A Statistical Study of Soft X-ray Flares on Solar-type Stars

Abstract

The statistical characteristic of stellar flares at optical bands has received an extensive study, but it remains to be studied at soft X-ray bands, in particular for solar-type stars. Here, we present a statistical study of soft X-ray flares on solar-type stars, which can help understand multi-wavelength behaviors of stellar flares. We mainly use Chandra Source Catalog Release 2.0, which includes a number of flaring stars with denoted variability, and Gaia Data Release 3, which includes necessary information for classifying stars. We also develop a set of methods for identifying and classifying stellar soft X-ray flares and estimating their properties. A detailed statistical investigation for 129 flare samples on 103 nearby solar-type stars as selected yields the following main results. (1) The flare energy emitted at the soft X-ray band in our sample ranges from $\sim 10^{33}$ to $\sim 10^{37}\mathrm{erg}$, and the majority of them are superflares with the most energetic one having energy of $6.0_{-4.7}^{+3.2}\times 10^{37}\mathrm{erg}$. (2) The flare duration is related to its energy as formulated by $T_{\mathrm{duration},\mathrm{SXR}}\propto E_{\mathrm{flare},\mathrm{SXR}}^{0.201\pm 0.024}$, which is different from those derived at optical and NIR bands, indicating distinct radiation mechanisms at different bands. (3) The frequency distribution of stellar flares as a function of energy is formulated as $\mathrm{d}{N}_{\mathrm{flare}}/\mathrm{d}{E}_{\mathrm{flare},\mathrm{SXR}}\propto E_{\mathrm{flare},\mathrm{SXR}}^{-1.77}$, which is similar to the results found at other bands and on other types of stars, indicating that the energy emitted at the soft X-ray band could be a constant fraction of the full-band bolometric energy.

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