LB-1 is inconsistent with the X-ray source population and pulsar-black hole binary searches in the Milky Way

TL;DR

This paper challenges the claimed detection of LB-1 as a black hole binary in the Milky Way, arguing that evolutionary models and lack of observational evidence make such systems highly unlikely.

Contribution

It provides a theoretical analysis of binary evolution and observational constraints, showing that LB-1-like systems are inconsistent with expected X-ray and pulsar observations in the Milky Way.

Findings

LBX sources from 8 M_sun stars around BHs are rare in the MW.

No pulsar-BH binaries have been detected, constraining their frequency.

The observed LB-1 system is inconsistent with evolutionary and observational expectations.

Abstract

If confirmed, a wide binary system of 70 $M_{\odot}$ black hole (BH) and an 8 $M_{\odot}$ main sequence star (LB-1) is observed to reside in the Milky Way (MW). We show that long term evolution of an 8 $M_{\odot}$ star around a BH with mass between 5-70 $M_{\odot}$ makes them visible as ultra-luminous X-ray (ULX) sources in the sky. Given the expected ULX phase lifetime ($\approx0.1$ Myr) and their lack of detection in the MW, we conclude that the frequency of an 8-20 $M_{\odot}$ star to be in binary around a stellar mass BH should be less than ($f<2\times10^{-3}$). This is in tension with Liu et al. (2019) claimed detection frequency of LB-1 like system around 8-20$M_{\odot}$ stars ($f\approx3\times10^{-2}$). Moreover, the 8 $M_{\odot}$ star is likely to end as a neutron star (NS) born with a very small kick from an electron capture supernova (ECSN), leaving behind a wide NS-BH binary. So far less than 1\% of all the detectable pulsars in the MW are mapped and there has been no detection of any pulsars in binary systems around BHs which sets an upper bound of about 100 possible pulsar-BH systems in the MW. We show if the NS is born from ECSN, a frequency upper limit of ($f=5\times10^{-4}$) for stars with masses $\approx 8-20~M_{\odot}$ in the MW to have a BH companion. The rate discrepancy will further increase as more pulsars are mapped in the MW, yet these searches would not be able to rule out the Liu et al. detection frequency if NSs are instead born in core collapse SNe with the commonly inferred high kick velocities.