Gamma-ray bursts

Discovered in 1968 by Vela spy satellitesOccur ~ 3 times a day at random positions in the sky

Variability on a less than 1 ms timescale – must be a very small object

Compton gamma-ray observatory discovered two puzzles:

• GRBs are distributed isotropically on the sky

• There is a deficiency of weak bursts – are we looking over the edge of their distribution?

Gamma-ray sky

GRB distribution

GRB duration: bimodal distribution

20% of GRBs last less than 2 s with a peak at 0.4 s80% of GRBs last more gthan 2 s with a peak at 40 s

Red: long-duration, bright burstsPurple: short-duration, dim bursts

Breakthrough: in 1997 when BeppoSAX satellite was able to detect the burst position at 1 arcmin resolution and coordinate with optical telescopes within 1 hour after the burst

An X-ray image of the gamma-ray burst GRB 970228, obtained by the team of Italian and Dutch scientists at 5:00 AM on Friday 28th February, 1997, using the BeppoSAX satellite.

Discovery of the optical and radio counterparts of GRBs

Spectral lines with redshift from 0.8 to over 6!

• GRBs are at the edge of the observable universe• They must be the most powerful explosions in the universe: ~ 1 solar mass is converted into gamma-rays in a few seconds!

Gamma-ray burst models

A hypernova model for long-duration bursts seems to be more or less successful.

A Wolf-Rayet star undergoing core collapse into a black hole

WR star: a very hot, massive, erratic star with sporadic outbursts

In the collapse, a blast wave propagates from the core outwards. When the star is rotating, the collapse is asymmetric. Hot gamma rays and ee+ plasma escape along the narrow cone parallel to the rotation axis. Erratic structure of the GRB is produced by shock waves propagating through the expelled material along the cone. We can see the burst only if the axis points at the Earth. A much slower expansion of the outer shell causes a usual supernova-type light curve and is usually invisible.

An afterglow at lower photon energies is produced when hot relativistic plasma starts coasting in the interstellar medium.

Theory #1: a peculiar supernova (hypernova)

Evidence?

2. A tentative link between GRB 980425 in 1998 and a supernova 1998bw

3. GRB 030329 on March 29, 2003, with an optical spectrum of an afterglow resembling a supernova

1. long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies.

Red giant: gore collapses and gets hotter, while the envelope expands and cools down

Fig. 10-18, p. 202

Known types of supernovae

Type II: hydrogen lines; collapse of a massive starType I: no hydrogen lines

Fig. 10-12, p. 197

Type Ia supernova

Hard to imagine a supernova without ejection of a star shell

Models for short-duration GRBs

Much shorter, dimmer, almost no afterglow, harder spectra

Mechanism is different from a collapsar/hypernova

Very few are associated with galactic hosts (??!)

Colliding neutron stars