The peculiar GRB 110731A: Lorentz factor, jet composition, central engine, and progenitor

The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from {\em Fermi} and {\em Swift} data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive $\Gamma_{0}>190$ during the prompt emission phase. Alternatively, we derive $\Gamma_{0} \approx 580$ and $\Gamma_{0} \approx 154$ by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle $\theta_{\rm j}>5.5^{\circ}$. Within a millisecond magnetar central engine scenario, we derive the period $P_{0}$ and polar magnetic field strength $B_{\rm p}$, which have extreme (but still allowed) values. The moderately short observed duration (7.3\,s) and relatively large redshift ($z=2.83$) places the burst as a "rest-frame short" GRB, so the progenitor of the burst is subject to debate. Its relatively large $f_{{\rm eff}, z}$ parameter (ratio of the 1\,s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger.