphan afterglow(Nakar & Piran ‘03)

Typical GRB

Nakar & Piran 03

Orphan afterglows –probing the jet structure

Extensive search for optical orphan afterglows didn’t detect any yet. GLAST has the potential to detect GeV orphans!

102

103

104

105

10-10

10-9

10-8

10-7

T (s)F

(er

g/c

m2/s

)

obs=0

obs

=0.05 rad

obs

=0.06 rad

obs

=0.07 rad

103

104

105

10-1

100

101

102

T (s)

N (

ph

)

1 false detection0.01 false detections

obs=0

obs

=0.05 rad

obs

=0.06 rad

obs

=0.07 rad

jet

=0.05 rad

Eiso=1054 erg, n=1 cm-3, e=0.3, B=0.01, z=1

Detectability of a very bright GRB by the LAT alone

jet=0.05 rad

EGRET GRBs

Earthoccultation

Hurley et al 1994

EGRET detected about a dozen GRBs both during the prompt emission and the afterglow

GeV detections by EGRET

Prompt emission Afterglow

From Ph.D. thesis by Maria Magdalena Gonzalez Sanchez

200s

-3 -2.5 -2 -1.5 -1

0.6

0.8

1

1.2

1.4

1.690%

1

T90

-3 -2.5 -2 -1.5 -1

0.6

0.8

1

1.2

1.4

1.6

90%

1

SSC predicts (to first order) a linear relation between BATSE and EGRET fluences: FEGRET=10·FBATSE where distributed normally

Likelihood contours for distribution ( and )

Ando, Nakar & Sari, in preparation

Prompt emission Afterglow

T90

-3 -2.5 -2 -1.5 -1

0.6

0.8

1

1.2

1.4

1.6

90%

1

C

A

B

Ando, Nakar & Sari, in preparation

Detection Rate (yr -1)>5 photons

prompt

AT90 15

BT90 20

CT90 10

Afterglow

A200 20

B200 30

C200 10

Summary •EGRET observations guarantee GRB detections by the LAT

•If the GeV emission source is synchrotron self-compton the predicted LAT detection rate is ~20 yr-1

•Determination of the MeV-GeV spectrum of the prompt emission:

• will constrain (and maybe measure) the Lorentz factor• may shed light on electrons acceleration in short GRBs• will help to determine Ep in many bursts

• The ratio of the GeV to MeV emission in the prompt and afterglow emission may constrain the magnetic field strength

• LAT triggering may detect the long sought for orphan afterglows.

• Simultaneous operation with Swift is very important

Thanks!