2010A&A...510A.105P

We present multiwavelength observations of one of the most distant gamma-ray bursts detected so far, GRB080913. 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. Multiwavelength X-ray, near IR and millimetre observations were made between 20.7h and ∼16.8-days after the event. 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 θ₀>3° blast wave with an energy injection at ∼0.5 days carrying 5∼10⁵²erg or approximately 12 times the initial energy of the blast wave. We find that GRB080913 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/3x10¹⁶G) (M_bh/7M_☉)∼1. Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand.