First eROSITA study of nearby M dwarfs

This unprecedented data base for X-ray emitting M dwarfs allowed us to put a quantitative constraint on the mass dependence of the X-ray luminosity, and to determine the change in the activity level with respect to pre-main-sequence stars. TESS observations are available for 489 of 687 X-ray detected M dwarfs, and applying standard period search methods we could determine the rotation period for 180 X-ray detected M dwarfs, about one forth of the X-ray sample.

With the joint eROSITA and TESS sample, and combining it with our compilation of historical X-ray and rotation data for M dwarfs, we examine the mass dependence of the "saturated" regime of the rotation-activity relation. A first comparison of eROSITA hardness ratios and spectra shows that 65% of the X-ray detected M dwarfs have coronal temperatures of ~0.5keV. We performed a statistical investigation of the long-term X-ray variability of M dwarfs comparing the eROSITA measurements to those obtained ~30 years earlier during the ROSAT all-sky survey (RASS).

Evidence for X-ray flares are found in various parts of our analysis: directly from an inspection of the eFEDS light curves, in the relation between RASS and eRASS1 X-ray luminosities, and in a subset of stars that displays X-ray emission hotter than the bulk of the sample according to the hardness ratios. Finally, we point out the need to obtain X-ray spectroscopy for more M dwarfs to study the coronal temperature-luminosity relation that is not well constrained by our eFEDS results.