Spectroscopy of candidate electromagnetic counterparts to gravitational wave sources

TL;DR

This paper discusses a coordinated multi-wavelength follow-up program for gravitational wave events, demonstrating the ability to eliminate most false optical counterparts, though some candidates remain unclassified.

Contribution

It presents a systematic approach to electromagnetic follow-up of GW sources and highlights the challenges in classifying all candidates.

Findings

Most candidates were supernovae at similar redshifts

No electromagnetic counterparts were confirmed for black hole mergers

A significant fraction of candidates remain unclassified

Abstract

A programme of worldwide, multi-wavelength electromagnetic follow-up of sources detected by gravitational wave detectors is in place. Following the discovery of GW150914 and GW151226, wide field imaging of their sky localisations identified a number of candidate optical counterparts which were then spectrally classified. The majority of candidates were found to be supernovae at redshift ranges similar to the GW events and were thereby ruled out as a genuine counterpart. Other candidates ruled out include AGN and solar system objects. Given the GW sources were black hole binary mergers, the lack of an identified electromagnetic counterpart is not surprising. However the observations show that is it is possible to organise and execute a campaign that can eliminate the majority of potential counterparts. Finally we note the existence of a "classification gap" with a significant fraction of candidates going unclassified.