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

Journal Article · Mon Jul 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA78F0· OSTI ID:22876033

Lü, HouJun; Wang, XiangGao; Lu, RuiJing; Lan, Lin; Liang, EnWei ^[1]; Gao, He ^[2]; Graham, Melissa L.; Zheng, WeiKang; Filippenko, Alexei V. ^[3]; Zhang, Bing ^[4]

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1. Guangxi Key Laboratory for Relativistic Astrophysics, Department of Physics, Guangxi University, Nanning 530004 (China)
2. Department of Astronomy, Beijing Normal University, Beijing 100875 (China)
3. Department of Astronomy, University of California, Berkeley, CA 94720-3411 (United States)
4. Department of Astronomy, School of Physics, Peking University, Beijing 100871 (China)

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 Γ{sub 0} > 190 during the prompt emission phase. Alternatively, we derive Γ{sub 0} ≈ 580 and Γ{sub 0} ≈ 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 θ {sub j} > 5.°5. Within a millisecond magnetar central engine scenario, we derive the period P {sub 0} and polar magnetic field strength B {sub p}, 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 f{sub 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.

• OSTI ID: 22876033
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 843; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
AFTERGLOW
COSMIC GAMMA BURSTS
EMISSION
GALAXIES
MAGNETIC FIELDS
NEUTRON STARS
OPACITY
RED SHIFT
SPECTRA
STELLAR WINDS
SYNCHROTRON RADIATION
X RADIATION