A Case Study of On-the-Fly Wide-Field Radio Imaging Applied to the Gravitational-wave Event GW 151226

TL;DR

This study demonstrates a real-time wide-field radio imaging technique with the VLA to search for electromagnetic counterparts to gravitational wave events, achieving sensitive, rapid, and efficient observations without detecting any afterglows from GW 151226.

Contribution

We developed and applied an On-the-Fly mosaicing method for real-time wide-field radio imaging, enabling efficient searches for gravitational wave afterglows.

Findings

No transients detected in the observed region.

The survey achieved RMS sensitivities better than 150 μJy/beam.

Low false positive rate suggests effectiveness of the method.

Abstract

We apply a newly-developed On-the-Fly mosaicing technique on the NSF's Karl G. Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive search for an afterglow from the Advanced LIGO binary black hole merger event GW 151226. In three epochs between 1.5 and 6 months post-merger we observed a 100 sq. deg region, with more than 80% of the survey region having a RMS sensitivity of better than 150 uJy/beam, in the northern hemisphere having a merger containment probability of 10%. The data were processed in near-real-time, and analyzed to search for transients and variables. No transients were found but we have demonstrated the ability to conduct blind searches in a time-frequency phase space where the predicted afterglow signals are strongest. If the gravitational wave event is contained within our survey region, the upper limit on any late-time radio afterglow from the merger event at an assumed mean distance of 440 Mpc is about 1e29 erg/s/Hz. Approximately 1.5% of the radio sources in the field showed variability at a level of 30%, and can be attributed to normal activity from active galactic nuclei. The low rate of false positives in the radio sky suggests that wide-field imaging searches at a few Gigahertz can be an efficient and competitive search strategy. We discuss our search method in the context of the recent afterglow detection from GW 170817 and radio follow-up in future gravitational wave observing runs.