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

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 hr 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) × 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 Two Micron All Sky Survey 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) × 10³⁵ 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 minutes and ~4 hr, 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 ~40-60 MK and a low-temperature component of ~10 MK, with a flare loop length of about 1.1-1.3 stellar radius.