High-velocity Blue-shifted Fe XXV Heα Line during a Superflare of the RS Canum Venaticorum–type Star IM Peg

The 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 (2‑30 keV). We conducted X-ray follow-up observations of the superflare with the 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 blueshifted 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 blueshifted 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 the reheating of the flare plasma was observed. We discuss the origin of the blueshift, a CME, or high-velocity chromospheric evaporation.