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

TL;DR

This study reports the first detailed multi-wavelength observation of a superflare on the RS CVn star HD22468, detected at hard X-ray energies, revealing insights into the flare's energy, plasma temperature, and associated chromospheric activity.

Contribution

It provides the first simultaneous hard X-ray and optical observations of a stellar superflare, including spectral analysis and mass estimates of ejected material, advancing understanding of stellar flare mechanisms.

Findings

Bolometric energy of the flare: ~7.2×10^{37} to 1.7×10^{38} erg.

Hard X-ray spectra fit by a plasma at ~106 MK.

Optical Hα line shows blueshift indicating chromospheric activity.

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 $\sim7.2\times10^{37}-1.7\times10^{38}\ \mathrm{erg}$. 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 $106^{+27}_{-22}$~MK. In the optical range, the H$\alpha$ emission-line profile obtained at $\sim1.7$ hrs after the trigger shows a bulk blueshift of $-96\pm20\ \mathrm{km\ s^{-1}}$, which can be explained by either a chromospheric evaporation or a prominence eruption. The ejected mass is estimated to be $3.9\times10^{20}$ g for the evaporating plasma, and to be $3.2\times10^{21}\ \mathrm{g}<M_{\mathrm{p}}<8.8\times10^{21}\ \mathrm{g}$ for the erupted prominence.