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Investigating the emission mechanism of the Fe Kα and Lα lines of stellar flares on RS CVn-type stars

Published online by Cambridge University Press:  02 October 2026

Shun Inoue Open the ORCID record for Shun Inoue [Opens in a new window] ,
Teruaki Enoto ,
Yuta Notsu Open the ORCID record for Yuta Notsu [Opens in a new window] ,
Wataru Buz Iwakiri ,
Tomoki Kimura ,
Hiroyuki Uchida ,
Kenji Hamaguchi ,
Kosuke Namekata Open the ORCID record for Kosuke Namekata [Opens in a new window] ,
Zaven Arzoumanian  and
Keith Gendreau
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Shun Inoue*
Affiliation:
Department of Physics, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
Teruaki Enoto
Affiliation:
Department of Physics, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan RIKEN Center for Advanced Photonics (RAP), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Yuta Notsu
Affiliation:
Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, 3665 Discovery Drive, Boulder, CO 80303, USA National Solar Observatory, 3665 Discovery Drive, Boulder, CO 80303, USA
Wataru Buz Iwakiri
Affiliation:
International Center for Hadron Astrophysics, Chiba University, Inage-ku, Chiba, 263-8522 Japan
Tomoki Kimura
Affiliation:
Department of Physics, Faculty of Science, Tokyo University of Science, Tokyo 162-8601, Japan
Hiroyuki Uchida
Affiliation:
Department of Physics, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
Kenji Hamaguchi
Affiliation:
CRESST II and X-ray Astrophysics Laboratory, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA Department of Physics, University of Maryland, Baltimore County, Baltimore, MD, USA
Kosuke Namekata
Affiliation:
Department of Physics, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan Division of Science, National Astronomical Observatory of Japan, NINS, Osawa, Mitaka, Tokyo, 181-8588, Japan Heliophysics Science Division, NASA’s Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
Zaven Arzoumanian
Affiliation:
Astrophysics Science Division, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
Keith Gendreau
Affiliation:
Astrophysics Science Division, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
Shin Toriumi
Affiliation:
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
Atsushi Yamazaki
Affiliation:
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
Go Murakami
Affiliation:
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
Fuminori Tsuchiya
Affiliation:
Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Sendai, Japan
Kazuo Yoshioka
Affiliation:
Department of Complexity Science and Engineering, Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Japan
*
email: inoue@cr.scphys.kyoto-u.ac.jp
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Abstract

The low-ionized Fe K$\alpha$ fluorescence line ($\sim$6.4 keV) has been observed during both solar and stellar flares. The emission mechanism of the Fe K$\alpha$ line has been attributed to the photoionization of neutral or low-ionized iron in the photosphere by coronal X-rays. Alternatively, some studies have suggested that collisional ionization by nonthermal electrons may also contribute. Determining whether photoionization or collisional ionization dominates is important because the Fe K$\alpha$ line has the potential to probe the spatially unresolved geometry of stellar flare loops. RS CVn-type stars are magnetically active systems that produce powerful superflares exceeding $10^{35}$ erg, providing X-ray spectra of stellar flares with high photon statistics. In this proceeding, we summarize the results of our systematic analysis of the Fe K$\alpha$ line in RS CVn-type stars (Inoue et al. 2025). We identified 25 new Fe K$\alpha$ detections and found a positive correlation between the Fe K$\alpha$ line luminosity ($L_{\mathrm{K \alpha}}$) and the 7.11-20 keV thermal plasma luminosity ($L_{\mathrm{HXR}}$), with a power-law index of $0.86 \pm 0.46$ (i.e., $L_{\mathrm{K \alpha}} \propto L_{\mathrm{HXR}}^{0.86\pm0.46}$), and a negative correlation between the Fe K$\alpha$ line equivalent width ($\mathrm{EW}_{\mathrm{K \alpha}}$) and the 7.11$-$20 keV thermal plasma luminosity with its powerlaw index of $-0.27 \pm 0.10$ (i.e., $\mathrm{EW}_{\mathrm{K \alpha}} \propto L_{\mathrm{HXR}}^{-0.27\pm0.10}$). These results support the dominance of photoionization from a new perspective that has not previously been explored in the context of stellar flares.

