A1 Refereed original research article in a scientific journal
The puzzling story of flare inactive ultra-fast-rotating M dwarfs - III. Investigating X-ray activity; 
Authors: Doyle, Lauren; King, George W.; Ramsay, Gavin; Corrales, Lía R.; Bagnulo, Stefano; Doyle, J. Gerry; Hakala, Pasi
Publisher: Oxford University Press
Publication year: 2026
Journal: Monthly Notices of the Royal Astronomical Society
Article number: stag681
Volume: 548
Issue: 3
ISSN: 0035-8711
eISSN: 1365-2966
DOI: https://doi.org/10.1093/mnras/stag681
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://academic.oup.com/mnras/article/548/3/stag681/8651297
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/526493316
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
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 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 < 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 × 1031 and 8.7 × 1034 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.
Keywords:
stars: activity, stars: flare, stars: low-mass, stars: rotation, X-rays: stars
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Funding information in the publication:
LD would like to acknowledge funding from the UK Space Agency. We include data in this paper collected by the TESS mission, where funding for the TESS mission is provided by the NASA Explorer Program. This work presented results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC was provided by national institutions, in particular, the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. The Gaia archive website is https://archives.esac.esa.int/gaia. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. We also used observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.
