Systematic NICER study of the low-ionized Fe Kα line on RS Canum Venaticorum-type stars

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The Fe Kα fluorescence line (~ 6.4 keV) has been observed during solar and stellar flares. Two emission mechanisms of the Fe Kα 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 Kα line flux and rough flare properties. Here, we systematically searched the Neutron Star Interior Composition ExploreR (0.2–12 keV) archive data for the Fe Kα line of RS Canum Venaticorum-type stars. Among our analysed 255 observation IDs with a total exposure of ~ 700 ks, we found 25 data sets (total ~ 40 ks) exhibiting the Fe Kα emission line at 6.37–6.54 keV with its equivalent width of 44.3–578.4 eV: 18 observations during flares, six observations during unconfirmed possible flare candidates, and one at a quiescent phase. These observations indicate a positive correlation between the Fe Kα line intensity (LK α) and the 7.11–20 keV thermal plasma luminosity (LHXR) with its power-law index of 0.86 ± 0.46 (i.e. LK α ∝ LHXR0.86± 0.46). This correlation in the range of the thermal plasma luminosity 10²⁹⁻³³ erg s⁻¹ is consistent with the photoionization origin of the line. On the other hand, the equivalent width of the Fe Kα line (EWK α) has a negative correlation with the 7.11–20 keV thermal plasma luminosity with its power-law index of -0.27 ± 0.10 (i.e. EWK α ∝ LHXR-0.27± 0.10). This anticorrelation 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+0.03-0.04 keV during a superflare of σ Gem. The equivalent width of the line was -34.7+2.03-1.58 eV. We discuss the density of the Fe ions from the equivalent width using the curve of growth analysis.