Context :

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 \sim 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 { \it Swift } and { \it 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 \sim 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 } 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 .

Conclusions :