2.3 On the connection between GRBs and compact binary mergers

Here we wish to mention the possibility of observational manifestations of NS/BH binary mergers other than the violent GW emission. First of all, it is the connection of relativistic binary mergers to some subclasses of cosmic GRBs. That catastrophic events like the coalescence of binary NSs or BHs can be related to GRBs has been suggested for quite a long time. First ideas can be found in the papers of Sergei Blinnikov and his coauthors [40] and Bohdan Paczyński [303] with subsequent studies in [92304260190], etc.

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 [118Jump To The Next Citation Point4132Jump To The Next Citation Point110]. The short-hard subclass of GRBs includes up to 30% of all GRBs [300]. The most important recent discovery is that these GRBs occur both in late-type [67148] and early-type galaxies [3218], 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 [147389].

The principal observational facts about several well-localized short GRBs (see [30Jump To The Next Citation Point] for more discussion) are:

  1. They occur at cosmological redshifts from 0.160 to 1.8 and may constitute up to 20% of the local short GRB population detected by the BATSE3 experiment on board of the Compton Gamma-ray Observatory.
  2. The isotropic energy release is typically lower than in long GRBs (from ∼ 1048 erg to ∼ 4 × 1051 erg).
  3. The opening angle of the ejecta in these GRBs is on average larger than in long GRBs [51].
  4. Short GRBs are found both in elliptical and star forming galaxies. Statistical analysis suggests that the occurrence rate of short bursts is roughly equal in early-type and late-type galaxies [30].

The short GRB rate inferred from these observations [318Jump To The Next Citation Point], ℛsGRB ∼ (10– 30)370 Gpc −3 yr−1 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 [266Jump To The Next Citation Point]. 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 [31Jump To The Next Citation Point], 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 [266], 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 [31].

We also emphasize the agreement of the observational estimates with population synthesis calculations of binary mergers in galaxies of different types [14306224725]. The analysis of luminosity function and statistics of short GRBs from the BATSE catalog [318] implies a delay relative to the star formation history, which can favour double NS systems dynamically formed in stellar clusters as progenitors [152123]. Theoretical issues related to the generation of short hard GRBs from binary NS and NS–BH mergers are discussed in [214215296].

Now let us see what theory says about the formation, evolution, and detection rates of close compact binaries and their properties.

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