Origin of the stellar Fe K{\alpha} line clarified with FUV and X-ray observations of a superflare on the RS Canum Venaticorum-type Star UX Arietis

TL;DR

This study uses simultaneous FUV and X-ray observations of a stellar superflare to conclusively determine that the Fe Kα line originates from photoionization, enhancing understanding of stellar flare mechanisms.

Contribution

It provides the first clear evidence that stellar Fe Kα lines are primarily produced by photoionization, supported by multi-wavelength observations and radiative transfer modeling.

Findings

Fe Kα line peaks with thermal X-ray emission, not FUV non-thermal activity.

Statistical significance of Fe Kα detection is 5.3 sigma.

Supports using Fe Kα line as a diagnostic for flare geometry.

Abstract

Fluorescence line diagnostics of the Fe K{\alpha} line at $\sim 6.4$ 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{\alpha} lines: i.e., it is unclear which of the two mechanisms-photoionization by hard X-ray photons or collisional ionization by non-thermal 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 Extreme ultraviolet spetrosCope for ExosphEric Dynamic (EXCEED; 900$-$1480 \r{A}) onboard Hisaki and Neutron Star Interior Composition Explorer (NICER; 0.2$-$12 keV). The flare started at 22:50 UT on 2018 November 15 and released $2 \times 10^{36}$ erg in the 900$-$1480 \r{A} band and $3 \times 10^{36}$ erg in the 0.3$-$4 keV band. The FUV emission, a proxy for non-thermal activity, peaked approximately 1.4 hours before the soft X-rays. In contrast, the Fe K{\alpha} line, detected at a statistical significance of $5.3 \sigma$ with an equivalent width of $67^{+28}_{-20}$ eV, peaked simultaneously with the thermal X-ray maximum rather than the non-thermal FUV peak-strongly supporting the photoionization hypothesis. Radiative transfer calculations, combined with the observed Fe K{\alpha} line intensity, further support the photoionization scenario and demonstrate the potential of this line to provide the flare geometry.