1. Gamma-Ray Bursts
Ehud Nakar
Caltech
APCTP 2007
Feb. 22

2. Gamma Ray Bursts (GRBs)
NASA web site
Twice a day energetic flash of g-rays hits the Earth

3. Afterglow X-rays – optical – radio
Following soft g-rays we observe:
X-rays (minutes-hours), optical emission (hours-days)
radio emission (weeks-years)
Fox et. al., 05
Afterglow  localization  Host galaxy  distance
g-ray Luminosity (isotropic equivalent) % Msunc2/s!

4. Why GRBs are interesting?
The most violent explosions in the Universe:
Luminosity of 108 Galaxies
Energy released within ~ km radius
Extreme physical conditions and possible sources of:
Ultra-high energy cosmic rays – 1011 GeV protons
High-energy neutrinos
Gravitational waves
Useful tool to probe various astrophysical aspects
High redshift universe
Winds from massive stars

5. ? Longs & Shorts Short GRBs Long GRBs
Kouveliotou et al. 1993 ?
Short GRBs
First afterglow detected in 2005 – Swift & HETE2 (Gehrels et. al., 05)
Long GRBs
First afterglow detected in BeppoSAX
(Costa et. al., 97)

6. Outline Main observations Compact Relativistic observed engine wind
g-rays + afterglow
Internal-external
fireball model
Central engine
properties and models
G=?
Viable progenitors of long and short GRBs and gravitational waves

7. Main observations Prompt g-rays Erratic light curves
variability time scale ~ 1-10ms
Duration ~ 10ms s
Photon counts
Time (s)

8. Main observations Prompt g-rays Erratic light curves
Non-thermal spectra
Typical photon energy ~ 0.3 MeV
Photon/keV
Energy (keV)
Hurley et. al., 02

9. Main observations Flux
Prompt g-rays
Erratic light curves
Non-thermal spectra
Afterglows
Power-law temporal decay
Power-law spectra
Bright in X-ray (hours-days)
Optical (days-weeks)
Radio (weeks-years)
Distances: redshift ~ (1-20 Gly)
Host galaxies
Optical
X-ray
Time
Flux
Soderberg et. al., 06
Fox et al ., 2005

10. Compact
engine
Relativistic
wind
observed
g-rays + afterglow
G=?

11. Relativistic outflow Thomson optical depth tT ~ sTneR < 1
Observations: non-thermal spectra  optically thin source
(Optical depth t - system length / mean free path)
Thomson optical depth tT ~ sTneR < 1
Thomson
cross-section
e- e+ density
source width
Typical observed photon energy ~ mec2  a large fraction of the photons annihilate to pairs (gg  e- e+)

12. Observations: non-thermal spectra  optically thin source
Relativistic outflow
Observations: non-thermal spectra  optically thin source
~1054 ~1
(cdt)2 ~ 1015
A conflict! (Schmidt 1978)

13. (Guilbert et. al, 83, Piran & Shemi 93) Source with a Lorentz factor G
Relativistic outflow
(Guilbert et. al, 83, Piran & Shemi 93)
Source with a Lorentz factor G
e’ph  eph / G
 f (e’ph>mec2) 
Rest frame
photon energy
Observed A A B R
~R/G2

14. Source with a Lorentz factor G
Relativistic outflow
(Guilbert et. al, ‘83, Piran & Shemi ’93)
Source with a Lorentz factor G
e’ph  eph / G
 f (e’ph>mec2) 
R  cdtG2 
G > 100 (e.g., Sari & Lithwick 2001, Nakar 2007)

15. Central engine properties and models
Compact
engine
Relativistic
wind
observed
g-rays + afterglow
Central engine
properties and models
G>100

16. Central engine properties
Isotropic equivalent g-ray luminosity: erg/s
Isotropic equivalent total emitted energy:
Long GRBs: erg
Short GRBs: erg
Variability time scales ~ 1ms  Engine size < 107 cm
Burst duration:
Long GRBs ~ 30 s  Engine activity time ~ 30 s
Short GRBs ~ 0.1 s  Engine activity time ~ 0.1 s
Probable beaming correction ~ 100

