X-ray activity of the RS CVn-type star σ Gem from the first-year observations of the Einstein Probe

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Abstract
Context. Stellar flares are energetic events driven by the sudden release of magnetic energy in the stellar atmosphere. Compared to the Sun, the diversity of stellar types and environments leads to a richer variety of flaring phenomena. Studying these flares is crucial for understanding their impact on exoplanets, the circumstellar environment, and stellar evolution itself. The launch of the Einstein Probe ( EP ) offers a unique opportunity to systematically detect such events. Aims. We present a systematic analysis of the flaring activity of the active RS CVn-type binary σ Gem, utilizing the first-year monitoring data from the Wide-field X-ray Telescope (WXT) aboard EP . Our goals are to demonstrate the unique capability of EP in monitoring stellar X-ray activity and detecting flares by identifying and characterizing extreme X-ray flares on σ Gem and estimating their occurrence rate. Methods. We developed a data-processing pipeline to select and extract EP -WXT observations, producing a background-subtracted, vignetting-corrected light curve. We employed the Bayesian blocks method to detect significant flares in the long-term X-ray light curve. The key detection parameter was optimized through extensive simulations. For each identified flare, we performed light curve and spectral fitting to derive its duration, peak luminosity, total energy, and plasma properties (e.g., temperature and emission measure). Results. Between October 2024 and April 2025, WXT detected six distinct flares from σ Gem. These lasted from 21 hours to 3 days, with peak X-ray luminosities (0.5–4 keV) of 3.7 × 10 31 to 7.0 × 10 32 erg s −1 and total energies of 1.1 × 10 36 to 4.4 × 10 37 erg, placing them among the “superflare” class. Conclusions. Using σ Gem as a case study, we demonstrate an analysis process for flare detection and analysis with EP -WXT data, which has rapidly expanded the catalog of known X-ray flares for this star and provides new statistical constraints on its flaring behavior. Applying this methodology to the growing EP stellar archive promises to yield a vast sample of X-ray flares, which will significantly advance our understanding of stellar magnetic activity.
Funding Information
  • National Key R&D Program of China (2025YFF0511100)
  • National Natural Science Foundation of China (12433005)

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