A new group of low-spin $50-70M_\odot$ Black Holes and the high pair-instability mass cutoff

Abstract

Pair-instability supernovae (PISN) will not leave compact remnants and hence yield a mass gap of the black holes. Though a transition point at $\approx 46M_\odot$, separating low- and high-spin black hole populations and interpreted as evidence for the PISN mass gap, was first identified in gravitational wave data by Wang et al. (2022, ApJL 941, L39) and later confirmed in follow-up studies, here we report the emergence of a new group of low-spin but massive ($\sim 50-70M_\odot$) black holes, which are hard to produce via hierarchical mergers, in the latest GWTC-4.0 data. Correspondingly, the mass cutoff of the low-spin black holes shifts to $68.5^{+19.8}_{-18.5}M_\odot$ (90\% credibility), which is consistent with the PISN model for a $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reaction rate of $S_{300} = 109^{+55}_{-27}~{\rm keV~b}$. Despite that the massive single-star collapse/dynamical capture origin can not be reliably tested at this moment, a high pair-instability mass cutoff $M_{\rm low}\sim 70M_\odot$ may be favored for its capability of accounting for the rather low observation rate of hydrogen-less super-luminous supernovae.