The Afterglow and Ulirg Host Galaxy of the Dark Short Grb 120804a

Abstract

We present the optical discovery and sub-arcsecond optical and X-ray localization of the afterglow of the short GRB 120804A, as well as optical, near-IR, and radio detections of its host galaxy. X-ray observations with Swift/XRT, Chandra, and XMM-Newton extending to δt ≈ 19 d reveal a single power law decline. The optical afterglow is faint and comparison to the X-ray flux indicates that GRB 120804A is “dark”, with a restframe extinction of A host V ≈ 2.5 mag (at z ≈ 1.3). The intrinsic neutral hydrogen column density inferred from the X-ray spectrum, NH,int(z = 1.3) ≈ 2×1022 cm−2, is commensurate with the large extinction. The host galaxy exhibits red optical/near-IR colors. Equally important, JVLA observations at ≈ 0.9−11 d reveal a constant flux density of Fν(5.8GHz) = 35 ± 4 µJy and an optically-thin spectrum, unprecedented for GRB afterglows, but suggestive instead of emission from the host galaxy. The optical/near-IR and radio fluxes are well fit with the scaled spectral energy distribution of the local ultra-luminous infrared galaxy (ULIRG) Arp 220 at z ≈ 1.3, with a resulting star formation rate of ≈ 300 M⊙ yr−1. The inferred extinction and small projected offset (2.2 ± 1.2 kpc) are also consistent with the ULIRG scenario, as is the presence of a companion galaxy at a separation of about 11 kpc. The limits on radio afterglow emission, in conjunction with the observed X-ray and optical emission, require a circumburst density of n ∼ 10−3 cm−3 an isotropic-equivalent energy scale of Eγ,iso ≈ EK,iso ≈ 7×1051 erg, and a jet opening angle of θj & 8 ◦. The expected fraction of luminous infrared galaxies in the short GRB host sample is ∼ 0.01 − 0.3 (for pure stellar mass and star formation weighting, respectively). Thus, the observed fraction of 2 events in about 25 hosts (GRBs 120804A and 100206A), provides additional support to our previous conclusion that short GRBs track both stellar mass and star formation activity