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We report the discovery by the Palomar Transient Factory (PTF) of thetransient source PTF11agg, which is distinguished by three primarycharacteristics: (1) bright, rapidly fading optical transient emission; (2) afaint, blue quiescent optical counterpart; and (3) an associated year-long,scintillating radio transient. We argue that these observed properties areinconsistent with any known class of Galactic transients, and instead suggest acosmological origin. The detection of incoherent radio emission at suchdistances implies a large emitting region, from which we infer the presence ofrelativistic ejecta. The observed properties are all consistent with thepopulation of long-duration gamma-ray bursts (GRBs), marking the first timesuch an outburst has been discovered in the distant universe independent of ahigh-energy trigger. We searched for possible high-energy counterparts toPTF11agg, but found no evidence for associated prompt emission. We thereforeconsider three possible scenarios to account for a GRB-like afterglow without ahigh-energy counterpart: an "untriggered" GRB (lack of satellite coverage), an"orphan" afterglow (viewing-angle effects), and a "dirty fireball" (suppressedhigh-energy emission). The observed optical and radio light curves appearinconsistent with even the most basic predictions for off-axis afterglowmodels. 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 serendipitousdiscovery by PTF and find that it is quite small (~ 2.6%). While notdefinitive, we nonetheless speculate that PTF11agg may represent a new, morecommon (> 4 times the on-axis GRB rate at 90% confidence) class of relativisticoutbursts lacking associated high-energy emission.

Discovery of a Cosmological, Relativistic Outburst via its Rapidly Fading Optical Emission