The massive host of the extreme long gamma-ray burst 110918A

Author: Jonathan Elliott

Abstract: Long gamma-ray bursts (GRBs) are the most powerful explosions in the Universe. Studying the most energetic and their underlying host galaxies may yield crucial results on the physics, progenitors and environments of these explosions.

The afterglow and host environment of the bright IPN long GRB 110918A, located at a redshift of z=0.984, is investigated to shed light on the population of long GRBs and their hosts.

Utilising simultaneous optical/near infra-red imaging of the GRB afterglow obtained with the Gamma-Ray Burst Optical Near-infrared Detector (GROND), temporal decay slopes, spectral energy distributions (SED), dust extinction and the jet opening angle are determined. Late deep follow-up detects the galaxy and allows its mass to be constrained from its photometric SED. The gas content of the host and GRB environments are determined from optical spectroscopy obtained with the OSIRIS and GMOS spectrographs.

The afterglow is seen to decay with a temporal slope of α =1.2, breaking at tbreak=252 ks to a steeper decay of α =2.0, yet remaining achromatic with an intrinsic spectral slope of β=0.7, requiring little extinction of AV=0.2 mag. Absorption lines of neutral gas, primarily MgII and MgI, with widths of up to EWrest ∼ 6 Åshow a large reservoir of neutral gas for star formation and hint at periods of starbursting or galactic winds. The host exhibits extended [OII] emission corresponding to an extinction corrected star formation rate of, SFR=50 MSun yr-1 for an AV=1.1 mag. The underlying host is red, r'-K∼ 2.5 magAB, with a large extent, ∼50 kpc, and a stellar mass of log(M /MSun )=10.68 ± 0.16.

The afterglow of GRB 110918A is well described by the classical GRB fireball model during its coverage, and is similar to that of the rest of the population. However, the host galaxy and local environment properties are seen to be more unique. The host galaxy has the largest mass in the redshift range of z.