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

TL;DR

This paper reports a multi-wavelength study of SVOM J00365+0033, a stellar superflare on the RS CVn-type star HD 22468 detected in hard X-rays by SVOM/ECLAIRs. Modeling the X-ray spectra with the APEC plasma model yields a peak temperature of about $kT\approx106$ MK, and the flare’s bolometric energy reaches up to $E_{bol}\sim(7.2\times10^{37}-1.7\times10^{38})$ erg, with an optical white-light counterpart of $E_R\approx1.2\times10^{37}$ erg. Time-resolved Hα spectroscopy shows a bulk blueshift of $\Delta v_{H\alpha} \approx -96\pm20$ km s⁻¹ around 1.7 hours post-trigger, consistent with chromospheric evaporation or a prominence eruption. The event is analyzed in the EM–T framework, yielding loop parameters such as a length $L\approx1.7\times10^{12}$ cm, electron density $n_e\approx5.2\times10^{9}$ cm⁻³, and magnetic field $B\approx66$ G, highlighting a powerful magnetic reconnection process in a close RS CVn binary. The work discusses discrepancies between X-ray and WL bolometric energy estimates and explores the nature of energy transport to the lower atmosphere, contributing to our understanding of extreme stellar flares and their impact on habitability considerations for exoplanets around active stars.

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$α$ 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.

Figures (5)

• Figure 1: The discovery image of SVOM J00365+0033 (ID$=$sb25010902) taken by SVOM/ECLAIRs. The source is marked by the blue circle. The bright source at the bottom right of SVOM J00365+0033 is the Crab. The blank at the right-bottom corner is due to the obscuration of the Earth.
• Figure 2: Upper-left panel: Hard X-ray spectrum of SVOM J00365+0033 observed by SVOM/ECLAIRs in the epoch of $[t_0,\ t_0+940]$s, where $t_0$ is the trigger time. The best-fit apec model is overplotted by the red line. A fixed abundance of $0.69Z_\odot$ is adopted in the modeling. The subpanel underneath the spectrum shows the residuals, in units of $\mathrm{counts\ s^{-1}\ keV^{-1}}$, of the observed data from the best-fit model. Upper-right and lower-left panels: the same as the left one, but for the epochs of $[t_0+1444,\ t_0+3500]$s and $[t_0+7273,\ t_0+9300]$s, respectively.
• Figure 3: Top panel: the hard X-ray light curve of SVOM J00365+0033 detected by SVOM/ECLAIRs in $4-120$ keV band. The dark-blue shadow region at the bottom of the panel marks the quiescent emission level estimated from the 3XMM catalog (see main text for the details). The sky-blue region corresponds to the $3\sigma$ ECLAIRs detection thresholds for a 1 000 seconds exposure. $T_0$ is the ECLAIRs trigger time. Middle panel: evolution of the modeled plasma temperature. Bottom panel: the white-light light curve of HD 22468 monitored by the SVOM/GWAC FFoV camera in 2025, January 09, after binning the data by averaging the measurements within every 10 minutes. The epoch of our first spectroscopic follow-up is marked by the downward magenta arrow.
• Figure 4: Upper panel: a comparison of the spectra of HD 22468 taken in the three different epochs. Note that the spectrum in the quiescent state (black line) obtained in 2025, February 05 is shifted vertically by an arbitrary amount for visibility. The insert panel compares the H$\alpha$ line profiles obtained in the three different epochs. Bottom panels: the modeling of the Fe$\;$$\lambda$6496 absorption feature and the H$\alpha$ emission line in the three epochs. Each line is reproduced by a Gaussian function. In each sub-panel, the observed and modeled line profiles are plotted by the black and red (blue) solid lines, respectively. The curves underneath each line spectrum present the residuals between the observed and modeled profiles.
• Figure 5: EM$-T$ diagram. The event SVOM J00365+0033 is marked with the red star. The triangles show the RS CVn-type stars (Tsuru et al. 1989; Endl et al. 1997; Franciosini et al. 2001; Pandey & Singh 2012; Tsuboi et al. 2016; Sasaki et al. 2021; Karmakar et al. 2023; Mao et al. 2025), and the crosses the main-sequence stars (Pye et al. 2015). The gray dots mark the positions of young stellar objects extracted from Getman & Feigelson (2021). The filled polygons denote the solar flares (Feldman et al. 1995) and solar microflares (Shimizu 1995). The dashed lines indicate the relation of $\mathrm{EM}\propto B^{-5}T^{17/2}$ for a constant magnetic field, and the dotted lines the relation of $\mathrm{EM}\propto L^{5/3}T^{8/3}$ at certain loop lengths (Shibata & Yokoyama 1999).