Keywords

Stellar flaresStellar coronal mass ejectionsstars: RS CVn-type

Information

Type
Contributed Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S400: Solar and Stellar Multi-scale Activity , October 2025 , pp. 129 - 134
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Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

The Fe K alpha $\alpha$ line ( tilde $\sim$ 6.4 keV) is produced when X-ray photons or electrons with energies above the Fe K-edge (7.11 keV) irradiate cold matter, ionizing iron atoms and leading to X-ray emission from transitions into the K-shell. The Fe K alpha $\alpha$ fluorescence line has been detected in flares from pre-main-sequence stars (e.g., Tsujimoto et al. Reference Tsujimoto, Feigelson, Grosso, Micela, Tsuboi, Favata, Shang and Kastner2005), the Sun (e.g., Tanaka et al. Reference Tanaka, Watanabe and Nitta1984), and active late-type stars (e.g., Osten et al. Reference Osten2010). In pre-main-sequence stars, this phenomenon is generally attributed to disk irradiation, whereas in late-type stars, it is thought to arise from photospheric iron ionized by coronal X-rays. In both cases, the dominant process is assumed to be photoionization, although collisions with nonthermal electrons may also contribute.

The question of whether Fe K alpha $\alpha$ emission in solar flares is dominated by photoionization or by nonthermal electron collisions was actively debated in the 1980s, when numerous Fe K alpha $\alpha$ detections were made by Hinotori (Tanaka et al. Reference Tanaka, Watanabe, Nishi and Akita1982, Reference Tanaka, Nitta, Akita and Watanabe1983, Reference Tanaka, Watanabe and Nitta1984) and the Solar Maximum Mission (SMM) (Parmar et al. Reference Parmar, Culhane, Rapley, Phillips, Wolfson, Acton and Dennis1982, Reference Parmar, Culhane, Rapley, Wolfson, Acton, Phillips and Dennis1984). Figure 1 summarizes the history of Fe K alpha $\alpha$ observations in solar and stellar flares. Since the termination of SMM in 1989, however, no X-ray spectroscopic observations covering the Fe K alpha $\alpha$ band have been performed for the Sun for more than 30 years, except for the recent indirect observations by XRISM (Suzuki et al. Reference Suzuki2025), leaving the origin of the line an unresolved question. In contrast, several stellar flares with Fe K alpha $\alpha$ emission have been reported in the 21st century using Chandra and Swift (e.g., Osten et al. Reference Osten, Drake, Tueller, Cummings, Perri, Moretti and Covino2007, Reference Osten2010). In the stellar case, if the line originates from photoionization, it can be used to estimate the latitude and size of spatially unresolved flare loops (Testa et al. Reference Testa, Drake, Ercolano, Reale, Huenemoerder, Affer, Micela and Garcia-Alvarez2008). Thus, identifying the emission mechanism is of critical importance.

Figure 1.

