# Masses of optical components and black holes in x-ray novae: the effects of components proximity

## TL;DR

This paper refines the mass estimates of optical stars in black hole low-mass X-ray binaries by calculating absorption profiles more accurately, revealing smaller stellar masses and challenging existing evolutionary models.

## Contribution

It provides an exact calculation of absorption profiles, correcting previous overestimations of stellar rotational velocities and masses in BH LMXBs.

## Key findings

- Refined optical star masses range from 0.1 to 1.6 solar masses.
- Mass of the optical star is about 1.5 times smaller than previous estimates.
- Companion masses peak around 0.35 solar masses, challenging standard formation scenarios.

## Abstract

An exact calculation of CaI $\lambda 6439.075$A absorption profiles in the spectra of optical stars in low-mass X-ray binary systems with black holes (BH LMXBs) is carried out. We show that the approximation of a real Roche lobe filling star as disk with uniform local line profile and linear limb darkening law leads to overestimation of projected equatorial rotational velocity $V_{rot} \sin i$ and accordingly, underestimation of mass ratio $q=M_x/M_v$. Refined value of $q$ does not affect the mass of a black hole, but the mass of an optical star has shrunk $\sim 1.5$ times. We present refined components masses in low-mass X-ray binaries with black holes. Companion masses in BH LMXBs are found in the mass range 0.1 - 1.6 $M_{\odot}$ with the peak at $M_v \simeq 0.35 M_{\odot}$. This finding poses additional problem for the standard evolutionary scenarios of BH LMXBs formation with a common envelope phase (CE). We also discuss the implications of these masses into the evolutionary history of the binary.