Kavli Affiliate: Michael Fausnaugh
| First 5 Authors: Hans Moritz Günther, Dheeraj Pasham, Alexander Binks, Stefan Czesla, Teruaki Enoto
| Summary:
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~d 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
$10^{34}$~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$alpha$ 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.
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