The population of massive X-ray binaries I. The Large Magellanic Cloud

TL;DR

This study provides detailed spectral classifications of massive X-ray binaries in the Large Magellanic Cloud, revealing similarities with Galactic populations and insights into their formation and evolution.

Contribution

It offers the first high-resolution spectral classifications for most optical counterparts in the LMC and compares their population characteristics with those in the Milky Way.

Findings

The population distribution in the LMC is similar to the Galactic one.

Be/X-ray binaries in the LMC likely form from moderately massive binaries.

Presence of large eccentricities suggests supernova kicks.

Abstract

We present high resolution blue spectroscopy of an almost complete sample of optical counterparts to massive X-ray binaries in the Large Magellanic Cloud (LMC) and derive their spectral classification. We find an spectral type B0II for the optical counterpart to RX J0532.5-6551, confirming it as the first wind-fed massive X-ray binary in the LMC. We also confirm the Be nature of the proposed counterpart to RX J0535.0-6700. The proposed optical counterpart to RX J0531.5-6518 is a B2V star with signs of emission in the Balmer lines. In total, we give accurate spectral types for 14 counterparts. We find that the overall observed population of massive X-ray binaries in the LMC has a distribution not very different from the observed Galactic population and we discuss different selection effects affecting our knowledge of this population. The spectral distribution of the Be/X-ray binary population is also rather similar to the Galactic one. This distribution implies that Be/X-ray binaries must have preferentially formed from moderately massive binaries undergoing semi-conservative evolution. The observation of several Be/X-ray binaries with large eccentricities implies then the existence of supernova kicks.