SRG/eROSITA survey of Be stars

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 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 similar to 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.