Quiescent and Flaring States of Three Active Stars: V834 Tau, LQ Hya, and BY Dra

We present a detailed X-ray study of the quiescent and flaring coronae of three active main-sequence K-type stars, V834 Tau, LQ Hya, and BY Dra, using XMM-Newton observations. The quiescent coronae are well described by two-temperature thermal-plasma models, with cool and hot components in the ranges of 0.26–0.30 keV and 0.93–1.01 keV, respectively. Despite similar coronal temperatures, X-ray luminosities ( 10 29.18–29.75 erg s −1 ), and overall abundances, the relative emission measures of the cool and hot components differ among the stars. High-resolution spectroscopy reveals significant iron depletion by factors of 5–10 relative to photospheric values, and an inverse first ionization potential effect in all three stellar coronae. Six energetic flares are detected, with peak temperatures of 30–133 MK and released energies of 0.6–4.2 × 10 33 erg, classifying them as superflares. Most flares exhibit decay times roughly twice their rise times, although one event shows a decay phase nearly 20 times longer than its rise. Time-resolved spectroscopy and loop scaling laws yield flare parameters consistent with previous studies of active stars. LQ Hya displays recurrent superflares at the same rotational phase in observations separated by 6 months, suggesting the presence of a long-lived, complex magnetic field structure. These results provide insights into the magnetic activity and flare energetics of active stars, and their implications for stellar and exoplanetary environments.