Detection of the high z GRB 080913 and its implications on progenitors and energy extraction mechanisms
Bremer, M.; Jóhannesson, G.; Pérez-Ramírez, D.; Riva, A.; Warwick, R. S.; Guerrero, M. A.; Gorosabel, J.; Castro-Tirado, A. J.; Jelínek, M.; Osborne, J. P.; Page, K. L.; de Ugarte Postigo, A.; Veres, P.; Winters, J. M.; Guziy, S.; Aloy, M. A.; Horváth, I.; Kubánek, P.
Spain, United Kingdom, Chile, United States, Hungary, Ukraine, France, Italy
Abstract
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 θ_0≳ 3° blast wave with an energy injection at ∼0.5 days carrying 5∼ 1052 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 (M_bh) are restricted within a relatively narrow range, such that (B / 3× 1016 G) (M_bh / 7 M_⊙ ) ∼ 1. Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand.