Limits on the Electro-Magnetic Counterpart of Binary Black Hole Coalescence at Visible Wavelengths

TL;DR

This study used the TAROT telescope network and machine learning algorithms to search for visible electromagnetic counterparts of three binary black hole mergers, setting strong limits on possible optical emissions.

Contribution

It developed an effective image analysis algorithm combining Gaia catalog matching and machine learning to identify unknown sources in large fields of view.

Findings

No confirmed electromagnetic counterparts were found.

The survey set a limiting magnitude of -23.8 for GW170814.

The results strongly constrain optical emissions associated with the black hole mergers.

Abstract

We used the TAROT network of telescopes to search for the electromagnetic counterparts of GW150914, GW170104 and GW170814, which were reported to originate from binary black hole merger events by the LIGO and Virgo collaborations. Our goal is to constrain the emission from a binary black hole coalescence at visible wavelengths. We developed a simple and effective algorithm to detect new sources by matching the image data with the Gaia catalog data release 1. Machine learning was used and an algorithm was designed to locate unknown sources in a large field of view image. The angular distance between objects in the image and in the catalog was used to find new sources; we then process the candidates to validate them as possible new unknown celestial objects. Though several possible candidates were detected in the three gravitational wave source error boxes studied, none of them were confirmed as a viable counterpart. The algorithm was effective for the identification of unknown candidates in a very large field and provided candidates for GW150914, GW170104 and GW170814. The entire 90\% GW170814 error box was surveyed extensively within 0.6 days after the GW emission resulting in an absolute limiting R magnitude of -23.8. This strong limit excludes to a great extent a possible emission of a gamma-ray burst with an optical counterpart associated with GW170814.