X-ray and Hα superflare on an RS CVn-type star, UX Arietis: Constraint on the flare location from radial velocity change during the flare

Abstract

We report on a giant stellar flare from the RS CVn-type binary UX Arietis, detected with the Monitor of All-sky X-ray Image (MAXI) and followed by a 12-day optical spectroscopic campaign using the 3.8~m Seimei Telescope. The flare released $5 \times 10^{37}$~erg in X-rays (0.1--100~keV) and $(2$--$6) \times 10^{36}$~erg in the H$α$ line, placing it among the most energetic events of its kind. The H$α$ light curve showed sinusoidal modulation atop an exponential decay, consistent with reappearance of the flaring region due to binary rotation. At orbital phase 0, when the primary star is farthest from the observer, 40\% of the H$α$ flux was obscured, while at phase 0.5 the full emission was visible. This suggests the H$α$ emitting region is located at a relatively low latitude and is comparable in size to the stellar disk. Radial velocity modulation implies that the region lies at $\sim19\,R_{\odot}$ from the system's rotation axis, farther out than the stellar limb at $14.4\,R_{\odot}$. Photometric monitoring with the Chuo-university Astronomical Telescope revealed a large low-latitude starspot covering $\sim25\%$ of the surface. These findings are consistent with a scenario in which the flare occurred above the starspot, and the H$α$-emitting plasma was magnetically confined in a loop extending at least $5\,R_{\odot}$ above the stellar surface. From the MAXI data and assuming a radiatively cooling plasma, the electron density and volume are estimated to be $10^{10}$~cm$^{-3}$ and $1 \times 10^{35}$~cm$^3$, respectively. If cubic in shape, this corresponds to $7\,R_{\odot}$, consistent with the H$α$ region height. These results provide direct constraints on the geometry of the plasma and its spatial relationship with the starspot in one of the most energetic stellar flares ever observed.

Figures (12)

• Figure 1: One example of the H$\alpha$ spectra normalized by continuum level. A black solid line represents a flaring spectrum on 2022 April 3, while a gray line is a quiescent spectrum obtained in the post flare phase of 2022 April 22. The standard date in the legend is from MJD 59670 as in Figure \ref{['fig:lc']}. Alt text: Line graph comparing two H$\alpha$ spectra during flare and quiescent phases.
• Figure 2: Light curve of a superflare in the X-ray and H$\alpha$ line bands. Black open circles with error bars represent the count rate in the 2.0-6.0 keV band observed with MAXI, while red filled circles with error bars represent the equivalent width of H$\alpha$ line observed with Seimei telescope. The error bars for the first seven data points for Seimei are too small and are buried within the symbol. All error bars represent 1 sigma confidence level. The bottom horizontal axis shows time from April 1, 2022 0:00 UT (= MJD-59670.0). The top horizontal axis indicates the phase of the orbital period (Orbital Period = 6.437888 days). Alt text: Line graph showing time variation of X-ray and H$\alpha$ emission
• Figure 3: Enlarged view of the light curve of the superflare in the 2.0--6.0 keV band obtained with MAXI. The data points are binned by 4 orbits. The black dashed line is the best fit model for the exponential decay model. The black arrow represents the range used for the fitting. Alt text: Line graph of an X-ray light curve with an exponential decay fit.
• Figure 4: X-ray spectrum observed with MAXI during the flare peak of MJD 59672.25–59672.65. (Upper) The data points with error bars are the observed X-ray spectrum and the blue line is the best-fit model spectrum. (Lower) Residuals for the best-fit model. Error bards represent 90 % confidence level. Alt text: Two-panel graph showing an X-ray spectrum with a model fit and residuals.
• Figure 5: Daily variations of the H$\alpha$ spectra from April 3 (top) to 2022 April 10 (bottom). The dashed line represents the central wavelength of the H$\alpha$ line in the stationary state, at 6562.8 Å. Dates indicate the time elapsed since MJD 59670.0. Alt text: Line graphs showing daily H$\alpha$ spectral variations