What is a GRB ?

Burst Optical Observer and Transient Exploring System    

( A world wide Network of Robotic Telescopes )

Theorical models

The most popular models fall into two broad cathegories: the explosion of a massive star and the coalescence of a compact binary system.

The "collapsar'' (or hypernova) model deals with a rotating massive star with a Fe core that collapses forming a rotaing  black hole (Kerr BH) and a 0.1-1 solar mass torus.

The matter is accreted at a very high rate and the energy is released amounting up 10^{54} erg. A ``dirty fireball'', is produced reaching a luminosity  300 times larger that than of a normal SN.

This would happen every  10$^6 yr on average. In this scenario, GRBs would be produced in dense enviroments near star forming regions and GRBs might be used for deriving the star-forming rate in the Universe.

The merging of a neutron star binary sistem giving rise to a GRB

The coalescence of neutron stars in a binary system has been also proposed (Narayan et al. 1992): lifetimes of such systems are of the order of  10^9 years, and large escape velocities are usual, putting them far away from the regions where their progenitors were born. The likely result is a Kerr BH, and the energy released energy during the merger process is 10^54 erg. It is also possible that a 0.1 solar mass accretion disk forms around the black hole and is accreted within a few dozen seconds, then producing internal shocks leading to the GRB.

In this scenario, GRBs would be produced far away from their host galaxies, and this could account for the 40 % of bursts not located in the optical window.

There are variations of this latter model where one or two components are substituted for black holes, white dwarfs or He stars.

A statitiscal study of the offsets of 20 long-duration GRBs from their apparent host galaxies centers favours the explosion of a massive star rather than the binary merger model.

It has been suggested that the short duration (< 1 s) bursts could be due to compact star mergers, whereas the longer ones are caused by the collapse of massive stars.