X-ray luminosity function of high-mass X-ray binaries: Studying the signatures of different physical processes using detailed binary evolution calculations

Abstract

Context. Many physical processes taking place during the evolution of binary stellar systems remain poorly understood. The ever-expanding observational sample of X-ray binaries (XRBs) makes them excellent laboratories for constraining binary evolution theory. Such constraints and useful insights can be obtained by studying the effects of various physical assumptions on synthetic X-ray luminosity functions (XLFs) and comparing them with observed XLFs. Aims. In this work we focus on high-mass X-ray binaries (HMXBs) and study the effects on the XLF of various, poorly constrained assumptions regarding physical processes, such as the common-envelope phase, core collapse, and wind-fed accretion. Methods. We used the new binary population synthesis code POSYDON, which employs extensive precomputed grids of detailed stellar structure and binary evolution models, to simulate the entire evolution of binaries. We generated 96 synthetic XRB populations corresponding to different combinations of model assumptions, including different prescriptions for supernova kicks, supernova remnant masses, common-envelope evolution, circularization at the onset of Roche-lobe overflow, and observable wind-fed accretion. Results. The generated HMXB XLFs are feature-rich, deviating from the commonly assumed single power law. We find a break in our synthetic XLF at luminosity ∼1038 erg s−1, similar to observed XLFs. However, we also find a general overabundance of XRBs (up to a factor of ∼10 for certain model parameter combinations) driven primarily by XRBs with black hole accretors. Assumptions about the transient behavior of Be XRBs, asymmetric supernova kicks, and common-envelope physics can significantly affect the shape and normalization of our synthetic XLFs. We find that less well-studied assumptions regarding the circularization of the orbit at the onset of Roche-lobe overflow and criteria for the formation of an X-ray-emitting accretion disk around wind-accreting black holes can also impact our synthetic XLFs and reduce the discrepancy with observations. Conclusions. Our synthetic XLFs do not always agree well with observations, especially at intermediate X-ray luminosities, which is likely due to uncertainties in the adopted physical assumptions. While some model parameters leave distinct imprints on the shape of the synthetic XLFs and can reduce this deviation, others do not have a significant effect overall. Our study reveals the importance of large-scale parameter studies, highlighting the power of XRBs in constraining binary evolution theory.

On the Formation of Double Neutron Stars in the Milky Way: Influence of Key Parameters

“…In addition, we assume that part of the accreting material is ejected out of the binary in the form of isotropic wind, while the actual situation for mass loss may be more complicated, which impacts the orbital evolution (Soberman et al 1997;Kruckow et al 2018;Vinciguerra et al 2020). Together with more detections of DNSs in the Milky Way, a thorough investigation on the evolutionary history of DNSs needs to incorporate the formation of massive binaries (e.g., Wang et al 2020), high-mass X-ray binaries (e.g., Misra et al 2023), and binary pulsars in a self-consistent way. This will be the subject of the future study.…”

A NuSTAR Census of the X-Ray Binary Population of the M31 Disk

“…Furthermore, the shape of HMXB XLFs and the total luminosity of HMXB populations in galaxies are linked to parameters like galaxy SFR, stellar mass, and metallicity (Kilgard et al 2002;Gilfanov 2004;Mineo et al 2012;Vulic et al 2018;Lehmer et al 2019Lehmer et al , 2020Lehmer et al , 2021, which are important in models of accreting binary evolution (Fragos et al 2009;Tzanavaris et al 2013;Zuo et al 2014;Giacobbo & Mapelli 2018;Misra et al 2023). Further characterization of X-ray point-source populations in galaxies allows for better comparisons to the outputs of these models.…”

Does Nature Allow the Formation of Ultra-compact Black Hole X-Ray Binaries via the Accretion-induced Collapse of Neutron Stars?

“…Following Misra et al (2023; see their Equations (1)-( 4)), one can estimate the bolometric X-ray luminosity of UCXBs. In Figure 3, we present the evolution of bolometric X-ray luminosity for the binary system presented in Figure 1.…”