You Can't Always Get What You Want: The Impact of Prior Assumptions on Interpreting GW190412
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 (≳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.
© 2020. The American Astronomical Society. Received 2020 June 19; revised 2020 July 7; accepted 2020 July 23; published 2020 August 11. The authors would like to thank Richard Udall and Richard O'Shaughnessy for performing parallel calculations using different pipelines, and Rory Smith for useful comments on this manuscript. The authors also thank the anonymous referee for helpful suggestions that improved this Letter. This work is supported by the National Science Foundation under grant No. PHY-1912648. M.Z. acknowledges support from CIERA and Northwestern University. C.P.L.B. is supported by the CIERA Board of Visitors Professorship. The Flatiron Institute is supported by the Simons Foundation. S.V. acknowledges support of the MIT physics department through the Solomon Buchsbaum Research Fund, the National Science Foundation, and the LIGO Laboratory. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation and operates under cooperative agreement PHY-0757058. This work used computing resources at CIERA funded by National Science Foundation under grant No. PHY-1726951, and the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. This research has made use of data obtained from the Gravitational Wave Open Science Center (www.gw-openscience.org; Abbott et al. 2019c), a service of LIGO Laboratory, the LIGO Scientific Collaboration and the Virgo Collaboration. LIGO is funded by the US National Science Foundation. Virgo is funded by the French Centre National de Recherche Scientifique (CNRS), the Italian Istituto Nazionale della Fisica Nucleare (INFN) and the Dutch Nikhef, with contributions by Polish and Hungarian institutes. Software: BILBY (Ashton et al. 2019; Romero-Shaw et al. 2020), iPython (Pérez & Granger 2007), Matplotlib (Hunter 2007), NumPy (Oliphant 2006; Van Der Walt et al. 2011), Pandas (McKinney 2010), SciPy (Virtanen et al. 2020).
Submitted - 2006.11293.pdf
Published - Zevin_2020_ApJL_899_L17.pdf