2011ApJ...730...26F

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 ∼3x10¹⁰ 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, leading to a specific star formation rate of ≲0.004/Gyr. 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 ~ 3x10^–4 ε^–3_e,–1ε^–1.75_B,–1/cm3. 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.