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

TL;DR

This paper reports the discovery of a distant, relativistic optical and radio transient, PTF11agg, likely a new class of gamma-ray burst-like outbursts without high-energy signals, expanding our understanding of cosmic relativistic explosions.

Contribution

It presents the first detection of a long-duration GRB-like event in the universe without associated high-energy emission, suggesting a new class of relativistic outbursts.

Findings

PTF11agg exhibits rapid optical fading and long-lasting radio emission.

The event is consistent with a long-duration GRB viewed on-axis, missed by high-energy detectors.

Estimated occurrence rate of such events is over four times that of typical GRBs.

Abstract

We report the discovery by the Palomar Transient Factory (PTF) of the transient source PTF11agg, which is distinguished by three primary characteristics: (1) bright, rapidly fading optical transient emission; (2) a faint, blue 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, 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.