Systematic NICER study of the low-ionized Fe K$\hat{I}\pm$ line on RS Canum Venaticorum type stars

Abstract

The Fe K$\hat{I}\pm$ fluorescence line ($\sim 6.4$ keV) has been observed during solar and stellar flares. Two emission mechanisms of the Fe K$\hat{I}\pm$ line, photoionization and collisional ionization, have been discussed, and the aim of this work is to collect evidences for each mechanism employing a statistical correlation approach between the Fe K$\hat{I}\pm$ line flux and rough flare properties. Here, we systematically searched the NICER (0.2$-$12 keV) archive data for the Fe K$\hat{I}\pm$ line of RS Canum Venaticorum type stars. Among our analyzed 255 observation IDs with a total exposure of $\sim 700$ ks, we found 25 data sets (total $\sim 40$ ks) exhibiting the Fe K$\hat{I}\pm$ emission line at 6.37$-$6.54 keV with its equivalent width of 44.3$-$578.4 eV: 18 observations during flares, 6 observations during unconfirmed possible flare candidates and one at a quiescent phase. These observations indicate a positive correlation between the Fe K$\hat{I}\pm$ line intensity and the 7.11$-$20 keV thermal plasma luminosity with its powerlaw index of $0.86\pm 0.46$. This correlation in the range of the thermal plasma luminosity $10^{29-33}$ erg s${}^{-1}$ is consistent with the photoionization origin of the line. On the other hand, the equivalent width of the Fe K$\hat{I}\pm$ line has a negative correlation with the 7.11$-$20 keV thermal plasma luminosity with its powerlaw index of $-0.27\pm 0.10$. This anti-correlation is consistent with the decline of the fluorescence efficiency with increasing the stellar flare loop height. Furthermore, we found a signature of an absorption line at $6.38_{-0.04}^{+0.03}$ keV during a superflare of $\ddot{I}ƒ$ Gem. The equivalent width of the line was $-34.7_{-1.58}^{+2.03}$ eV. We discuss the density of the Fe ions from the equivalent width using the curve of growth analysis.

19 pages, 12 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society