DISCOVERY OF A COSMOLOGICAL, RELATIVISTIC OUTBURST VIA ITS RAPIDLY FADING OPTICAL EMISSION.

We report the discovery by the Palomar Transient Factory (PTF) of the transient source PTF11agg, which is distinguished by three primary characteristics: (1) bright (Rpeak = 18.3mag), rapidly fading (ΔR = 4mag inΔt = 2 days) optical transient emission; (2) a faint (R = 26.2 ± 0.2mag), blue (g' − R = 0.17 ± 0.29 mag)quiescent optical counterpart; and (3) an associated year-long, scintillating radio transient. We argue that the seobserved 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 distance simplies a large emitting region, from which we infer the presence of relativistic ejecta. The observed properties areall 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 counter parts to PTF11agg, but found no evidence for associated prompt emission. We therefore consider three possible scenarios to account for a GRB-like after glow 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-axislong-duration GRB for which the associated high-energy emission was simplymissed. 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. [ABSTRACT FROM AUTHOR]