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Discovery of a cosmological, relativistic outburst via its rapidly fading optical emission

Cenko, SB, Kulkarni, SR, Horesh, A, Corsi, A, Fox, DB, Carpenter, J, Frail, DA, Nugent, PE, Perley, DA, Gruber, D, Gal-Yam, A, Groot, PJ, Hallinan, G, Ofek, EO, Rau, A, MacLeod, CL, Miller, AA, Bloom, JS, Filippenko, AV, Kasliwal, MM , Law, NM, Morgan, AN, Polishook, D, Poznanski, D, Quimby, RM, Sesar, B, Shen, KJ, Silverman, JM and Sternberg, A (2013) Discovery of a cosmological, relativistic outburst via its rapidly fading optical emission. The Astrophysical Journal, 769 (2). ISSN 1538-4357

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We report the discovery by the Palomar Transient Factory (PTF) of the transient source PTF11agg, which is distinguished by three primary characteristics: (1) bright (R peak = 18.3 mag), rapidly fading (ΔR = 4 mag in Δt = 2 days) optical transient emission; (2) a faint (R = 26.2 ± 0.2 mag), blue (g′-R = 0.17 ± 0.29 mag) quiescent optical counterpart; and (3) an associated year-long, scintillating radio transient. We argue that these observed properties are inconsistent with any known class of Galactic transients (flare stars, X-ray binaries, dwarf novae), and instead suggest a cosmological origin. The detection of incoherent radio emission at such distances implies a large emitting region, from which we infer the presence of relativistic ejecta. The observed properties are all consistent with the population of long-duration gamma-ray bursts (GRBs), marking the first time such an outburst has been discovered in the distant universe independent of a high-energy trigger. We searched for possible high-energy counterparts to PTF11agg, but found no evidence for associated prompt emission. We therefore consider three possible scenarios to account for a GRB-like afterglow without a high-energy counterpart: an "untriggered" GRB (lack of satellite coverage), an "orphan" afterglow (viewing-angle effects), and a "dirty fireball" (suppressed high-energy emission). The observed optical and radio light curves appear inconsistent with even the most basic predictions for off-axis afterglow models. The simplest explanation, then, is that PTF11agg is a normal, on-axis long-duration GRB for which the associated high-energy emission was simply missed. However, we have calculated the likelihood of such a serendipitous discovery by PTF and find that it is quite small (≈2.6%). While not definitive, we nonetheless speculate that PTF11agg may represent a new, more common (>4 times the on-axis GRB rate at 90% confidence) class of relativistic outbursts lacking associated high-energy emission. If so, such sources will be uncovered in large numbers by future wide-field optical and radio transient surveys. © 2013. The American Astronomical Society. All rights reserved.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society
Date Deposited: 18 May 2017 08:31
Last Modified: 26 Apr 2022 14:15
DOI or ID number: 10.1088/0004-637X/769/2/130
URI: https://researchonline.ljmu.ac.uk/id/eprint/6461
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