ehud nakar california institute of technology gamma-ray bursts and glast glast at ucla may 22
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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!