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 GRB100117A. 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 [OII]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, GRB100117A is only the second short burst to date with a secure early-type host (the other being GRB050724 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 ε^-3 _e,-1ε^-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.