iPTF14yb: The First Discovery of a GRB Afterglow Independent of a High-Energy Trigger

TL;DR

This paper reports the first discovery of a gamma-ray burst afterglow independent of high-energy triggers, using optical transient observations, and estimates the rate of such events across the sky.

Contribution

It presents the first unambiguous optical afterglow detection of a GRB without a prior high-energy trigger and estimates the occurrence rate of similar events.

Findings

First independent optical afterglow detection of a GRB

Estimated all-sky rate of relativistic explosions: 610 per year

Implications for GRB beaming and orphan afterglows

Abstract

We report here the discovery by the Intermediate Palomar Transient Factory (iPTF) of iPTF14yb, a luminous ($M_{r}\approx-27.8$ mag), cosmological (redshift 1.9733), rapidly fading optical transient. We demonstrate, based on probabilistic arguments and a comparison with the broader population, that iPTF14yb is the optical afterglow of the long-duration gamma-ray burst GRB 140226A. This marks the first unambiguous discovery of a GRB afterglow prior to (and thus entirely independent of) an associated high-energy trigger. We estimate the rate of iPTF14yb-like sources (i.e., cosmologically distant relativistic explosions) based on iPTF observations, inferring an all-sky value of $\Re_{\mathrm{rel}}=610$ yr$^{-1}$ (68% confidence interval of 110-2000 yr$^{-1}$). Our derived rate is consistent (within the large uncertainty) with the all-sky rate of on-axis GRBs derived by the Swift satellite. Finally, we briefly discuss the implications of the nondetection to date of bona fide "orphan" afterglows (i.e., those lacking detectable high-energy emission) on GRB beaming and the degree of baryon loading in these relativistic jets.