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, 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.
