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

Abstract

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 ∼10 33 to ∼10 37 erg, and the majority of them are superflares, with the most energetic one having energy of 6.0 − 4.7 + 3.2 × 10 37 erg . (2) The flare duration is related to its energy as formulated by T duration , SXR ∝ E flare , SXR 0.201 ± 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 flare / dE flare , SXR ∝ E flare , 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.