Gamma-Ray Bursts: I. Observations and Overview* Brian Metzger - PowerPoint PPT Presentation

Gamma-Ray Bursts: I. Observations and Overview* Brian Metzger Columbia University *select slides borrowed from Chuck Dermer, Jim Lattimer, Stan Woosley

Blazar Pulsar AGN Globular Blazar Cluster

Gamma-Ray Bursts (GRBs)  Variable `bursts’ of gamma-rays lasting milliseconds to minutes.  Discovered by the VELA satellites in 1967 when monitoring nuclear test ban treaty (declassified 1972)  GRBs occur about once per day across the whole sky. counts per second “When you’ve seen one GRB…. you’ve see one GRB”

Barraud et al. 2002 (keV)

Gamma-Ray Burst Durations long short Duration BATSE Bursts (from Nakar 2007)

High E peak ⇒

The Dark Ages (1972-1991)  Gamma-rays are difficult to focus. The precise location of a GRB on the sky is difficult to pin down accurately.  ‘Consensus’ opinion in the 1970s & 80s: GRBs come from within our Galaxy.

The Dark Ages (1972-1991)  Gamma-rays are difficult to focus. The precise location of a GRB on the sky is difficult to pin down accurately.  ‘Consensus’ opinion in the 1970s & 80s: Compton Gamma-Ray GRBs come from within our Galaxy. Observatory (1991-2000) The Milky Way “The Great Debate”

GRBs as Ultra-Relativistic Explosions Large (cosmological) distances ⇒ huge energies E γ ~ 10 51 -10 54 ergs ~ 10 -3 - 1M  c 2 fast variability δ t var ~ 10 ms ⇒ compact source R max ~ c/ δ t var ~10 7 cm Γ ~ 100-1000 GRB Spectrum v ~ 0.99999 c E peak γ " + " # e $ + e +

Why GRBs must originate from relativistic outflows Zhang & Meszaros (2004) radius of 1st photon : R 1 = ct 1st photon leaves at t=0 radius of 2nd photon : R 2 = c ( t " # t ) + $ c # t …over which time the gamma-emitting 2nd photon leaves Δ t later.. shell has moved this distance $ ~1 2 # t / % 2 # t obs ~ ( R 1 " R 2 )/ c ~ (1 " $ ) # t ~

The Afterglow Revolution (~1997) • If GRBs originate from far away, they must be very energetic explosions. • Space is filled with tenuous gas. When the explosion runs into the gas, the resulting shock collisionless shock - A. Spitkovsky will accelerate electrons. This powers synchrotron emission from radio to X-rays.

The Afterglow Revolution (~1997) • If GRBs originate from far away, they must be very energetic explosions. • Space is filled with tenuous gas. When the explosion runs into the gas, the resulting shock collisionless shock - A. Spitkovsky will accelerate electrons. This powers synchrotron emission from radio to X-rays. Beppo Sax z > 0.835!

Deceleration of a Relativistic Jet Zhang & MacFadyen 2009 t 1 t 2 t 3 2 " syn # B $ e E = " # 2 r 3 = const t ~ r/2c # 2 $

Resolving the Radio Afterglow => Relativistic Motion Gehrels, Ramirez-Ruiz, & Fox 2009; data from Pihlstrom et al. 2008 z = 0.168; D A ~ 600 Mpc " = # ct ( ) mas D A ~ 0.2 # t 100 d $ # ~ 1 (days)

GRBs as Jetted Relativistic Explosions Once corrected for beaming, the true energies of GRBs are less extreme Jet Break jet opening angle Frail+01; Bloom+03 Θ jet ~ 5-10 o (ergs) (ergs)

Implications for the “Central Engine” • Rapid variability dt ~ 10 ms ⇒ R < c dt ~ 10 3 km • Relativistic velocities v ~ c • Huge energy release ~10 50 -10 52 ergs ~ 10 -4 -10 -2 M  c 2 ⇒ Catastrophic birth or destruction of stellar mass compact objects (neutron stars or black holes) NS Circinus X-1 BH ~day 1 ~day 3 NS ~day 5 Fender et al. 2004

GRB Em Emission: S Still El Elusive! Relativistic Outflow ( Γ >> 1) ~ 10 7 cm Central Engine GRB / Flaring Afterglow 1. What is jet’s composition? (kinetic or magnetic?) 2. Where is dissipation occurring? (photosphere? deceleration radius?) 3. How is radiation generated? (synchrotron, inverse-compton, hadronic?)

Prompt Emission Models Internal Shocks (Synchrotron) • jet variability ⇒ internal collisions ⇒ shocks ⇒ particle acceleration + B field Piran 2004 amplification ⇒ synchrotron pros: shocks inevitable in variable flows cons: low radiative efficiency, requires fine tuning of shock parameters “Photospheric” Dissipation (IC Scattering) • GRB emission = thermal spectrum Comptonized (producing high energy power-law tail) by hot electrons near Giannios 2011 photosphere. pros: ~MeV spectral peak set by photosphere temperature (robust) cons: source of electron heating uncertain (shocks, collisions, reconnection)

~50 MeV - 100 GeV GLAST = Fermi

GBM (8 keV - 40 MeV) LAT (20MeV - 300 GeV) • Extreme Bursts (e.g. 080916C: E iso = 8.8 x 10 54 ergs ) • Distinct GeV Component – Delayed wrt MeV photons – Slow Decay ( ~ t -1.5 ) - Origin: Prompt? Afterglow? (e.g. Kumar & Barniol Duran 09; Ghirlanda+09) 090510 090902B (Fermi Collaboration 09) Ghirlanda et al. 2009

Beloborodov et al. 2011

X-Ray Afterglows in the Swift Era GRB Here Steep Decay Phase Gehrels, Ramirez-Ruiz & Fox 2009 Plateau Phase Late Flares ‘Canonical’ X-Ray Light Curve

Gamma-Ray Burst Durations long short Duration BATSE Bursts (from Nakar 2007)

GRB 030329 and the Supernova Connection Exploding “Wolf-Rayet” Star radius R~10 11 cm (3 light-seconds).

GRB 030329 and the Supernova Connection ⇒ Long GRBs come from the deaths of massive Stars Exploding “Wolf-Rayet” Star radius R~10 11 cm Gamma-Ray Burst Galaxies (3 light-seconds). (courtesy A. Fruchter)