“Forbidden” stars in the eROSITA all-sky survey: X-ray emission from very late-type giants

Abstract

We present the results of the first X-ray all-sky survey (eRASS1) performed by the eROSITA instrument aboard the Spectrum-Roentgen-Gamma (SRG) mission on X-ray emitting red giants and supergiants. Focusing on stars positioned at high galactic latitudes above 20°, we constructed a complete sample of such objects using the Gaia DR3 catalog and identified a sample 96 stars that appear as bona fide entries in the eRASS1 source catalog. By again restricting the sample to objects nearer than 1300 pc and eliminating all catalog entries that are due to optical contamination, we ended up with a sample of 16 genuine red giant or supergiant X-ray sources, most of which represent new X-ray detections. We present a low signal-to-noise X-ray spectrum of the nearby low-activity giant Arcturus obtained from a pointed observation with the XMM-Newton satellite and give a detailed account of our data analysis. We show that Arcturus-like X-ray emission cannot be the explanation for the X-ray emissions observed by eROSITA and provide a discussion of the possible nature of the detected X-ray sources.