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Many giant stars are magnetically active, which causes rotational variability, chromospheric emission lines, and X-ray emission. Large outbursts in these emission features can set limits on the magnetic field strength and thus constrain the mechanism of the underlying dynamo. HD 251108 is a Li-rich active K-type giant. We find a rotational period of 21.3 days with color changes and additional long-term photometric variability. Both can be explained with very stable stellar spots. We followed the decay phase of a superflare for 28 days with NICER and from the ground. We track the flare decay in unprecedented detail in several coronal temperature components. With a peak flux around 1034 erg s−1 (0.5–4.0 keV) and an exponential decay time of 2.2 days in the early decay phase, this is one of the strongest flares ever observed, yet it follows trends established from samples of smaller flares, for example, for the relations between H α and X-ray flux, indicating that the physical process that powers the flare emission is consistent over a large range of flare energies. We estimate a flare loop length about 2–4 times the stellar radius. No evidence is seen for abundance changes during the flare. [ABSTRACT FROM AUTHOR]

Low-mass (≲1.2 M ⊙) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional H α and X-ray luminosities, L H α / L bol and L X/ L bol, and to calculate Rossby numbers R o. We found that at ≈700 Myr almost all M dwarfs exhibit H α emission, with binaries having the same overall color–H α equivalent width distribution as single stars. In the R o– L H α / L bol plane, unsaturated single stars follow a power law with index β = −5.9 ± 0.8 for R o > 0.3. In the R o– L X/ L bol plane, we see evidence for supersaturation for single stars with R o ≲ 0.01, following a power law with index β sup = 0.5 − 0.1 + 0.2 , supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical R o value at which activity saturates is smaller for L X/ L bol than for L H α / L bol. Finally, we observed an almost 1:1 relation between L H α / L bol and L X/ L bol, suggesting that both the corona and the chromosphere experience similar magnetic heating. [ABSTRACT FROM AUTHOR]