Multi-wavelength Study of A Superflare on RS CVn-type Star HD22468 Triggered at Hard X-ray by SVOM

Abstract

Detection of stellar flares at hard X-ray is still rare at the current stage. A transient was recently detected by the hard X-ray camera, ECLAIRs onboard the SVOM mission at 11:39:01.2UT on 2025, January 09. Simultaneous monitor in the optical band on the ground by SVOM/GWAC and follow-up spectroscopy enable us to confirm that the transient is caused by a superflare on HD~22468, a RS CVn-type star. The bolometric energy released in the flare is estimated to be $\sim 7.2\times 10^{37}-1.7\times 10^{38}\mathrm{erg}$. The hard X-ray spectra of the event at the peak can be reproduced by the ``apec'' model of a hot plasma with a temperature of $106_{-22}^{+27}$~MK. In the optical range, the H$\hat{I}\pm$ emission-line profile obtained at $\sim 1.7$ hrs after the trigger shows a bulk blueshift of $-96\pm 20\mathrm{km}{\mathrm{s}}^{-1}$, which can be explained by either a chromospheric evaporation or a prominence eruption. The ejected mass is estimated to be $3.9\times 10^{20}$ g for the evaporating plasma, and to be $3.2\times 10^{21}\mathrm{g}<M_{\mathrm{p}}<8.8\times 10^{21}\mathrm{g}$ for the erupted prominence.

14 pages, 5 figures, and 3 tables. Accepted by ApJ