Transcript Slide 1
Gamma-Ray Bursts and GLAST
Ehud Nakar
California Institute of Technology GLAST at UCLA May 22
Outline
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GRBs: observations and model –very very brief overview
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Sources of GeV emission in GRBs
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Some physics probed by GLAST
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The Lorentz factor during the prompt emission
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The magnetic field strength
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The jet structure
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Predictions based on EGRET observations
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Summary
Observations prompt emission
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Fluence ~ 10 -7 - 10 -4 erg/cm Isotropic Energy ~ 10 50 -10 54 2 erg
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Duration 0.01- 1000 s
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Non-thermal spectrum (peaking at ~0.1-1Mev)
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Highly variable temporal structure
Time
Afterglow Radio – optical – X-rays
Fox et. al. ‘05 Following soft g -rays we observe: X-rays (minutes-weeks), optical emission (hours-months) radio emission (weeks-years)
Longs & shorts
Kouveliotou et al. 1993
?
Shorts A merger of compact binary ???
(Eichler et al 1989; …) (Review by Nakar 07)
Longs Collapsar (
Woosley et al., …) (Review by Piran 05, Meszaros 06)
Goodman 86’ Paczynski 86’ Shemi & Piran 90’, …
The Fireball Model
Prompt emission
(Rees & Meszaros 94, …)
Compact Source synchrotron
g
-rays Internal Shocks 10 13 -10 15 cm
Thompson 94’, Usov 94’, Katz 97’, Meszaros & Rees 97’, …
EM instabilities
Particle acceleration (~10 16 cm)
Lyutikov & Blandford 02, Thompson 06
synchrotron
g
-rays
Afterglow (in the fireball model)
Reverse shock
††
(~10 17 cm) Relativistic ejecta X-rays Optical Radio Forward shock
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(10 17 -10 18 cm) External medium Magnetic
†††
bubble
† ††
Meszaros & Rees 92… Meszaros & Rees 92; Katz 94; Sari & Piran 95…
†††
Luytikov & Blandford 02 X-rays Optical Radio
GeV-TeV photons
Gev-TeV photons are expected to result from
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Inverse compton: Comptonization of the self synchrotron emission (SSC) in
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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
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Wang et al. 2006, Fan & Piran 2006) p 0
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
G
>~100-300
(e.g., Lithwick & Sari 01)
Short GRBs – Observatoins hint on a spectral cutoff (indication of particle acceleration cutoff???) around 300 keV implying
G
>~15
(Nakar 07)
Detection of opacity spectral cutoff will provide a measurement of
G
Synchrotron Self-Compton constraining the magnetic field strength
SSC emission is predicted to dominate at GeV
L IC L syn
~
e B e B
1 / 2 if if
B e B e
1 1
e – fractional electron energy
B – fractional magetic field energy Afterglow observation indicate
e ~0.1 and
B ~10 -3 -10 -2 In the prompt emission
e >0.1,
B is not well constrained
Orphan afterglows –probing the jet structure
A collimated relativistic jets predict: On-axis orphan afterglow
(Nakar & Piran ‘03)
Typical GRB Off-axis orphan afterglow
(Rhoads ‘97) Nakar & Piran 03
Extensive search for optical orphan afterglows didn’t detect any yet.
GLAST has the potential to detect GeV orphans!
Detectability of a very bright GRB by the LAT alone 10 -7 10 2
jet =0.05 rad 10 1 10 0 10 -1 10 -8 10 3 T (s) 10 4 1 false detection 0.01 false detections
obs =0
obs =0.05 rad
obs =0.06 rad
obs =0.07 rad 10 5 10 -9 10 -10 10 2
jet =0.05 rad 10 3 T (s) 10 4 E iso =10 54 erg, n=1 cm -3 ,
e =0.3,
B =0.01, z=1
obs =0
obs =0.05 rad
obs =0.06 rad
obs =0.07 rad 10 5
EGRET GRBs
Earth occultation Hurley et al 1994
EGRET detected about a dozen GRBs both during the prompt emission and the afterglow
GeV detections by EGRET
From Ph.D. thesis by Maria Magdalena Gonzalez Sanchez
Prompt emission Afterglow
SSC predicts (to first order) a linear relation between BATSE and EGRET fluences: F
EGRET
=10
h
·F BATSE where
h
distributed normally
T90 200s Afterglow
1.6
Prompt emission 1.6
90%
1.4
1.4
90% 1
1.2
1.2
1
1
1
0.8
0.6
0.8
0.6
-3 -2.5
-2
-1.5
-1
-3 -2.5
-2 -1.5
-1 Ando, Nakar & Sari, in preparation
Likelihood contours for
h
distribution (
and
)
T90
1.6
1.4
1.2
1 0.8
0.6
-3 B
90%
-2.5
1
-2
A -1.5
C -1 Detection Rate (yr -1 ) >5 photons
prompt A T90 B T90 C T90 15 20 10 Afterglow A 200 B 200 C 200 20 30 10 Ando, Nakar & Sari, in preparation
Summary
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EGRET observations guarantee GRB detections by the LAT
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If the GeV emission source is synchrotron self-compton the predicted LAT detection rate is ~20 yr -1
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Determination of the MeV-GeV spectrum of the prompt emission:
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will constrain (and maybe measure) the Lorentz factor
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may shed light on electrons acceleration in short GRBs will help to determine E p in many bursts
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The ratio of the GeV to MeV emission in the prompt and afterglow emission may constrain the magnetic field strength
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LAT triggering may detect the long sought for orphan afterglows.
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Simultaneous operation with Swift is very important