Detection of the high z GRB 080913 and its implications on progenitors and energy extraction mechanisms

TL;DR

This study presents multiwavelength observations of the distant GRB 080913, analyzes its properties, and discusses implications for its progenitor type and energy extraction mechanisms, suggesting it may be a short-duration GRB from a compact binary merger.

Contribution

It provides detailed multiwavelength data and modeling of GRB 080913, proposing its classification as a short GRB and constraining the properties of its central black hole and progenitor system.

Findings

GRB 080913's afterglow was detected in X-ray but faint in near IR.

Data modeling suggests a collimated blast wave with energy injection of about 10^52 erg.

Constraints on the black hole's magnetic field and mass support a compact binary merger progenitor.

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 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.