The optical afterglow and z = 0.92 early-type host galaxy of the short GRB 100117A

We present the discovery of the optical afterglow and early-type host galaxy of the short-duration GRB 100117A. The faint afterglow is detected 8.3 hr after the burst with r AB = 25.46 ± 0.20 mag. Follow-up optical and near-infrared observations uncover a coincident compact red galaxy, identified as an early-type galaxy at a spectroscopic redshift of z 0.915 with a mass of ~3 × 1010 M ☉, an age of ~1 Gyr, and a luminosity of L B 0.5 L *. From a possible weak detection of [O II]λ3727 emission at z = 0.915 we infer an upper bound on the star formation rate of ~0.1 M ☉ yr–1, leading to a specific star formation rate of 0.004 Gyr–1. Thus, GRB 100117A is only the second short burst to date with a secure early-type host (the other being GRB 050724 at z = 0.257) and it has one of the highest short gamma-ray burst (GRB) redshifts. The offset between the host center and the burst position, 470 ± 310 pc, is the smallest to date. Combined with the old stellar population age, this indicates that the burst likely originated from a progenitor with no significant kick velocity. However, from the brightness of the optical afterglow we infer a relatively low density of n 3 × 10–4 –3 e,–1–1.75 B,–1 cm–3. The combination of an optically faint afterglow and host suggests that previous such events may have been missed, thereby potentially biasing the known short GRB host population against z 1 early-type hosts