Histogram showing the number of solar (orange) and stellar (blue) flares in which the Fe K alpha $\alpha$ line has been detected. Solar flare samples are taken from Neupert et al. (Reference Neupert, Gates, Swartz and Young1967); Neupert (Reference Neupert1971); Doschek et al. (Reference Doschek, Meekins, Kreplin, Chubb and Friedman1971); Bai (Reference Bai1979); Feldman et al. (Reference Feldman, Doschek and Kreplin1980); Culhane et al. (Reference Culhane1981); Parmar et al. (Reference Parmar, Culhane, Rapley, Phillips, Wolfson, Acton and Dennis1982); Tanaka et al. (Reference Tanaka, Watanabe, Nishi and Akita1982, Reference Tanaka, Nitta, Akita and Watanabe1983); Parmar et al. (Reference Parmar, Culhane, Rapley, Wolfson, Acton, Phillips and Dennis1984); Tanaka et al. (Reference Tanaka, Watanabe and Nitta1984); Tanaka & Zirin (Reference Tanaka and Zirin1985); Emslie et al. (Reference Emslie, Phillips and Dennis1986); Zarro et al. (Reference Zarro, Dennis and Slater1992); Phillips et al. (Reference Phillips, Pike, Lang, Zarro, Fludra, Watanabe and Takahashi1995); Phillips (Reference Phillips2012); Suzuki et al. (Reference Suzuki2025), and stellar flare samples from Osten et al. (Reference Osten, Drake, Tueller, Cummings, Perri, Moretti and Covino2007); Testa et al. (Reference Testa, Drake, Ercolano, Reale, Huenemoerder, Affer, Micela and Garcia-Alvarez2008); Osten et al. (Reference Osten2010); Huenemoerder et al. (Reference Huenemoerder, Schulz, Testa, Drake, Osten and Reale2010); Karmakar et al. (Reference Karmakar, Pandey, Airapetian and Misra2017); Inoue et al. (Reference Inoue, Enoto, Notsu, Uchida, Iwakiri, Namekata and Gendreau2025). Note that pre-main-sequence stars are excluded, as their Fe K alpha $\alpha$ lines originate from disk irradiation and are distinct in nature from those observed in solar and late-type stellar flares.

A bar graph showing the number of solar and stellar flares with detected Fe Kα lines over various time periods.

The Neutron Star Interior Composition Explorer (NICER; Gendreau et al. Reference Gendreau2016) has observed many superflares on RS CVn-type stars since 2017, benefiting from its large effective area of tilde 600 normal c normal m squared $\sim600\ \mathrm{cm}^{2}$ around 6 keV (Gendreau et al. Reference Gendreau, Arzoumanian and Okajima2012; Arzoumanian et al. Reference Arzoumanian and Takahashi2014). RS CVn-type stars are magnetically active binaries that provide X-ray spectra of large superflares with high photon statistics (e.g., Kurihara et al. Reference Kurihara2024; Inoue et al. Reference Inoue2024). Despite this, no Fe K alpha $\alpha$ detections from RS CVn-type stellar flares with NICER have been reported to date. Motivated by this, we hypothesized that many unreported Fe K alpha $\alpha$ events remain in the NICER stellar flare archive, and we carried out a systematic analysis. In this work, we present the data analysis, results, and implications of our study.

2. Data Reduction & Analyses

We analyzed all NICER archival data of RS CVn-type stars (UX Ari, GT Mus, sigma $\sigma$ Gem, HD 251108, HR 1099, VY Ari, and DS Tuc), totaling 255 ObsIDs and tilde $\sim$ 700 ks of exposure. The data were processed with <monospace>nicerl2</monospace> (HEASoft 6.32.1, CALDB xti20240206), applying overshoot rate and rigidity filters. Source and background spectra were generated with <monospace>nicerl3-spect</monospace> and the 3C50 model, and GTI-resolved spectra were also extracted using our automated Python pipeline.Footnote 1

Quiescent phases were defined from the lowest non-variable observations. Using 0.3–4 keV light curves, we calculated median count rates upper C Subscript normal q $C_{\mathrm{q}}$ and standard deviation sigma Subscript normal q $\sigma_{\mathrm{q}}$ to set flare thresholds at upper C Subscript normal q Baseline plus 1.65 sigma Subscript normal q $C_{\mathrm{q}}+1.65\sigma_{\mathrm{q}}$ , corresponding to a 90% confidence excess above quiescent variability.

Spectral fitting was carried out in 5–8 keV using <monospace>Xspec</monospace> and <monospace>PyXspec</monospace>. Models included <monospace>apec</monospace> and <monospace>apec+gauss</monospace>, with the Gaussian line fixed at zero width since rotational and Doppler broadening are negligible compared with NICER’s energy resolution (137 eV at 6 keV). The line center energy ( upper E Subscript l $E_{l}$ ) and normalization ( upper F Subscript normal upper K alpha $F_{\mathrm{K\alpha}}$ ) of Gaussian were free parameters, and a detection of the Fe K alpha $\alpha$ line was claimed when upper E Subscript l Baseline equals 6.4 $E_{l} = 6.4$ –6.7 keV with a positive 90% lower limit of upper F Subscript normal upper K alpha $F_{\mathrm{K\alpha}}$ . We measured equivalent widths of the Fe K alpha $\alpha$ line with <monospace>eqwidth</monospace> command and calculated the 7.11–20 keV thermal plasma luminosity ( upper L Subscript normal upper H normal upper X normal upper R $L_{\mathrm{HXR}}$ ) using the <monospace>flux</monospace> command.

