Multiwavelength Study of a Superflare on RS CVn–Type Star HD 22468 Triggered at Hard X-Ray by SVOM

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 on board the SVOM mission at 11:39:01.2 UT on 2025 January 09. Simultaneous monitoring in the optical band on the ground by SVOM/GWAC and follow-up spectroscopy enabled us to confirm that the transient is caused by a superflare on HD 22468, an RS CVn–type star. The bolometric energy released in the flare is estimated to be ∼7.2 × 1037–1.7 × 1038 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 Hα emission-line profile obtained at ∼1.7 hr after the trigger shows a bulk blueshift of −96 ± 20 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 × 1020 g for the evaporating plasma and to be 3.2 × 1021 g < Mp < 8.8 × 1021 g for the erupted prominence.