XRISM view of a stellar flare: High-resolution Fe K spectra of HR 1099, an RS CVn-type star

TL;DR

This study presents the first stellar flare observed with an X-ray microcalorimeter, revealing detailed Fe K spectra and DEM distribution, and showing elemental abundance changes during the flare.

Contribution

It provides high-resolution Fe K spectra of a stellar flare with XRISM, enabling detailed DEM and abundance analysis without assuming elemental abundances.

Findings

Resolved inner-shell and outer-shell Fe lines across a wide temperature range

Constructed a bimodal DEM distribution showing hotter component increase during flare

Observed elemental abundance increase for Ca and Fe during the flare

Abstract

A high-resolution X-ray spectroscopic observation was made of the RS CVn-type binary star HR 1099 using the Resolve instrument onboard XRISM for its calibration purposes. During the $\sim$400 ks telescope time covering 1.5 binary orbit, a flare lasting for $\sim$100 ks was observed with a released X-ray radiation energy of $\sim 10^{34}$ erg, making it the first stellar flare ever observed with an X-ray microcalorimeter spectrometer. The flare peak count rate is 6.4 times higher than that in quiescence and is distinguished clearly in time thanks to the long telescope time. Many emission lines were detected in the 1.7--10 keV range both in the flare and quiescent phases. Using the high spectral resolution of Resolve in the Fe K band (6.5--7.0 keV), we resolved the inner-shell lines of Fe XIX--XXIV as well as the outer-shell lines of Fe XXV--XXVI. These lines have peaks in the contribution functions at different temperatures over a wide range, allowing us to construct the differential emission measure (DEM) distribution over the electron temperature of 1--10 keV (roughly 10--100 MK) based only on Fe lines, thus without an assumption of the elemental abundance. The reconstructed DEM has a bimodal distribution, and only the hotter component increased during the flare. The elemental abundance was derived based on the DEM distribution thus constructed. A significant abundance increase was observed during the flare for Ca and Fe, which are some of the elements with the lowest first ionization potential among those analyzed, but not for Si, S, and Ar. This behavior is seen in some giant solar flares and the present result is a clear example in stellar flares.