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 magnitudes 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 works suggest that black hole spins are small but nonzero and exhibit a wide range of misalignment angles relative to binaries’ orbital angular momenta. Other works conclude that the majority of black holes are nonspinning 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 subpopulation of black holes with vanishing spins is not required by current data. Should such a subpopulation exist, we conclude 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 subpopulation.

Spin it as you like: The (lack of a) measurement of the spin tilt distribution with LIGO-Virgo-KAGRA binary black holes

“…Therefore, the fact that the LVK default model finds support for a peak at cos τ = 1 can be explained because it can only add support at cos τ = 1 when trying to match any population features in addition to the isotropic distribution. Our results agree with previous results for population models fitting the distribution of χ eff , which find only a small fraction of sources with negative χ eff , implying negative tilts (Roulet & Zaldarriaga 2019;Miller et al 2020;Roulet et al 2021;Callister et al 2022). Indeed, if we recast our inferred distributions for cos τ and spin magnitude to the resulting χ eff distribution, we would obtain results consistent with Abbott et al (2021e).…”

“…The only conclusion that is consistently found across all models is that there is no excess of systems with negative tilts, relative to what is expected in an isotropic distribution. Our results agree with the literature (e.g., Callister et al 2022;Tong et al 2022;Abbott et al 2021e; as to the lack of an excess of cos τ −1, but disagree as to other details (e.g., whether there is a hard cutoff in the cos τ distribution at cos τ < 0 (cfr Callister et al 2022)). The point of this work is to show that those disagreements are to be expected, given the information in the current dataset.…”

“…The impact of modeling on astrophysical inference of gravitational-wave sources has already become apparent in recent months. Several groups have investigated whether there is evidence for a fraction of black hole spin magnitude to be vanishingly small, finding results that depend on the model to a large extent (Callister et al 2022;Galaudage et al 2021;Tong et al 2022;Roulet et al 2021;). We note that Callister et al (2022) also provide a comprehensive summary of the status of that measurement.…”

“…While this is a reasonable model, we are interested in verifying whether it is actually supported by the available data, or whether we are instead getting posteriors that are driven by that model. Callister et al (2022) and Tong et al (2022) recently considered alternative models for the tilt angles, but they focused on whether there is a cutoff at negative cosine tilts (i.e., for antialigned spins), and if that answer depends on the model for the spin magnitude. On the other hand, we do not limit our investigation to the existence of negative tilts, but instead are interested in what-if anything-can be said about the tilt inference that is not strongly dependent on the model being used.…”

Tossing Black Hole Spin Axes

“…However, in those scenarios the expected χ eff distribution is less clear. For further discussion on the interpretation of the empirical χ eff data, see, e.g., Stevenson et al (2017), Qin et al (2018), Bavera et al (2020), Olejak & Belczynski (2021), Callister et al (2022).…”