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

Cenko, SB and Kulkarni, SR and Horesh, A and Corsi, A and Fox, DB and Carpenter, J and Frail, DA and Nugent, PE and Perley, DA and Gruber, D and Gal-Yam, A and Groot, PJ and Hallinan, G and Ofek, EO and Rau, A and MacLeod, CL and Miller, AA and Bloom, JS and Filippenko, AV and Kasliwal, MM and Law, NM and Morgan, AN and Polishook, D and Poznanski, D and Quimby, RM and Sesar, B and Shen, KJ and 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: 18 May 2017 08:31
DOI or Identification number: 10.1088/0004-637X/769/2/130
URI: http://researchonline.ljmu.ac.uk/id/eprint/6461

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