Now these ideas gained strong observational support from the accurate localization of short GRBs with hard spectrum by the Swift and HETE-II space missions [118, 41, 32, 110]. The short-hard subclass of GRBs includes up to 30% of all GRBs . The most important recent discovery is that these GRBs occur both in late-type [67, 148] and early-type galaxies [32, 18], suggesting old stellar population progenitors. This is in sharp contrast to long GRBs, some of which are definitely associated with peculiar type Ib/c supernovae produced by the core collapse of massive stars [147, 389].
The principal observational facts about several well-localized short GRBs (see  for more discussion) are:
The short GRB rate inferred from these observations , agrees with the double NS merger rate derived from binary pulsar statistics. Depending on the unknown beaming factor a possible upper limit of about 105 events yr–1 Gpc–3 was obtained in . That paper also extensively discusses the application of the rate of short GRBs to LIGO/VIRGO detections of double NS binary mergings if they are associated with short GRBs, and gives very good prospects for the Advanced LIGO sensitivity (up to hundreds detections per year). However, recent deep optical observations of several short GRBs provide evidence for their association with very faint galaxies , suggesting the intrinsic luminosity of a significant part of short GRBs to be much higher than 1048 – 1049 erg, as inferred from observations of close short GRBs by , close to that of classical long GRBs 1051 – 1052 erg. Taking this finding into account decreases the expected detection rate of NS mergers (if they are associated with short GRBs) down to several events per year by the Advanced LIGO detector .
We also emphasize the agreement of the observational estimates with population synthesis calculations of binary mergers in galaxies of different types [14, 306, 22, 47, 25]. The analysis of luminosity function and statistics of short GRBs from the BATSE catalog  implies a delay relative to the star formation history, which can favour double NS systems dynamically formed in stellar clusters as progenitors [152, 123]. Theoretical issues related to the generation of short hard GRBs from binary NS and NS–BH mergers are discussed in [214, 215, 296].
Now let us see what theory says about the formation, evolution, and detection rates of close compact binaries and their properties.
© Max Planck Society and the author(s)