  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 hours 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 $\theta_0$ $\gtrsim$ 3$^\circ$ blast
wave with an energy injection at 0.5 days carrying $5\sim 10^{52}$ 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 $(B / 3\times 10^{16} \rm{G}) (M_{bh} / 7 M_\odot) \sim
1$. Similar constraints on the central BH hold for collapsar-like progenitor
systems if the BZ-mechanism works for the system at hand.
