A Peculiar GRB 110731A: Lorentz Factor, Jet Composition, Central Engine, and Progenitor

• Published in: The Astrophysical journal Vol. 843; no. 2; pp. 114 - 124
• Main Authors: Lü, HouJun, Wang, XiangGao, Lu, RuiJing, Lan, Lin, Gao, He, Liang, EnWei, Graham, Melissa L., Zheng, WeiKang, Filippenko, Alexei V., Zhang, Bing
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10.07.2017; IOP Publishing
• Subjects: AFTERGLOW; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Broadband; Composition; COSMIC GAMMA BURSTS; EMISSION; Emission spectra; Field strength; Fluctuations; Flux; GALAXIES; Gamma ray bursts; Gamma rays; gamma-ray burst: general; Lorentz factor; Magnetars; MAGNETIC FIELDS; NEUTRON STARS; OPACITY; Questions; Radiation; radiation mechanisms: non-thermal; RED SHIFT; SPECTRA; STELLAR WINDS; SYNCHROTRON RADIATION; X RADIATION
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa78f0

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 Fermi and 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 Γ0 > 190 during the prompt emission phase. Alternatively, we derive Γ0 580 and Γ0 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 θj > 5 5. Within a millisecond magnetar central engine scenario, we derive the period P0 and polar magnetic field strength Bp, which have extreme (but still allowed) values. The moderately short observed duration (7.3 s) and relatively large redshift (z = 2.83) place the burst as a "rest-frame short" GRB, so the progenitor of the burst is subject to debate. Its relatively large 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.