SRG/eROSITA survey of Be stars

Abstract

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 γ Cas phenomenon. Its actual incidence has been particularly difficult to assess. Thanks to four semesters of sky survey data taken by the Spectrum-Roentgen-Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA), we revisit the question of the X-ray properties of Be stars. Amongst a large catalogue of Be stars, eROSITA achieved 170 detections (20 per cent of the 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 100–1000 pc appears to be ∼12 per cent, which is far from negligible. At the other extreme, no bright supersoft X-ray emission seems to be associated with any of our targets.