The puzzling story of flare inactive ultra-fast-rotating M dwarfs – III. Investigating X-ray activity

ABSTRACT According to activity-rotation relations, rapid rotators are expected to show high levels of magnetic activity. However, recent studies with TESS have found ultra-fast-rotating (UFR) M dwarfs with periods $\lt 1$ d displaying low levels of flaring activity. There have been efforts to explore their magnetic field strengths through spectropolarimetric measurements and to assess the potential for binarity. However, neither could fully explain the lack of observed flaring activity despite their rapid rotation. Another avenue for investigation is to measure their coronal emission for signs of supersaturation: an underluminosity in X-rays observed for some rapidly rotating FGK stars. Therefore, in this study, we utilize X-ray observations from Swift and XMM–Newton of 10 M dwarf ultra-fast rotators with P$_{\rm {rot}}$ < 1 d to determine their X-ray luminosities. Overall, we do not find evidence for supersaturation amongst our UFR M dwarf stars, instead determining them to be at the saturated level, or perhaps even enhanced. Therefore, supersaturation seems not to be the main driver behind the reduced level of flaring activity observed in these stars, and the mystery behind the magnetic activity of UFR low-mass stars remains. Additionally, we provide an updated analysis on the long-term variability within our sample using TESS light curves taken during Cycles 5 and 7. We identify 352 optical flares from our sample with energies between $1.2\times 10^{31}$ and $8.7\times 10^{34}$ erg. We determine flare rates for each TESS cycle, compare them, identifying variations across a 7-yr timespan and attribute this to potential activity cycles.