The Afterglow and Ulirg Host Galaxy of the Dark Short Grb 120804a
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Author
Levan, A.
Margutti, R.
Laskar, T.
Fong, W.
Mangano, V.
Fox, D. B.
Tunnicliffe, R. L.
Chornock, R.
Tanvir, N. R.
Menten, K. M.
Hjorth, J.
Roth, K.
Dupuy, T. J.
Note: Order does not necessarily reflect citation order of authors.
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https://doi.org/10.1088/0004-637x/765/2/121Metadata
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Berger, E., B. A. Zauderer, A. Levan, R. Margutti, T. Laskar, W. Fong, V. Mangano, et al. 2013. The Afterglow and Ulirg Host Galaxy of the Dark Short Grb 120804a. The Astrophysical Journal 765, no. 2: 121. doi:10.1088/0004-637x/765/2/121.Abstract
We present the optical discovery and sub-arcsecond optical and X-ray localization of the afterglow of theshort 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
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