The Afterglow and Environment of the Short Grb 111117a

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 d and Fν(5.8GHz) . 18 µJy at δt ≈ 0.50 d, respectively. However, using a Chandra observation at δt ≈ 3.0 d we locate the absolute position of the X-ray afterglow to an accuracy of 0.22′′ (1σ), a factor of about 6 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 GRB, and 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 Eγ,iso ≈ 3.0×1051 erg (rest-frame 23 − 2300 keV) with a peak energy of Epk ≈ 850 − 2300 keV (rest-frame). In conjunction with the isotropic X-ray energy, GRB 111117A appears to follow our recently-reported Ex,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 EK,iso ≈ Eγ,iso erg, the circumburst density is n0 ≈ 3×10−4 − 1 cm−3 (for a range of ǫB = 0.001 − 0.1). Similarly, from the non-detection of a break in the X-ray light curve at δt . 3 d, we infer a minimum opening angle for the outflow of θj & 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.
Astronomy