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

TL;DR

This study investigates a high-latitude flare on a rapidly rotating M7 dwarf star, revealing insights into stellar magnetic activity, flare mechanisms, and coronal properties, with implications for understanding magnetic flux migration in low-mass stars.

Contribution

It provides the first detailed analysis of a near-polar flare on a fully convective star, linking magnetic flux migration to flare occurrence at high latitudes.

Findings

High-latitude flare observed at 81° latitude.

Star's corona similar to other low-mass stars but with lower X-ray luminosity.

Flux emergence migration to poles explains high-latitude flares and low X-ray flux.

Abstract

In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly rotating M7 dwarf, TIC 277539431, produce a flare at 81{\deg} 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{\deg}. 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 TIC 277539431's corona 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.