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

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

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) - 0.01-1% Msunc2/s!

Why GRBs are interesting? The most violent explosions in the Universe: Luminosity of 108 Galaxies Energy released within ~ 10-100 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

? 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 1997 - BeppoSAX (Costa et. al., 97)

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

Main observations Prompt g-rays Erratic light curves variability time scale ~ 1-10ms Duration ~ 10ms - 1000 s Photon counts Time (s)

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

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 ~ 0.1- 6 (1-20 Gly) Host galaxies Optical X-ray Time Flux Soderberg et. al., 06 Fox et al ., 2005

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

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+)

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)

(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

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)

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

Central engine properties Isotropic equivalent g-ray luminosity: 1050-52erg/s Isotropic equivalent total emitted energy: Long GRBs: 1052-54erg Short GRBs: 1049-51erg 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

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

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

Internal-external fireball model Compact engine Internal Dissipation External Dissipation Relativistic wind G > 100 g-rays Afterglow 106-107 cm 1013-1016 cm 1016-1018 cm The afterglow – a blast wave in the ambient medium

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 1020 eV cosmic rays Possible source of >1014 eV neutrinos

(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

Viable progenitors of long and short GRBs

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.

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

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; …

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

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)

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

Thanks!