On similarity of binary black hole gravitational-wave skymaps: to observe or to wait?

TL;DR

This paper compares gravitational-wave skymaps for binary black holes from different algorithms, highlighting when to observe promptly or wait for refined localizations, with implications for electromagnetic follow-up strategies.

Contribution

It demonstrates that unmodeled and CBC-specific skymaps often differ and provides a method to decide optimal timing for electromagnetic observations.

Findings

CBC skymaps are generally more precise than unmodeled ones.

Differences between skymaps can inform observation timing decisions.

A method to determine whether to observe immediately or wait for refined skymaps.

Abstract

Localization estimates for GW150914, the first binary black hole detected by the LIGO instruments, were shared with partner facilities for electromagnetic follow-up. While the source was a compact binary coalescence (CBC), it was first identified by algorithms that search for unmodeled signals, which produced the skymaps that directed electromagnetic observations. Later on, CBC specific algorithms produced refined versions, which showed significant differences. In this paper we show that those differences were not accidental and that CBC and unmodeled skymaps for binary black holes will frequently be different; we thus provide a way to determine whether to observe electromagnetically as promptly as possible (following a gravitational-wave detection), or to wait until CBC skymaps become available, should they not be available in low latency. We also show that, unsurprisingly, CBC algorithms can yield much smaller searched areas.