High-velocity Blue-shifted Fe xxv Heα Line during a Superflare of the RS Canum Venaticorum–type Star IM Peg
Shun Inoue, W. Iwakiri, Teruaki Enoto, Hiroyuki Uchida, Miki Kurihara, Masahiro Tsujimoto, Yuta Notsu, Kenji Hamaguchi, Keith C. Gendreau, Zaven Arzoumanian, Takeshi Go Tsuru
Abstract
Abstract The Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing 5 × 10 37 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 He α and Fe xxvi Ly α for ∼1.5 days since the MAXI detection. The Fe XXV He α 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 He α 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.