No evidence that the majority of black holes in binaries have zero spin

Abstract

The spin properties of merging black holes observed with gravitational waves can offer novel information about the origin of these systems. The magnitude and orientations of black hole spins offer a record of binaries' evolutionary history, encoding information about massive stellar evolution and the astrophysical environments in which binary black holes are assembled. Recent analyses of the binary black hole population have yielded conflicting portraits of the black hole spin distribution. Some work suggests that black hole spins are small but non-zero and exhibit a wide range of misalignment angles relative to binaries' orbital angular momenta. Other work concludes that the majority of black holes are non-spinning while the remainder are rapidly rotating and primarily aligned with their orbits. We revisit these conflicting conclusions, employing a variety of complementary methods to measure the distribution of spin magnitudes and orientations among binary black hole mergers. We find that the existence of a sub-population of black hole with vanishing spins is not required by current data. Should such a sub-population exist, we find that it must contain 60% of binaries. Additionally, we find evidence for significant spin-orbit misalignment among the binary black hole population, with some systems exhibiting misalignment angles greater than 90 • , and see no evidence for an approximately spin-aligned sub-population.

Characterization of merging black holes with two precessing spins

“…For the case of GW190521, a strong precession signature was also reported [14], though potential degeneracies with the eccentricity still need to be fully understood [20]. Collective evidence for spin precession was reported in the context of BH binary populations, with all current fits requiring some misaligned spins at high confidence [21][22][23].…”

Do high-spin high mass X-ray binaries contribute to the population of merging binary black holes?

“…Other important but contended features of the BBH spin distribution include the possibility of a zerospin excess (Roulet et al 2021;Galaudage et al 2021), and the presence of systems with spin-orbit misalign-ments larger than 90 • (implying χ eff < 0) (Abbott et al 2021c,d). Implementing a series of hierarchical analyses of the BBH population, Callister et al (2022) found preference for significant spin-orbit misalignment among the merging BBH population, but show that there is no evidence that GW data includes an excess of zero-spin systems. This latter point is in agreement with other studies (Kimball et al 2020(Kimball et al , 2021Mould et al 2022), and indicates that the majority of merging BBHs have small but non-zero spin (Abbott et al 2021c).…”

Limits on Hierarchical Black Hole Mergers from the Most Negative χ eff Systems

“…Combining information across multiple events, Abbott et al (2021bAbbott et al ( , 2021c and Callister et al (2022) found that the BBH population likely contains systems with (small) negative χ eff (χ eff  −0.02), but this evidence weakens under population models that include a correlation between χ eff and mass ratio (Callister et al 2021) or a nonspinning subpopulation that would create a narrow peak at zero in the χ eff distribution (Galaudage et al 2021;Roulet et al 2021). Disentangling small negative χ eff from nonspinning systems requires resolving the χ eff distribution to very small scales, -( )  10 2 (Callister et al 2022). If dynamically assembled BBHs always consist of slowly spinning, or even nonspinning, BHs (Fuller & Ma 2019), the ambiguity around the presence of systems with negative χ eff will persist.…”

“…However, a few GWTC-3 events, GW191109 010717 and GW200225 060421 have χ eff < 0 with 90% and 85% credibility, respectively (Abbott et al 2021a), and lower-significance candidate events with negative χ eff have been reported in independent analyses (Venumadhav et al 2020;Olsen et al 2022). Combining information across multiple events, Abbott et al (2021c,b) and Callister et al (2022) find that the BBH population likely contains systems with (small) negative χ eff (χ eff −0.02), but this evidence weakens under population models that include a correlation between χ eff and mass ratio (Callister et al 2021), or a nonspinning subpopulation that would create a narrow peak at zero in the χ eff distribution (Roulet et al 2021a;Galaudage et al 2021). Disentangling small negative χ eff from nonspinning systems requires resolving the χ eff distribution to very small scales O(10 −2 ) (Callister et al 2022).…”