2024ApJ...961..130Z

The statistical characteristics of stellar flares at optical bands have received extensive study, but they remain 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 in understanding the multiwavelength 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 the 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 of 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 ∼10³³ to ∼10³⁷ 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 near-IR bands, indicating distinct radiation mechanisms at different bands. (3) The frequency distribution of stellar flares as a function of energy is formulated as ${{dN}}_{\mathrm{flare}}/{{dE}}_{\mathrm{flare},\mathrm{SXR}}\propto {E}_{\mathrm{flare},\mathrm{SXR}}^{\ -1.77}$, 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.