3. Results & Discussion

Using the criteria described in Section 2, we identified 24 cases of Fe K alpha $\alpha$ emission during flare phases and one detection during a quiescent phase. Figure 2 presents the light curve and spectrum of a superflare observed in November 2018 on UX Ari as a representative example, in which the Fe K alpha $\alpha$ line was clearly detected.

Figure 2.

Light curves and X-ray spectrum of UX Ari during the superflare in November 2018 taken from Inoue et al. (Reference Inoue, Enoto, Notsu, Uchida, Iwakiri, Namekata and Gendreau2025). (a) 64 s binned 0.3 minus $-$ 4 keV light curve. The vertical is count rate (cps; counts s^-1). The one standard deviation statistical error bars are smaller than the symbol size. The red shaded area indicates the time when the low-ionized Fe emission line is detected. (b)(c) Background-subtracted 5 minus $-$ 8 keV NICER spectra of UX Ari duing the time shown as the red-shaded area in panel a fitted with a CIE model (<monospace>apec</monospace>; panel b) and additionaly with the Gaussian emission line at tilde 6.4 $\sim 6.4$ keV (<monospace>apec+gauss</monospace>; panel c).

Three graphs depict light curves and X-ray spectrum of UX Ari during a superflare.

From these newly identified samples, we obtained the following results:

  1. (1) As shown in Figure 3a, we find a positive correlation between the Fe K alpha $\alpha$ line photon intensity and the 7.11–20 keV thermal plasma luminosity, with a Spearman rank correlation coefficient of 0.74 plus or minus 0.08 $0.74 \pm 0.08$ and a power-law index of upper L Subscript normal upper K alpha Baseline proportional to upper L Subscript normal upper H normal upper X normal upper R Superscript 0.86 plus or minus 0.46 $L_{\mathrm{K \alpha}} \propto L_{\mathrm{HXR}}^{0.86 \pm 0.46}$ ( 1 sigma $1\sigma$ error).

    Figure 3.

    Correlation obtained by our systematic analysis taken from Inoue et al. (Reference Inoue, Enoto, Notsu, Uchida, Iwakiri, Namekata and Gendreau2025). (a) The Fe K alpha $\alpha$ line luminosity vs. the 7.11 minus $-$ 20 keV luminosity of the thermal plasma. The purple shaded area shows the range of upper L Subscript normal upper K alpha Baseline divided by upper L Subscript normal upper H normal upper X normal upper R $L_{\mathrm{K \alpha}} / L_{\mathrm{HXR}}$ from 0.01 $0.01$ to 0.05 $0.05$ . The green dashdot line shows the slope upper L Subscript normal upper K alpha Baseline divided by upper L Subscript normal upper H normal upper X normal upper R Baseline equals 1 $L_{\mathrm{K \alpha}} / L_{\mathrm{HXR}} = 1$ . Green square, blue circle, and orange diamond symbols are the data obtained by NICER during the impulsive phase of a flare, the decay phase of a flare, and the quiescent phase, respectively. The crosses represent observations of the stellar flares reported by Osten et al. (Reference Osten, Drake, Tueller, Cummings, Perri, Moretti and Covino2007, Reference Osten2010) and Karmakar et al. (Reference Karmakar, Pandey, Airapetian and Misra2017). (b) The equivalent width of the Fe K alpha $\alpha$ line vs. the 7.11 minus $-$ 20 keV luminosity of the thermal plasma. The same symbols as in panel a are used.

