High-velocity blue-shifted Fe XXV Heα line during a superflare of the RS CVn-type star IM Peg

Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing 5× 10³⁷ erg in 2$-$10 keV, of the RS CVn-type star IM Peg at 10:41 UT on 2023 July 23 with its Gas Slit Camera (GSC; 2$-$30 keV). We conducted X-ray follow-up observations of the superflare with Neutron Star Interior Composition ExploreR (NICER; 0.2$-$12 keV) starting at 16:52 UT on July 23 until 06:00 UT on August 2. NICER X-ray spectra clearly showed emission lines of the Fe XXV Heα and Fe XXVI Lyα for ~ 1.5 days since the MAXI detection. The Fe XXV Heα line was blue-shifted with its maximum Doppler velocity reaching -2200 ± 600 km \: s⁻¹, suggesting an upward-moving plasma during the flare, such as a coronal mass ejection (CME) and/or chromospheric evaporation. This is the first case that the Fe XXV Heα line is blue-shifted during a stellar flare and its velocity overwhelmingly exceeds the escape velocity of the star ($-230$ km \: s⁻¹). One hour before the most pronounced blueshift detection, a signature of reheating the flare plasma was observed. We discuss the origin of the blueshift, a CME or high-velocity chromospheric evaporation.