Progenitors of Long-Duration Gamma-ray Bursts

We review the current scenario of long-duration Gamma-ray burst (LGRB) progenitors, and in addition, present models of massive stars for a mass range of 10–150M⊙ with ΔM=10M⊙ and rotation rate v/vcrit=0 to 0.6 with a velocity resolution Δv/vcrit=0.1. We further discuss possible metallicity and rotation rate distribution from our models that might be preferable for the creation of successful LGRB candidates given the observed LGRB rates and their metallicity evolution. In the current understanding, LGRBs are associated with Type-Ic supernovae (SNe). To establish LGRB-SN correlation, we discuss three observational paths: (i) space-time coincidence, (ii) evidence from photometric light curves of LGRB afterglows and SN Type-Ic, (iii) spectroscopic study of both LGRB afterglow and SN. Superluminous SNe are also believed to have the same origin as LGRBs. Therefore, we discuss constraints on the progenitor parameters that can possibly dissociate these two events from a theoretical perspective. We further discuss the scenario of single star versus binary star as a more probable pathway to create LGRBs. Given the limited parameter space in the mass, mass ratio and separation between the two components in a binary, binary channel is less likely to create LGRBs to match the observed LGRB rate. Despite effectively-single massive stars are fewer in number compared to interacting binaries, their chemically homogeneous evolution (CHE) might be the major channel for LGRB production.

A very energetic supernova associated with the gamma-ray burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (>2 s) gamma-ray bursts (GRBs)-the most luminous of all astronomical explosions-signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the 'collapsar' model.

The Late Afterglow and Host Galaxy of GRB 990712

We present deep Hubble Space Telescope (HST) imaging, as well as ground-based imaging and spectroscopy, of the optical afterglow associated with the long-duration gamma-ray burst GRB 990712 and its host galaxy. The data were obtained 48-123 days after the burst occurred. The magnitudes of the host (R=21.9, V=22.5) and optical afterglow (R=25.4, V=25.8, 47.7 days after the burst) favor a scenario in which the optical light follows a pure power-law decay with an index of alpha approximately -1.0. We find no evidence for a contribution from a supernova like SN 1998bw. This suggests that either there are multiple classes of long-duration gamma-ray bursts or that the peak luminosity of the supernova was more than 1.5 mag fainter than SN 1998bw. The HST images and EFOSC2 spectra indicate that the gamma-ray burst was located in a bright, extended feature (possibly a star-forming region) 1.4 kpc from the nucleus of a 0.2L*B galaxy at z=0.434, possibly a Seyfert 2 galaxy. The late-time afterglow and host galaxy of GRB 990712 bear some resemblance to those of GRB 970508.

The Ejection of Relativistic Bullets from Supernovae and the Generation of Gamma-Ray Bursts

It is generally believed that cosmological gamma-ray bursts (GRBs) are produced by the deceleration of relativistic objects with Gamma greater, similar100. We study the possibility that some GRBs are produced along with relativistic matter ejected from supernovae. In this model, it is quite likely that the matter has to travel through the progenitor's thick envelope before generating GRBs. Under the assumption that the ejected matter is described as a single collective piece of matter, we obtain constraints on the matter having Gamma greater, similar100 at the breakout of the progenitor. One advantage of considering this type of model is that the expected GRB energy is sufficiently large, in contrast to the GRB-generation model of the shock breakout in the energetic supernova explosion. We find that, in general, the cross section of the matter has to be very small compared with the progenitor's radius, and thus the matter has to be bullet-like (or jetlike) rather than shell-like.