Constraining the parameters of GW150914 and GW170104 with numerical relativity surrogates

Abstract

Gravitational-wave (GW) detectors have begun to observe coalescences of heavy black hole binaries (M ≳ 50  M_⊙) at a consistent pace for the past few years. Accurate models of gravitational waveforms are essential for unbiased and precise estimation of source parameters, such as masses and spins of component black holes. Recently developed surrogate models based on high-accuracy numerical relativity (NR) simulations provide ideal models for constraining physical parameters describing these heavy black hole merger events. In this paper, we first demonstrate the viability of these multi-modal surrogate models as reliable parameter estimation tools. We show that within a fully Bayesian framework, NR surrogates can help extract additional information from GW observations that is inaccessible to traditional models. We demonstrate this by analyzing a set of synthetic signals with NR surrogate templates and comparing against contemporary approximate models. We then consider the case of two of the earliest binary black holes detected by the LIGO observatories, GW150914 and GW170104. We reanalyze their data with the generically precessing NR-based surrogate model and freely provide the resulting posterior samples as supplemental material. We find that our refined analysis is able to extract information from sub-dominant GW harmonics in data, and therefore better resolve the degeneracy in measuring source luminosity distance and orbital inclination for both events. Our analysis estimates the sources of both events to be 20%–25% further away than was previously estimated. Our analysis also constrains their orbital orientations more tightly around face-on or face-off configurations than before. Additionally, for GW150914 we constrain the effective inspiral spin χ_(eff) more tightly around zero. This work is one of the first to unambiguously extract sub-dominant GW mode information from real events. It is also a first step toward eliminating the approximations used in semi-analytic waveform models from GW parameter estimation. It strongly motivates that NR surrogates be extended to cover more of the binary black hole parameter space.