The corona of a fully convective star with a near-polar flare

Abstract

In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly rotating M7 dwarf, TIC 277539431, producing a flare at $81^ circ $ latitude, the highest latitude flare located to date. This is in stark contrast to solar flares that occur much closer to the equator, typically below $30^ circ $. The mechanisms that allow flares at high latitudes to occur are poorly understood. We studied five sectors of TESS monitoring, and obtained 36 ks of XMM-Newton observations to investigate the coronal and flaring activity of TIC 277539431. From the observations, we infer the optical flare frequency distribution; flare loop sizes and magnetic field strengths; the soft X-ray flux, luminosity, and coronal temperatures; as well as the energy, loop size, and field strength of a large flare in the XMM-Newton observations. We find that the corona of TIC 277539431 does not differ significantly from other low-mass stars on the canonical saturated activity branch with respect to coronal temperatures and flaring activity, but shows lower luminosity in soft X-ray emission by about an order of magnitude, consistent with other late M dwarfs. The lack of X-ray flux, the high-latitude flare, the star's viewing geometry, and the otherwise typical stellar corona taken together can be explained by the migration of flux emergence to the poles in rapid rotators like TIC 277539431 that drain the star's equatorial regions of magnetic flux, but preserve its ability to produce powerful flares.