A search for an optical counterpart to the gravitational wave event GW151226

TL;DR

This study conducted a wide-area optical search for electromagnetic counterparts to the GW151226 gravitational wave event using Pan-STARRS1 and ATLAS, finding no likely counterpart but demonstrating the survey's potential for future nearby neutron star mergers.

Contribution

The paper presents the first extensive optical follow-up search for GW151226, highlighting the survey's capabilities and limitations in detecting kilonovae at distances up to 100 Mpc.

Findings

No likely optical counterpart was identified for GW151226.

The survey was sensitive to kilonovae within 100 Mpc.

A peculiar transient, PS15dpn, was detected but unlikely related to GW151226.

Abstract

We present a search for an electromagnetic counterpart of the gravitational wave source GW151226. Using the Pan-STARRS1 telescope we mapped out 290 square degrees in the optical i_ps filter starting 11.5hr after the LIGO information release and lasting for a further 28 days. The first observations started 49.5hr after the time of the GW151226 detection. We typically reached sensitivity limits of i_ps = 20.3-20.8 and covered 26.5% of the LIGO probability skymap. We supplemented this with ATLAS survey data, reaching 31% of the probability region to shallower depths of m~19. We found 49 extragalactic transients (that are not obviously AGN), including a faint transient in a galaxy at 7Mpc (a luminous blue variable outburst) plus a rapidly decaying M-dwarf flare. Spectral classification of 20 other transient events showed them all to be supernovae. We found an unusual transient, PS15dpn, with an explosion date temporally coincident with GW151226 which evolved into a type Ibn supernova. The redshift of the transient is secure at z=0.1747 +/- 0.0001 and we find it unlikely to be linked, since the luminosity distance has a negligible probability of being consistent with that of GW151226. In the 290 square degrees surveyed we therefore do not find a likely counterpart. However we show that our survey strategy would be sensitive to NS-NS mergers producing kilonovae at D < 100 Mpc which is promising for future LIGO/Virgo searches.