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, 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. Methods. Multiwavelength X-ray, near IR and millimetre observations were made between 20.7 h and similar to 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 theta(0) greater than or similar to 3 degrees blast wave with an energy injection at similar to 0.5 days carrying 5 similar to 10(52) 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 x 10(16) G)(M-bh/7 M-circle dot) similar to 1. Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand.