Host Galaxies and Redshifts

As it is shown in fig. , all the bursts, whose redshift has been measured, range between $0.4$ and $4.5$, except for GRB980425 ($z \sim 0.0085$), that is likely to be connected with the peculiar SN 1998bw (see next paragraph).

Figure: Redshift Distribution (Nov 2001).

The presence of a host galaxy can be useful for two reasons: first, in most cases redshifts are measured for the galaxy, rather than the afterglow itself (in some cases both), and this is due to the fact that an effective absorption-line spectroscopy for an afterglow, especially at late times, is more difficult, and also because in some cases, despite of a non-detection of the optical afterglow, the host galaxy is identified thanks to a combination of X-ray and radio transients, when available. Second, the location of the Optical Transient (OT) within the host galaxy, in addition to properties of the galaxy itself, may provide important clues about the progenitor's nature.

As it is apparent from fig. , the set of GRB with known redshifts is poor yet; on this subject, there has been an interest in trying to find out a relationship between redshift and some properties of the gamma light curves of the prompt emission, like time variability ([Reichart et al., 2001], [Fenimore and Ramirez-Ruiz, 2000], [Schaefer et al., 2001a], [Schaefer, 2001b]).

Figure: Isotropic energy, fluence and host magnitude ($R$ and $V$ filters) as a function of redshift (Nov 2001; from [Ghisellini, 2001]).

In fig.  the energy released (as calculated in the isotropic case), the fluence and the magnitude of the host galaxy are expressed as a function of the redshift $z$ for the all the bursts with known redshift to date (Nov 2001).