High-velocity blue-shifted Fe XXV He$\alpha$ line during a superflare of the RS CVn-type star IM Peg

Shun Inoue; Wataru Buz Iwakiri; Teruaki Enoto; Hiroyuki Uchida; Miki Kurihara; Masahiro Tsujimoto; Yuta Notsu; Kenji Hamaguchi; Keith Gendreau; Zaven Arzoumanian; Takeshi Go Tsuru

arXiv · 2406.06940

High-velocity blue-shifted Fe XXV He$\alpha$ line during a superflare of the RS CVn-type star IM Peg

Shun Inoue·Wataru Buz Iwakiri·Teruaki Enoto·Hiroyuki Uchida·Miki Kurihara·Masahiro Tsujimoto·Yuta Notsu·Kenji Hamaguchi·Keith Gendreau·Zaven Arzoumanian·Takeshi Go Tsuru

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Abstract

Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing $5\times 10^{37}$ erg in 2$-$10 keV, of the RS CVn-type star IM Peg at 10:41 UT on 2023 July 23 with its Gas Slit Camera (GSC; 2$-$30 keV). We conducted X-ray follow-up observations of the superflare with Neutron Star Interior Composition ExploreR (NICER; 0.2$-$12 keV) starting at 16:52 UT on July 23 until 06:00 UT on August 2. NICER X-ray spectra clearly showed emission lines of the Fe XXV He$\alpha$ and Fe XXVI Ly$\alpha$ for $\sim 1.5$ days since the MAXI detection. The Fe XXV He$\alpha$ line was blue-shifted with its maximum Doppler velocity reaching $-2200 \pm 600$ $\mathrm{km \: s^{-1}}$, 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$\alpha$ line is blue-shifted during a stellar flare and its velocity overwhelmingly exceeds the escape velocity of the star ($-230$ $\mathrm{km \: s^{-1}}$). 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.

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Shun Inoue, Wataru Buz Iwakiri, Teruaki Enoto, Hiroyuki Uchida, Miki Kurihara, Masahiro Tsujimoto, Yuta Notsu, Kenji Hamaguchi, Keith Gendreau, Zaven Arzoumanian, Takeshi Go Tsuru. 2024-06-11. High-velocity blue-shifted Fe XXV He$\alpha$ line during a superflare of the RS CVn-type star IM Peg. https://arxiv.org/abs/2406.06940

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