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 of $\sim 45M_\odot$ between low-spin and high-spin black holes groups had been inferred with gravitational wave data since 2022 and then interpreted as the signature of the PISN mass gap, 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 low $^{12}C(α,γ)^{16}O$ reaction rate of $S_{300{\rm keV}} \sim 110~{\rm keV~b}$. 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.

Figures (5)

• Figure 1: Top panel: The mass-spin distribution of the black holes, the points (black for GWTC-3, while purple for O4a) are for the median values ($\bar{m}$,$\bar{\chi}$), and the shaded regions are for the 90% credible intervals. An attractive feature we find is the emergence of a new group of $\gtrsim 50M_\odot$ black holes likely with low spins in the O4a data. Bottom panel: component-mass and spin-magnitude distributions of events in GWTC-4, reweighed by a population-informed prior inferred in this work. The points, diamonds, and stars are for the low-spin low-mass ($\bar{\chi}<0.4$, $\bar{m}<50M_\odot$), low-spin high-mass ($\bar{\chi}<0.4$, $\bar{m}>50M_\odot$), and high-spin ($\bar{\chi}>0.4$) BHs. Orange and black are for primary and secondary component, respectively.
• Figure 2: The reconstructed component-mass and spin-magnitude distributions. The solid lines (dashed lines / shaded regions) indicate the median values and $90\%$ credible regions for different subpopulations.
• Figure 3: The left panel is the probability distribution of the maximum mass ($m_{\rm max,1}$) of the low-spin group, and we have $m_{\rm max,1}=68.5^{+19.8}_{-18.5}M_\odot$. In comparison to 2024PhRvL.133e1401L, the peak of the current probability distribution of the maximum mass is significantly higher. The right panel is the evaluated $S$ factor of $^{12}C(\alpha,\gamma)^{16}O$ reaction at 300 keV assuming that $m_{\rm max,1}$ represents $M_{\rm low}$. The value of $S_{\rm 300keV}$ recommended by 2017RvMP...89c5007D (1$\sigma$ region including the model uncertainty) is also plotted for comparison.
• Figure 4: Primary-mass versus secondary-mass distributions of events in GWTC-3 and O4a, reweighed by a population-informed prior inferred in this work. The points, diamonds, and stars / squares are for the low-spin low-mass ($\bar{\chi_1}<0.4$, $\bar{m_1}<50M_\odot$), low-spin high-mass ($\bar{\chi_1}<0.4$, $\bar{m_1}>50M_\odot$), and high-spin ($\bar{\chi_1}>0.4$, $\bar{\chi_2}<0.4$) / ($\bar{\chi_1}>0.4$, $\bar{\chi_2}>0.4$) BBHs.
• Figure 5: Parameters of mass functions for the two subpopulations. The values are for median and 90% credible intervals.