2012ApJ...756...63M

We present multi-wavelength observations of the afterglow of the short GRB 111117A, and follow-up observations of its host galaxy. From rapid optical and radio observations, we place limits of r ≳ 25.5 mag at δt ~ 0.55 days and F_ν(5.8 GHz) ≲ 18 µJy at δt ~ 0.50 days, respectively. However, using a Chandra observation at δt ~ 3.0 days we locate the absolute position of the X-ray afterglow to an accuracy of 0".22 (1σ), a factor of about six times better than the Swift/XRT position. This allows us to robustly identify the host galaxy and to locate the burst at a projected offset of 1".25±0".20 from the host centroid. Using optical and near-IR observations of the host galaxy we determine a photometric redshift of z = 1.3^+0.3_- 0.2_, one of the highest for any short gamma-ray burst (GRB), leading to a projected physical offset for the burst of 10.5±1.7 kpc, typical of previous short GRBs. At this redshift, the isotropic γ-ray energy is E_γ, iso_~ 3.0x10⁵¹ erg (rest-frame 23-2300 keV) with a peak energy of E_pk~ 850-2300 keV (rest-frame). In conjunction with the isotropic X-ray energy, GRB 111117A appears to follow our recently reported E_x, iso_–Eγ, iso_–Epk_ universal scaling. Using the X-ray data along with the optical and radio non-detections, we find that for a blastwave kinetic energy of E_K, iso_ ~ E_γ, iso_erg, the circumburst density is n₀~ 3x10^–4 - 1/cm3 (for a range of ε_B= 0.001-0.1). Similarly, from the non-detection of a break in the X-ray light curve at δt ≲ 3 days, we infer a minimum opening angle for the outflow of θ_j ≳ 3-10° (depending on the circumburst density). We conclude that Chandra observations of short GRBs are effective at determining precise positions and robust host galaxy associations in the absence of optical and radio detections.