Experimentally, How Dark Are Black Hole Mergers?

TL;DR

This paper reviews optical followup efforts for black hole mergers detected by Advanced LIGO, analyzing the limits of optical emission detection and the probability of imaging these events, concluding current efforts are far from definitive.

Contribution

It systematically assesses the experimental upper limits on optical emission from black hole mergers and evaluates the probability of successful imaging during followup campaigns.

Findings

No optical emission detected from black hole mergers.

Current followup efforts are below the threshold for confident detection.

Quantitative assessment of imaging probabilities for each followup group.

Abstract

The first Advanced LIGO observing run detected two black hole merger events with confidence and likely a third. Many groups organized to followup the events in the optical even though the strong theoretical prior that no optical emission should be seen. We carry through the logic of this by asking about the experimental upper limits to the optical light from Advanced LIGO black hole mergere events. We inventory the published optical searches for transient events associated with the black hole mergers. We describe the factors that go into a formal limit on the visibility of an event (sky area coverage, the coverage factor of the camera, the fraction of sky not covered by intervening objects), and list what is known from the literature of the followup teams quantitative assessment of each factor. Where possible we calculate the total probability from each group that the source was imaged. The calculation of confidence level is reviewed for the case of no background. We find that an experimental 95% upper limit on the magnitude of a black hole requires the sum of the total probabilities over all events to be more than 3. In the first Advanced LIGO observing run we were far from reaching that threshold.