Origin of the Stellar Fe K <i>α</i> Line Clarified with Far-ultraviolet and X-Ray Observations of a Superflare on the RS Canum Venaticorum–type Star UX Arietis

Abstract Fluorescence line diagnostics of the Fe K α line at ∼6.4 keV observed in both solar and stellar flares can constrain the latitude and size of the flare loop, even in the absence of imaging observations. However, they are hampered by the unresolved origin of stellar Fe K α lines; i.e., it is unclear which of two mechanisms—photoionization by hard X-ray photons or collisional ionization by nonthermal electrons—is the dominant process. We present clear evidence for the photoionization origin based on simultaneous far-ultraviolet (FUV) and soft X-ray observations of a superflare on the RS Canum Venaticorum–type star UX Arietis with the Extreme Ultraviolet Spectroscope for Exospheric Dynamics instrument (900−1480 Å) on board the Hisaki and Neutron Star Interior Composition Explorer (0.2−12 keV). The flare started at 22:50 UT on 2018 November 15 and released 2 × 10 36 erg in the 900−1480 Å band and 3 × 10 36 erg in the 0.3−4 keV band. The FUV emission, a proxy for nonthermal activity, peaked approximately 1.4 hr before the soft X-rays. In contrast, the Fe K α line, detected at a statistical significance of 5.3 σ with an equivalent width of 6 7 − 20 + 28 eV, peaked simultaneously with the thermal X-ray maximum rather than the nonthermal FUV peak—strongly supporting the photoionization hypothesis. Radiative transfer calculations, combined with the observed Fe K α line intensity, further support the photoionization scenario and demonstrate the potential of this line to provide the flare geometry.