Inferring the pair-instability mass gap from gravitational wave data

We use hierarchical Bayesian inference with nonparametric Gaussian process models to investigate the effective inspiral spin parameter, χeff, as a function of primary black hole mass in the third gravitational-wave transient catalog (GWTC-3). Our analysis reveals a transition in the population at a primary mass of 46−5+7M⊙. Beyond this mass, the χeff distribution broadens, becomes consistent with being symmetric around zero, and has a median of −0.03−0.59+0.36 (90% credibility). These results are consistent with the presence of a pair-instability mass gap that is repopulated by black holes that are the remnant of a previous merger, formed in dense star clusters. However, asymmetric distributions skewed toward positive χeff are not excluded by current data. Below the inferred transition mass, we constrain the fraction of second-generation black holes to be ≲10%. These results provide model-independent support for a high-mass and high-spin population of black holes in the data, consistent with earlier work using parametric models. Imminent gravitational-wave data releases will be essential to sharpen constraints on spin symmetry and clarify the origin of the black holes.