THE OPTICAL AFTERGLOW ANDz= 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 × 10 10 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.