SRG/eROSITA survey of Be stars

TL;DR

This study uses SRG/eROSITA survey data to analyze X-ray emissions from Be stars, revealing a significant subset with hard X-ray properties and providing new insights into their incidence and characteristics.

Contribution

It offers the first large-scale assessment of X-ray properties of Be stars using eROSITA data, identifying the incidence of hard X-ray emission among early-type Be stars.

Findings

170 Be stars detected in X-ray (20% of sample)

At least 34 sources exhibit hard X-ray spectra

Incidence of hard X-ray emission among early-type Be stars is ~12%

Abstract

Massive stars are known X-ray emitters and those belonging to the Be category are no exception. One type of X-ray emission even appears specific to that category, the gamma Cas phenomenon. Its actual incidence has been particularly difficult to assess. Thanks to four semesters of sky survey data taken by SRG (Spectrum Roentgen Gamma)/eROSITA, we revisit the question of the X-ray properties of Be stars. Amongst a large catalog of Be stars, eROSITA achieved 170 detections (20% of sample), mostly corresponding to the earliest spectral types and/or close objects. While X-ray luminosities show an uninterrupted increasing trend with the X-ray-to-bolometric luminosity ratios, the X-ray hardness was split between a large group of soft (and fainter on average) sources and a smaller group of hard (and brighter on average) sources. The latter category gathers at least 34 sources, nearly all displaying early spectral types. Only a third of them were known before to display such X-ray properties. The actual incidence of hard and bright X-rays amongst early-type Be stars within 100--1000pc appears to be ~12%, which is far from negligible. At the other extreme, no bright supersoft X-ray emission seem to be associated to any of our targets.