Fe K alpha and hydrodynamic loop model diagnostics for a large flare on II Peg

TL;DR

This study models Fe K alpha emission during a large stellar flare on II Peg, constraining flare height, loop properties, and energetics, and confirms fluorescence as the emission mechanism through comparison with observational data.

Contribution

It provides the first detailed modeling of Fe K alpha emission in a stellar flare, constraining flare geometry and energetics, and demonstrates fluorescence as the likely emission process.

Findings

Flare height constrained to 0.15 R_* with solar abundances.

Estimated plasma density of ~4 x 10^12 cm^-3.

Flare energy output approximately 10^33 erg/sec.

Abstract

The observation by the Swift X-ray Telescope of the Fe K alpha_1, alpha_2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe K alpha from a star other than our Sun. We present models of the Fe K alpha equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionisation of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15 R_* assuming solar abundances for the photospheric material, and 0.1 R_* and 0.06 R_* assuming depleted photospheric abundances ([M/H]=-0.2 and [M/H]=-0.4, respectively). Accounting for an extended loop geometry has the effect of increasing the estimated flare heights by a factor of ~3. These predictions are consistent with those derived using results of flaring loop models, which are also used to estimate the flaring loop properties and energetics. From loop models we estimate a flare loop height of 0.13 R_*, plasma density of ~4 * 10^12 cm^-3 and emitting volume of ~6 * 10^30 cm^3. Our estimates for the flare dimensions and density allow us to estimate the conductive energy losses to E_cond <= 2 * 10^36 erg, consistent with upper limits previously obtained in the literature. Finally, we estimate the average energy output of this large flare to be ~10^33 erg sec^-1, or 1/10th of the stellar bolometric luminosity.