Apples and Oranges: Comparing Black Holes in X-Ray Binaries and Gravitational-wave Sources

Abstract

The component black holes (BHs) observed in gravitational-wave (GW) binary black hole (BBH) events tend to be more massive and slower spinning than those observed in black hole X-ray binaries (BH-XRBs). Without modeling their evolutionary histories, we investigate whether these apparent tensions in the BH populations can be explained by GW observational selection effects alone. We find that this is indeed the case for the discrepancy between BH masses in BBHs and the observed high-mass X-ray binaries (HMXBs), when we account for statistical uncertainty from the small sample size of just three HMXBs. On the other hand, the BHs in observed low-mass X-ray binaries (LMXBs) are significantly lighter than the astrophysical BBH population, but this may just be due to a correlation between component masses in a binary system. Given their light stellar companions, we expect light BHs in LMXBs. The observed spins in HMXBs and LMXBs, however, are in tension with the inferred BBH spin distribution at the >99.9% level. We discuss possible scenarios behind the significantly larger spins in observed BH-XRBs. One possibility is that a small subpopulation (conservatively <30%) of BBHs have rapidly spinning primary components, indicating that they may have followed a similar evolutionary pathway to the observed HMXBs. In LMXBs, it has been suggested that BHs can spin up by accretion. If LMXB natal spins follow the BBH spin distribution, we find LMXBs must gain an average dimensionless spin of 0.47 − 0.11 + 0.10 , but if their natal spins follow the observed HMXB spins, the average spin-up must be <0.03.

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

“…These differences, which can lead to high-spin BHs, are important to consider when interpreting observations. Our conclusions are in agreement with Fishbach & Kalogera (2022), who found that a subpopoulation comprising of at most 30% of BBHs may have features resembling rapidly spinning HMXB-like systems, where one BH component is high-spin. This is also in agreement with Neijssel et al (2021), who, following a case study of Cygnus X-1 and finding a 5% probability that it will result in a merging BBH within a Hubble time, infer that a small fraction of HMXBs like Cygnus X-1 may form BBHs.…”

“…However, considering BH spins, the merging population of BBHs may include only a small subpopulation of systems that are HMXB-like (systems containing a rapidly spinning component with χ 0.8, and preferentially aligned with the orbital angular momentum axis, as expected from isolated binary evolution). Conservatively, Fishbach & Kalogera (2022) find that a HMXB-like population can make up at most 30% of merging BBH systems. It is therefore important to understand how the specific evolutionary pathways of merging BBHs and HMXBs shape their observed spins distributions (Liotine et al 2022, in prep.…”

“…Of the three HMXBs with confident BH spin measurements (M33 X-7, Cygnus X-1 and LMC X-1), all BHs are observed to be highly spinning, with spin magnitudes 0.8 (Liu et al 2008;Miller-Jones et al 2021;Reynolds 2021). Although there are only three of these systems, it is clear that they have a distinct spin distribution compared to merging BBHs (Roulet & Zaldarriaga 2019;Reynolds 2021;Fishbach & Kalogera 2022).…”

“…We must consider the possibility that these two classes of binaries may only appear different due to the limitations of how they are observed. Fishbach & Kalogera (2022) investigated whether the differences in the mass and spin distributions of HMXBs and merging BBHs may be a result of GW observational selection effects alone. Based upon GWTC-2 observations (Abbott et al 2021e), they found that, accounting for GW observational selection effects and the small-number statistics of the observed HMXBs, the masses of the observed HMXBs are consistent with the BBH mass distribution.…”

Black hole mass and spin measurements through the relativistic precession model: XTE J1859+226

“…On the one hand, our results stress once more that the inconsistency of the spin measurements obtained via any two (or more) of the available 'electromagnetic' methods (timing, continuum fitting, reflection spectroscopy, and reverberation) is symptomatic of an inexact, or at least inaccurate, modelling of the data. On the other hand, if the RPM is correct, our results alleviate the tension between the gravitational waves (see Wysocki & Ligo-VIRGO-Kagra Collaboration 2022;Golomb & Ligo-VIRGO-Kagra Scientific Collaboration 2022) and the electromagnetic spin measurements (Fishbach & Kalogera 2022), and supports the hypothesis that LIGO/Virgo BHs and X-ray binary BHs do not necessarily form two distinct populations (Belczynski et al 2021).…”