The Discovery of an X-ray/UV Stellar Flare from the Late-K/Early-M Dwarf LMC 335

TL;DR

This paper reports the discovery and detailed analysis of a long-duration X-ray/UV flare from a late-K/early-M dwarf star, providing insights into stellar flare properties and energetics.

Contribution

It presents the longest continuous observation of a stellar flare from a late-K/early-M dwarf, with detailed spectral and temporal analysis of the event.

Findings

Flare lasted over 10 hours with flux increases over 100 times.

X-ray spectra show a high-temperature component of 40-60MK.

Flare energy output estimated between 0.4 and 2.9 x 10^35 erg.

Abstract

We report the discovery of an X-ray/UV stellar flare from the source LMC 335, captured by XMM-Newton in the field of the Large Magellanic Cloud. The flare event was recorded continuously in X-ray for its first 10 hours from the precursor to the late decay phases. The observed fluxes increased by more than two orders of magnitude at its peak in X-ray and at least one in the UV as compared to quiescence. The peak 0.1-7.0 keV X-ray flux is derived from the two-temperature APEC model to be ~(8.4 +/- 0.6) x 10^-12 erg cm-2 s-1. Combining astrometric information from multiple X-ray observations in the quiescent and flare states, we identify the NIR counterpart of LMC 335 as the 2MASS source J05414534-6921512. The NIR color relations and spectroscopic parallax characterize the source as a Galactic K7-M4 dwarf at a foreground distance of (100 - 264) pc, implying a total energy output of the entire event of ~(0.4 - 2.9) x 10^35 erg. This report comprises detailed analyses of this late-K / early-M dwarf flare event that has the longest time coverage yet reported in the literature. The flare decay can be modeled with two exponential components with timescales of ~28 min and ~4 hours, with a single component decay firmly ruled out. The X-ray spectra during flare can be described by two components, a dominant high temperature component of ~40-60MK and a low temperature component of ~10MK, with a flare loop length of about 1.1-1.3 stellar radius.