    Two scatter plots depict the relationship between Fe Kα line properties and thermal plasma luminosity in RS CVn-type stars.
    Figure 3 Long description

    Panel A: A scatter plot shows the Fe Kα line luminosity (erg s^-1) on the vertical axis and the thermal plasma luminosity at 7.11-20 keV (erg s^-1) on the horizontal axis. The purple shaded area indicates the range of L_Kα/L_HXR from 0.01 to 0.05. The green dashdot line represents the slope L_Kα/L_HXR = 1. Data points are marked with green squares, blue circles, and orange diamonds, representing observations during the impulsive phase, decay phase, and quiescent phase, respectively. Crosses represent observations from previous studies. Panel B: Another scatter plot shows the equivalent width of the Fe Kα line (keV) on the vertical axis and the thermal plasma luminosity at 7.11-20 keV (erg s^-1) on the horizontal axis. The same symbols as in Panel A are used to mark the data points.

  2. (2) As shown in Figure 3b, we also find a negative correlation between the equivalent width of the Fe K alpha $\alpha$ line and the 7.11–20 keV thermal plasma luminosity, with a Spearman rank correlation coefficient of negative 0.44 plus or minus 0.12 $-0.44 \pm 0.12$ and a power-law index of normal upper E normal upper W Subscript normal upper K alpha Baseline proportional to upper L Subscript normal upper H normal upper X normal upper R Superscript negative 0.27 plus or minus 0.10 $\mathrm{EW}_{\mathrm{K \alpha}} \propto L_{\mathrm{HXR}}^{-0.27 \pm 0.10}$ ( 1 sigma $1\sigma$ error).

Both of these results support the photoionization origin of the Fe K alpha $\alpha$ line. First, result (1) is consistent with Bai (Reference Bai1979), who showed that the photoionizing flux of the Fe K alpha $\alpha$ emission line, upper F Subscript normal upper K alpha $F_{\mathrm{K \alpha}}$ , can be expressed as

(1) upper F Subscript normal upper K alpha Baseline equals StartFraction normal upper Gamma left parenthesis upper T comma h right parenthesis f left parenthesis theta right parenthesis Over 4 pi d squared EndFraction integral Subscript 7.11 normal k normal e normal upper V Superscript normal infinity Baseline upper L left parenthesis epsilon right parenthesis d epsilon comma \begin{equation}F_{\mathrm{K \alpha}} = \frac{\Gamma (T, h) f (\theta)}{4 \pi d^{2}} \int_{7.11 \mathrm{keV}}^{\infty} L (\varepsilon) d\varepsilon,\end{equation}

where normal upper Gamma left parenthesis upper T comma h right parenthesis $\Gamma (T, h)$ is the fluorescence efficiency, which depends on the plasma temperature T and flare loop height h; f left parenthesis theta right parenthesis $f (\theta)$ describes the angular dependence of fluorescence; d is the distance to the observer (e.g., Sun-Earth distance); and the integral term represents the total luminosity above the Fe K-edge at tilde $\sim$ 7.11 keV. This formulation naturally explains the positive correlation between the Fe K alpha $\alpha$ photon intensity and the 7.11–20 keV thermal plasma luminosity.

Second, result (2) reflects the decline of the fluorescence efficiency, epsilon equals 2 upper L Subscript normal upper K alpha Baseline divided by upper L Subscript normal upper H normal upper X normal upper R $\varepsilon = 2 L_{\mathrm{K\alpha}} / L_{\mathrm{HXR}}$ , and the equivalent width normal upper E normal upper W Subscript normal upper K alpha $\mathrm{EW}_{\mathrm{K\alpha}}$ with increasing loop height h, due to the 1 divided by h squared $1/h^{2}$ dilution of the irradiating flux at the stellar surface (Testa et al. Reference Testa, Drake, Ercolano, Reale, Huenemoerder, Affer, Micela and Garcia-Alvarez2008; Ercolano et al. Reference Ercolano, Drake, Reale, Testa and Miller2008).

4. Summary

We systematically searched the NICER archival data of RS CVn-type stars and identified 25 detections of the Fe K alpha $\alpha$ emission line, along with a positive correlation between the Fe K alpha $\alpha$ line intensity and the 7.11-20 keV thermal plasma luminosity. This indicates that many stellar flares exhibit Fe K alpha $\alpha$ emission and that its dominant production mechanism is photoionization by thermal plasma.

