Insights into binary black hole formation from gravitational waves
Wed 01 Nov, 2017 @12PM, Level 7
Dr. Simon Stevenson, OzGrav postdoctoral fellow
In its first observing run (O1), Advanced LIGO detected gravitational waves from two binary black hole mergers, GW150914 and GW151226, along with a statistically less significant candidate LVT151012. The Advanced LIGO detectors were joined by the Advanced Virgo detector in the recently concluded second observing run (O2). Another two binary black hole mergers, GW170104 and GW170814, have been announced so far from this analysis, with the analysis of the data ongoing. These observations confirmed the existence of merging stellar mass black hole binaries originating in low metallicity environments. How to form merging binary black holes remains an open question theoretically. There are many proposed mechanisms, falling broadly into two families: 1) Isolated binary evolution and 2) dynamical formation in dense stellar environments. We show that comparing the distribution of observed black hole masses and spins to those predicted from population models will allow us to gain insight into how binary black holes form. We examine the measurements of the effective spin parameters for the 5 binary black hole observations published thus far and show that the data already exhibit a mild (2.7 sigma) preference for an isotropic distribution of spins over spins aligned with the binary orbital angular momentum. Alternatively, dimensionless black hole spin magnitudes in these systems could be intrinsically small (average spin magnitude < 0.2), at odds with measurements of black hole spin magnitudes in high mass X-ray binaries. Both of these conclusions have implications for the formation of binary black holes, and the formation of heavy black holes in supernovae.