You Can't Always Get What You Want: The Impact of Prior Assumptions on Interpreting GW190412

TL;DR

This paper examines how prior assumptions influence the interpretation of GW190412, a gravitational wave event with unequal black hole masses, highlighting the importance of prior choices in parameter estimation.

Contribution

It systematically analyzes the impact of different prior assumptions on the inferred parameters and Bayes factors for GW190412, emphasizing the robustness of key features.

Findings

Mass asymmetry and positive effective spin are qualitatively robust.

Prior choices significantly affect the Bayes factors for black hole spin.

Results underscore the importance of astrophysically motivated priors.

Abstract

GW190412 is the first observation of a black hole binary with definitively unequal masses. GW190412's mass asymmetry, along with the measured positive effective inspiral spin, allowed for inference of a component black hole spin: the primary black hole in the system was found to have a dimensionless spin magnitude between 0.17 and 0.59 (90% credible range). We investigate how the choice of priors for the spin magnitudes and tilts of the component black holes affect the robustness of parameter estimates for GW190412, and report Bayes factors across a suite of prior assumptions. Depending on the waveform family used to describe the signal, we find either marginal to moderate (2:1-6:1) or strong ($\gtrsim$ 20:1) support for the primary black hole being spinning compared to cases where only the secondary is allowed to have spin. We show how these choices influence parameter estimates, and find the asymmetric masses and positive effective inspiral spin of GW190412 to be qualitatively, but not quantitatively, robust to prior assumptions. Our results highlight the importance of both considering astrophysically motivated or population-based priors in interpreting observations and considering their relative support from the data.