Potential Chromospheric Evaporation in the M Dwarf’s Flare Triggered by Einstein Probe Mission

Abstract

Although flares from late-type main-sequence stars have been frequently detected in the multiwavelength, the associated dynamical process has been rarely reported so far. Here, we report follow-up observations of an X-ray transient triggered by Wide-field X-ray Telescope onboard the Einstein Probe at UT08:45:08 in 2024, May 7. The photometry in multibands and time-resolved spectroscopy started at 3 hr and 7.5 hr after the trigger, which enables us to identify the transient as a flare of the M-dwarf 2MASS J12184187−0609123. The bolometric energy released in the flare is estimated to be ∼1036 erg from its X-ray light curve. The Hα emission-line profile obtained at about 7 hr after the trigger shows an evident blue asymmetry with a maximum velocity of 200–250 km s−1. The blue wing can be likely explained by the chromospheric temperature (cool) upflow associated with chromospheric evaporation, in which the mass of the evaporating plasma is estimated to be 1.2 × 1018 g. In addition, a prominence eruption with an estimated mass of 7 × 1015 g < M p < 7 × 1018 g cannot be entirely excluded.