17. Central engine – most popular model accretion disk – black hole system
Powered by gravity
Activity time = Accretion time
Outflow is launched by neutrino annihilation (e.g., Goodman, et. al., 87; …) or by electromagnetic processes (e.g., Blandford & Znajek 77, …)
Accretion rate ~ 0.01 Msun/s  Neutrino cooled accretion

18. Central engine – alternative model
Hyper-magnetized (>1015G) msec neutron star
(e.g., Usov 92, Thompson et al 04)
Powered by rotational energy
Activity time  spin down time

19. Internal-external fireball model
Compact
engine
Internal
Dissipation
External
Dissipation
Relativistic
wind
G > 100
g-rays
Afterglow cm cm cm
The afterglow – a blast wave in the ambient medium

20. Shocks are collisionless~G ~G G ~G ~G ~G ~G ~G
Collisionless
Shock
Unshocked
plasma
Shocked
plasma
Relativistic electrons + magnetic field = synchrotron radiation
Observations suggest that electrons accelerated at least to TeV
Collisionless shocks - natural particle accelerators:
Possible source of eV cosmic rays
Possible source of >1014 eV neutrinos

21. (Meszaros & Rees; Waxman; Katz & Piran; Sari, Piran & Narayan)
Afterglow theory
(Meszaros & Rees; Waxman; Katz & Piran; Sari, Piran & Narayan)
A self-similar relativistic blast-wave in perfect fluid (Blandford & McKee ‘76) .
GR-3/2
Shocked plasma
(downstream)
Pressure
R/G2
ambient medium
(upstream)
Radius
Typical electron energy and Magnetic field  G
The peak of the afterglow shifts to lower wavelengths with time

22. Viable progenitors of
long and short GRBs

23. Long GRBs are probably originate from gravitational collapse of massive stars
Observed signature - association with core-collapse supernova (Stanek et. al., 03; Hjorth et. al., 03)
The infalling gas feeds an accretion-disk on a newly born
black hole.

24. Some Viable progenitor of short GRBs and central engine systems
Slowly
accreting NS
NS-NS
NS-BH NS
Quark star
WD - WD
Merger
(AIC)
Phase
transition
AIC
Merger
Accretion disk +
Black whole
msec
magnetar
>1016 G
magnetar
Quark star

25. Merger of double neutron stars
Rosswog et. al.
Simulations: Rosswog et al; Ruffert, Janka et al; Lee et al ; Shibata et al; Freyer et al; Faber et al; …

26. Gravitational-wave detection
The Laser Interferometer Gravitational-
Wave Observatory (LIGO)
From Kip Thorne
Inspiraling NS binary detection ranges:
Initial LIGO (operational): 45 Million light years
Intermediate LIGO (2008-9): ~ 120 Million light years
Advanced LIGO (2013+ ): ~ Billion light years

27. If short GRBs are NS-NS mergers:
Short GRB rate  LIGO detection rate: detection is “guarantied” for advanced LIGO (Nakar et. al., 06)
Coincident EM+GW detection increases LIGO range by a factor of ~2 (Kochanek & Piran 93)
A valuable source of information on the binary evolution and short GRB physics
A strong cosmological probe – Within 1 yr next generation GW observatories may improve the constraints on the universe expansion rate (Hubble constant) to ~2% (Dalal et al., 06)

28. Summary GRBs are the most violent explosions known in the Universe
The bursts are produced by ultra-relativistic wind ejected during the death of a stellar size object/system and the probable birth of a stellar mass black hole
The prompt g-rays are produced by internal dissipation within the relativistic wind
The afterglow is produced by the external shock driven into the ambient medium by the relativistic wind
The collisionless shocks that produce the observed emission are considered as promising factories of high energy cosmic rays and neutrinos

29. Thanks!