1. Gamma-ray bursts Discovered in 1968 by Vela spy satellites
Occur ~ 3 times a day at random positions in the sky

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

4. 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?

5. GRB distribution
Gamma-ray sky

7. GRB duration: bimodal distribution
20% of GRBs last less than 2 s with a peak at 0.4 s
80% of GRBs last more gthan 2 s with a peak at 40 s

8. Red: long-duration, bright bursts
Purple: short-duration, dim bursts

9. 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 , obtained by the team of Italian and Dutch scientists at 5:00 AM on Friday 28th February, 1997, using the BeppoSAX satellite.

10. 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!

11. Gamma-ray burst models
Theory #1: a peculiar supernova (hypernova)
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.

12. Evidence?
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.
2. A tentative link between GRB in 1998 and a supernova 1998bw
3. GRB on March 29, 2003, with an optical spectrum of an afterglow resembling a supernova

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

15. Known types of supernovae
Type II: hydrogen lines; collapse of a massive star
Type I: no hydrogen lines
Figure 10.18: Type I supernovae decline rapidly at first and then more slowly, but type II supernovae pause for about 100 days before beginning a steep decline. Supernova 1987A was odd in that it did not rise directly to maximum brightness. These light curves have been adjusted to the same maximum brightness. Generally, type II supernovae are about 2 magnitudes fainter than type I.
Fig , p. 202

16. Type Ia supernova Fig. 10-12, p. 197
Figure 10.12: Matter falling into a compact object forms a whirling accretion disk. Friction and tidal forces can make the disk very hot.
Fig , p. 197

17. Hard to imagine a supernova without ejection of a star shell

18. Models for short-duration GRBs
Much shorter, dimmer, almost no afterglow, harder spectra
Very few are associated with galactic hosts (??!)
Mechanism is different from a collapsar/hypernova

19. Colliding neutron stars