Detection of the high z GRB 080913 and its implications on progenitors and energy extraction mechanisms

Aims. We present multiwavelength observations of one of the most distant gamma-ray bursts detected so far, GRB 080913. Based on these observations, we consider whether it could be classified as a short-duration GRB and discuss the implications for the progenitor nature and energy extraction mechanisms. Methods. Multiwavelength X-ray, near IR and millimetre observations were made between 20.7 h and ~16.8 days after the event. Results. Whereas a very faint afterglow was seen at the 3.5m CAHA telescope in the nIR, the X-ray afterglow was clearly detected in both Swift and XMM-Newton observations. An upper limit is reported in the mm range. We have modeled the data assuming a collimated ≳ 3° blast wave with an energy injection at days carrying erg or approximately 12 times the initial energy of the blast wave. We find that GRB 080913 shares many of the gamma-ray diagnostics with the more recent burst GRB 090423 for being classified as short had they ocurred at low redshift. If the progenitor were a compact binary merger, it is likely composed by a NS and BH. The Blandford-Znajek (BZ) mechanism is the preferred one to extract energy from the central, maximally-rotating BH. Both the magnetic field close to the event horizon (B) and the BH mass () are restricted within a relatively narrow range, such that . Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand

Variable Lyα sheds light on the environment surrounding GRB 090426

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionization state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z= 2.609, whose optical spectrum indicates an almost fully ionized medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Lyα absorption line. Photoionization modelling shows that we are probing material from the vicinity of the burst (∼80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies