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

TL;DR

This study investigates the X-ray activity of ultra fast rotating M dwarfs, finding no evidence of supersaturation and analyzing long-term flare variability over seven years.

Contribution

It provides the first X-ray luminosity measurements for UFR M dwarfs and explores their activity levels and variability, addressing the flare inactivity mystery.

Findings

UFR M dwarfs are at saturated or enhanced X-ray activity levels.

No evidence of supersaturation in the X-ray emission of these stars.

Detected 352 optical flares with energies up to 8.7×10^{34} erg over seven years.

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 $<1$ 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 utilise X-ray observations from Swift and XMM-Newton of ten M dwarf UFRs 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 $1.2\times10^{31}$ and $8.7\times10^{34}$ erg. We determine flare rates for each TESS cycle, compare them, identifying variations across a 7 year timespan and attribute this to potential activity cycles.