X-ray and H{\alpha} superflare on an RS CVn-type star, UX Arietis: Constraint on the flare location from radial velocity change during the flare
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$\alpha$ line, placing it among the most energetic events of its kind. The H$\alpha$ 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$\alpha$ flux was obscured, while at phase 0.5 the full emission was visible. This suggests the H$\alpha$ 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$\alpha$-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$\alpha$ 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.