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

Abstract

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 rAB = 25.46 ± 0.20 mag. Follow-up optical and near-IR observations uncover a coincident compact red galaxy, identified as an early-type galaxy at a photometric redshift of z ≈ 0.6 − 0.9 (2σ) with a mass of ∼ 3 × 1010 M⊙, an age of ∼ 1 Gyr, and a luminosity of LB ≃ 0.5L∗. Spectroscopic observations of the host reveal a notable break corresponding to the Balmer/4000Å break at z ≈ 0.9, and stellar population spectral evolution template fits indicate z ≈ 0.915, which we adopt as the redshift of the host, with stellar population ages of ∼ 1−3 Gyr. 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 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 ǫ −3 e,−1 ǫ−1.75 B,−1 cm−3. The combination of an optically faint afterglow and host suggest that previous such events may have been missed, thereby potentially biasing the known short GRB host population against z & 1 early-type hosts.