In addition, although the details are not included in this proceeding, simultaneous far-ultraviolet and X-ray observations (Inoue et al., submitted) also support photoionization as the primary mechanism of the Fe K alpha $\alpha$ line, providing an independent perspective complementary to this systematic analysis. Together, these results offer a resolution to the long-standing question dating back to solar X-ray observations in the 1980s.

References

Arzoumanian, Z., et al., 2014, in Takahashi, T., den Herder J.-W. A., Bautz M., eds, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series Vol. 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray. p. 914420, doi:10.1117/12.2056811CrossRefGoogle Scholar
Bai, T., 1979, Sol. Phys., 62, 113 CrossRefGoogle Scholar
Culhane, J. L., et al., 1981, ApJL, 244, L141 CrossRefGoogle Scholar
Doschek, G. A., Meekins, J. F., Kreplin, R. W., Chubb, T. A., Friedman, H., 1971, ApJ, 170, 573 CrossRefGoogle Scholar
Emslie, A. G., Phillips, K. J. H., Dennis, B. R., 1986, Sol. Phys., 103, 89 CrossRefGoogle Scholar
Ercolano, B., Drake, J. J., Reale, F., Testa, P., Miller, J. M., 2008, ApJ, 688, 1315 CrossRefGoogle Scholar
Feldman, U., Doschek, G. A., Kreplin, R. W., 1980, ApJ, 238, 365 CrossRefGoogle Scholar
Gendreau, K. C., Arzoumanian, Z., Okajima, T., 2012, in Takahashi T., Murray S. S., den Herder J.-W. A., eds, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series Vol. 8443, Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray. p. 844313, doi:10.1117/12.926396CrossRefGoogle Scholar
Gendreau, K. C., et al., 2016, in den Herder J.-W. A., Takahashi T., Bautz M., eds, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series Vol. 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray. p. 99051H, doi:10.1117/12.2231304CrossRefGoogle Scholar
Huenemoerder, D. P., Schulz, N. S., Testa, P., Drake, J. J., Osten, R. A., Reale, F., 2010, ApJ, 723, 1558 CrossRefGoogle Scholar
Inoue, S., et al., 2024, ApJL, 969, L12 CrossRefGoogle Scholar
Inoue, S., Enoto, T., Notsu, Y., Uchida, H., Iwakiri, W. B., Namekata, K., Gendreau, K., 2025, MNRAS, 541, 1403 CrossRefGoogle Scholar
Karmakar, S., Pandey, J. C., Airapetian, V. S., Misra, K., 2017, ApJ, 840, 102 CrossRefGoogle Scholar
Kurihara, M., et al., 2024, ApJ, 965, 135 CrossRefGoogle Scholar
Neupert, W. M., 1971, Sol. Phys., 18, 474 CrossRefGoogle Scholar
Neupert, W. M., Gates, W., Swartz, M., Young, R., 1967, ApJL, 149, L79 CrossRefGoogle Scholar
Osten, R. A., Drake, S., Tueller, J., Cummings, J., Perri, M., Moretti, A., Covino, S., 2007, ApJ, 654, 1052 CrossRefGoogle Scholar
Osten, R. A., et al., 2010, ApJ, 721, 785 CrossRefGoogle Scholar
Parmar, A. N., Culhane, J. L., Rapley, C. G., Phillips, K. J. H., Wolfson, C. J., Acton, L. W., Dennis, B. R., 1982, Advances in Space Research, 2, 151 CrossRefGoogle Scholar
Parmar, A. N., Culhane, J. L., Rapley, C. G., Wolfson, C. J., Acton, L. W., Phillips, K. J. H., Dennis, B. R., 1984, ApJ, 279, 866 CrossRefGoogle Scholar
Phillips, K. J. H., 2012, MNRAS, 421, 1757 CrossRefGoogle Scholar
Phillips, K. J. H., Pike, C. D., Lang, J., Zarro, D. M., Fludra, A., Watanabe, T., Takahashi, M., 1995, Advances in Space Research, 15, 33 CrossRefGoogle Scholar
Suzuki, H., et al., 2025, arXiv e-prints, p. arXiv:2509.05029 Google Scholar
Tanaka, K., Zirin, H., 1985, ApJ, 299, 1036 CrossRefGoogle Scholar
Tanaka, K., Watanabe, T., Nishi, K., Akita, K., 1982, ApJL, 254, L59 CrossRefGoogle Scholar
Tanaka, K., Nitta, N., Akita, K., Watanabe, T., 1983, Sol. Phys., 86, 91 CrossRefGoogle Scholar
Tanaka, K., Watanabe, T., Nitta, N., 1984, ApJ, 282, 793 CrossRefGoogle Scholar
Testa, P., Drake, J. J., Ercolano, B., Reale, F., Huenemoerder, D. P., Affer, L., Micela, G., Garcia-Alvarez, D., 2008, ApJL, 675, L97 CrossRefGoogle Scholar
Tsujimoto, M., Feigelson, E. D., Grosso, N., Micela, G., Tsuboi, Y., Favata, F., Shang, H., Kastner, J. H., 2005, ApJS, 160, 503 CrossRefGoogle Scholar
Zarro, D. M., Dennis, B. R., Slater, G. L., 1992, ApJ, 391, 865 CrossRefGoogle Scholar
Figure 0

