Contemporaneous XMM-Newton Investigation of a Giant X-ray Flare and Quiescent State from a Cool M-Class Dwarf in the Local Cavity

We report the serendipitous detection of a giant X-ray flare from the source 2XMM J043527.2-144301 during an XMM-Newton observation of the high latitude molecular cloud MBM20. The source has not been previously studied at any wavelength. The X-ray flux increases by a factor of more than 52 from quiescent state to peak of flare. A 2MASS counterpart has been identified (2MASS J04352724-1443017), and near-infrared colors reveal a spectral type of M8-M8.5 and a distance of (67 ± 13) pc, placing the source in front of MBM20. Spectral analysis and source luminosity are also consistent with this conclusion. The measured distance makes this object the most distant source (by about a factor of four) at this spectral type detected in X-rays. The X-ray flare was characterized by a peak X-ray luminosity of ~8.2 × 10²⁸ erg s^-1 and integrated X-ray energy of ~2.3 × 10³² erg. The flare emission has been characterized with a two-temperature model with temperatures of ~10 and 46 MK (0.82 and 4.0 keV) and is dominated by the higher temperature component.