7 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
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 burstAn 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 universeThey must be the most powerful explosions in theuniverse: ~ 1 solar mass is converted into gamma-raysin 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 holeWR star: a very hot, massive, erratic star with sporadic outburstsIn 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 1998bw3. 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 starType I: no hydrogen linesFigure 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 spectraVery few are associated with galactic hosts (??!)Mechanism is different from a collapsar/hypernova