Figure 1. Histogram showing the number of solar (orange) and stellar (blue) flares in which the Fe Kα$\alpha$ line has been detected. Solar flare samples are taken from Neupert et al. (1967); Neupert (1971); Doschek et al. (1971); Bai (1979); Feldman et al. (1980); Culhane et al. (1981); Parmar et al. (1982); Tanaka et al. (1982, 1983); Parmar et al. (1984); Tanaka et al. (1984); Tanaka & Zirin (1985); Emslie et al. (1986); Zarro et al. (1992); Phillips et al. (1995); Phillips (2012); Suzuki et al. (2025), and stellar flare samples from Osten et al. (2007); Testa et al. (2008); Osten et al. (2010); Huenemoerder et al. (2010); Karmakar et al. (2017); Inoue et al. (2025). Note that pre-main-sequence stars are excluded, as their Fe Kα$\alpha$ lines originate from disk irradiation and are distinct in nature from those observed in solar and late-type stellar flares.

Figure 1

Figure 2. Light curves and X-ray spectrum of UX Ari during the superflare in November 2018 taken from Inoue et al. (2025). (a) 64 s binned 0.3−$-$4 keV light curve. The vertical is count rate (cps; counts s^-1). The one standard deviation statistical error bars are smaller than the symbol size. The red shaded area indicates the time when the low-ionized Fe emission line is detected. (b)(c) Background-subtracted 5−$-$8 keV NICER spectra of UX Ari duing the time shown as the red-shaded area in panel a fitted with a CIE model (apec; panel b) and additionaly with the Gaussian emission line at ∼6.4$\sim 6.4$ keV (apec+gauss; panel c).

Figure 2

Figure 3. Figure 3 long description.Correlation obtained by our systematic analysis taken from Inoue et al. (2025). (a) The Fe Kα$\alpha$ line luminosity vs. the 7.11−$-$20 keV luminosity of the thermal plasma. The purple shaded area shows the range of LKα/LHXR$L_{\mathrm{K \alpha}} / L_{\mathrm{HXR}}$ from 0.01$0.01$ to 0.05$0.05$. The green dashdot line shows the slope LKα/LHXR=1$L_{\mathrm{K \alpha}} / L_{\mathrm{HXR}} = 1$. Green square, blue circle, and orange diamond symbols are the data obtained by NICER during the impulsive phase of a flare, the decay phase of a flare, and the quiescent phase, respectively. The crosses represent observations of the stellar flares reported by Osten et al. (2007, 2010) and Karmakar et al. (2017). (b) The equivalent width of the Fe Kα$\alpha$ line vs. the 7.11−$-$20 keV luminosity of the thermal plasma. The same symbols as in panel a are used.