Inferring spin tilts of binary black holes at formation with plus-era gravitational wave detectors

TL;DR

This paper investigates how the spin tilts of binary black holes inferred from gravitational wave data differ between the time near merger and at formation, especially with future advanced detectors, to better understand their origins.

Contribution

It introduces a method to infer the initial spin tilts of binary black holes by evolving observed tilts back to infinite separation, aiding in understanding their formation scenarios.

Findings

Future detectors can distinguish tilt differences at formation and merger.

The method identifies up-down instability cases from tilts at infinity.

Simulated data shows significant tilt evolution effects for certain configurations.

Abstract

The spin orientations of spinning binary black hole (BBH) mergers detected by ground-based gravitational wave detectors such as LIGO and Virgo can provide important clues about the formation of such binaries. However, these spin tilts, i.e., the angles between the spin vector of each black hole and the binary's orbital angular momentum vector, can change due to precessional effects as the black holes evolve from a large separation to their merger. The tilts inferred at a frequency in the sensitive band of the detectors by comparing the signal with theoretical waveforms can thus be significantly different from the tilts when the binary originally formed. These tilts at the binary's formation are well approximated in many scenarios by evolving the BBH backwards in time to a formally infinite separation. Using the tilts at infinite separation also places all binaries on an equal footing in analyzing their population properties. In this paper, we perform parameter estimation for simulated BBHs and investigate the differences between the tilts one infers directly close to merger and those obtained by evolving back to infinite separation. We select simulated observations such that their configurations show particularly large differences in their orientations close to merger and at infinity. While these differences may be buried in the statistical noise for current detections, we show that in future plus-era (A$+$ and Virgo$+$) detectors, they can be easily distinguished in some cases. We also consider the tilts at infinity for BBHs in various spin morphologies and at the endpoint of the up-down instability. In particular, we find that we are able to easily identify the up-down instability cases as such from the tilts at infinity.