X-ray flares and quiescent coronae of V834 Tau, BY Dra, and LQ Hya

We present an X-ray study of the quiescent and flaring coronae of the active main-sequence stars V834 Tau, LQ Hya, and BY Dra based on XMM-Newton observations. In quiescence, the X-ray spectra of all three stars are well reproduced by two-temperature thermal plasma models, with cool and hot components at 0.26-0.30 keV and 0.93-1.01 keV, respectively, and X-ray luminosities in the range $10^{29.18-29.75}$ erg s$^{-1}$. Although the characteristic temperatures and luminosities are similar, the relative emission measures of the cool and hot components differ between the stars. High-resolution spectroscopy reveals substantial coronal iron depletion by factors of ~5-10 relative to photospheric values, and the elemental abundance patterns as a function of first ionization potential indicate the presence of the inverse-FIP effect in all three coronae.
We detect six flares with peak temperatures of 30–133 MK and total radiated energies of $(0.6-4.2)\times10^{33}$ erg, placing them in the superflare regime. The inferred peak fluxes correspond to GOES X190-X432 class events. For most flares, decay times are approximately twice the rise times, while one event exhibits an unusually prolonged decay phase. Using time-resolved spectroscopy together with loop scaling relations, we estimate the physical properties of the flaring plasma and compare them with solar and stellar flares reported in the literature. Recurrent superflares at the same rotational phase in LQ Hya suggest long-lived magnetic structures. In addition to characterising the coronae of three active stars, including a prototype system (BY Dra), this study places observational constraints on the strength and variability of their high-energy emission, providing useful context for studies of exoplanet environments around magnetically active stars.