First eROSITA-TESS results for M dwarfs: Mass dependence of the X-ray activity rotation relation and an assessment of sensitivity limits

TL;DR

This study analyzes the relationship between X-ray activity and rotation in M dwarf stars using eROSITA X-ray data and TESS rotation periods, revealing mass-dependent behaviors and sensitivity limits of current observations.

Contribution

It provides new insights into the activity-rotation relation for M dwarfs across different masses and assesses eROSITA's sensitivity to slower rotators in the unsaturated regime.

Findings

Positive slope in activity-rotation relation for lowest mass bin due to lack of intermediate rotators

eROSITA detects more stars than TESS, including slower rotators in the unsaturated regime

Mass dependence observed in the activity-rotation relation for M dwarfs

Abstract

We present a study of the activity-rotation relation for M dwarf stars, using new X-ray data from the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG), combined with photometric rotation periods from the Transiting Exoplanet Survey Satellite (TESS). The stars used in this work are selected from the superblink proper motion catalog of nearby M dwarfs. We study the 135 stars with both a detection in the first eROSITA survey (eRASS1) and a rotation period measurement from TESS jointly with the sample of 197 superblink M dwarfs re-adapted from our previous work. We fit the activity-rotation relation for stars with rotation periods shorter than ~10 d (saturated regime) using three mass bins. The surprising positive slope for stars in our lowest mass bin ($M_{\star} \leq 0.4 {\rm M_\odot}$) is due to a paucity of stars with intermediate rotation periods (~ 1-10 d), probably caused by fast period evolution. The much higher fraction of eRASS1 detections compared to stars that have also rotation periods from TESS shows that eROSITA is also sensitive for slower rotating M dwarfs that are in the unsaturated regime with periods inaccessible to TESS.