[Paper Review] Confusing head-on and precessing intermediate-mass binary black hole mergers

J. Calderón Bustillo, N. Sanchis-Gual+2 more|arXiv (Cornell University)|Sep 2, 2020

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper reveals a degeneracy between gravitational-wave signals from quasi-circular precessing intermediate-mass black hole binaries and those from head-on mergers, showing that head-on collisions with total masses between 130 and 310 solar masses are indistinguishable from precessing binaries in standard parameter estimation, leading to incorrect distance and mass inferences unless specialized waveform models and astrophysical priors are used.

ABSTRACT

We report a degeneracy between the gravitational-wave signals from quasi-circular precessing black-hole mergers and those from extremely eccentric mergers, namely head-on collisions. Performing model selection on numerically simulated signals of head-on collisions using models for quasi-circular binaries we find that, for signal-to-noise ratios consistent with Advanced LIGO observations, head-on mergers with total mass $M\in (130,310)M_\odot$ would be identified as a precessing quasi-circular intermediate-mass black hole binary, located at a much larger distance. Ruling out the head-on scenario would require to perform model selection using currently nonexistent waveform models for head-on collisions, together with the application of astrophysically motivated priors on the (rare) occurrence of those events. We show that in situations where standard parameter inference of compact binaries may report component masses inside (outside) the pair-instability supernova gap, the true object may be a head-on merger with masses outside (inside) this gap. We briefly discuss the potential implications of these findings for the recent gravitational-wave detection GW190521.

Motivation & Objective

• To investigate whether head-on binary black hole mergers can be mistaken for precessing quasi-circular binaries in gravitational-wave data analysis.
• To assess the implications of this degeneracy for parameter estimation, particularly in the context of the pair-instability supernova gap.
• To evaluate the necessity of specialized waveform models and astrophysical priors to rule out the head-on scenario in real detections.
• To examine the potential impact of this degeneracy on the interpretation of GW190521 and similar intermediate-mass black hole merger events.

Proposed method

• Simulating gravitational-wave signals from head-on collisions of black holes with total masses in the range 130–310 M☉.
• Performing model selection using standard quasi-circular precessing binary waveform models on these simulated signals.
• Applying standard parameter inference techniques to assess inferred masses and distances under typical Advanced LIGO signal-to-noise ratios.
• Evaluating the robustness of inference results when the true signal is a head-on merger versus a precessing binary.
• Assessing the need for future waveform models of head-on collisions and the role of astrophysical priors in distinguishing the two scenarios.

Experimental results

Research questions

• RQ1 Can head-on binary black hole mergers be statistically indistinguishable from precessing quasi-circular binaries in gravitational-wave data?
• RQ2 To what extent does this degeneracy affect the inferred component masses and luminosity distances in parameter estimation?
• RQ3 How does this degeneracy influence the interpretation of events near or across the pair-instability supernova mass gap?
• RQ4 What role do astrophysical priors and specialized waveform models play in resolving this ambiguity?
• RQ5 How might this degeneracy affect the interpretation of GW190521 and similar intermediate-mass black hole merger candidates?

Key findings

• Head-on mergers with total masses between 130 and 310 M☉ are statistically indistinguishable from precessing quasi-circular binaries in standard parameter estimation at Advanced LIGO sensitivity levels.
• These head-on mergers are consistently misclassified as precessing binaries at significantly larger luminosity distances than their true distance.
• When component masses fall inside the pair-instability supernova gap, the true system may actually be a head-on merger with masses outside the gap.
• Conversely, when masses are inferred to lie outside the gap, the true system could be a head-on merger with masses inside the gap.
• Ruling out the head-on scenario requires waveform models for head-on collisions and astrophysically motivated priors, which are currently unavailable in standard analysis pipelines.