What’s in a binary black hole’s mass parameter?

ABSTRACT The black hole (BH) masses measured from gravitational wave observations appear to cluster around specific mass values. Consequently, the primary (and chirp) mass distribution of binary black holes (BBHs) inferred using these measurements shows four emerging peaks. These peaks are approximately located at a primary (chirp) mass value of 10 $\, \mathrm{M}_\odot$ (8$\, \mathrm{M}_\odot$), 20 $\, \mathrm{M}_\odot$ (14 $\, \mathrm{M}_\odot$), 35 $\, \mathrm{M}_\odot$ (28 $\, \mathrm{M}_\odot$), and 63 $\, \mathrm{M}_\odot$ (49 $\, \mathrm{M}_\odot$). Although the presence of the first and third peaks has been attributed to BBH formation in star clusters or due to the evolution of stellar binaries in isolation, the second peak has received relatively less attention because it lacks significance in the primary mass distribution. In this article, we report that confidence in the second peak depends on the mass parameter we choose to model the population on. Unlike primary mass, this peak is significant when modelled on the chirp mass. We discuss the disparity as a consequence of mass asymmetry in the observations that cluster at the second peak. Finally, we report this asymmetry as part of a potential trend in the mass ratio distribution manifested as a function of the chirp mass, but not as a function of primary mass, when we include the observation GW190814 in our modelling. The chirp mass is not a parameter of astrophysical relevance. Features present in the chirp mass, but not in the primary mass, are relatively difficult to explain and expected to garner significant interest.