  The Neutron star Interior Composition ExploreR (NICER) X-ray observatory
observed two powerful X-ray flares equivalent to superflares from the nearby
young solar-like star, kappa1 Ceti, in 2019. NICER follows each flare from the
onset through the early decay, collecting over 30 cts s-1 near the peak,
enabling a detailed spectral variation study of the flare rise. The flare in
September varies quickly in ~800 sec, while the flare in December has a few
times longer timescale. In both flares, the hard band (2-4 keV) light curves
show typical stellar X-ray flare variations with a rapid rise and slow decay,
while the soft X-ray light curves, especially of the September flare, have
prolonged flat peaks. The time-resolved spectra require two temperature plasma
components at kT ~0.3-1 keV and ~2-4 keV. Both components vary similarly, but
the cool component lags by ~200 sec with a 4-6 times smaller emission measure
(EM) compared to the hot component. A comparison with hydrodynamic flare loop
simulations indicates that the cool component originates from X-ray plasma near
the magnetic loop footpoints, which mainly cools via thermal conduction. The
time lag represents the travel time of the evaporated gas through the entire
flare loop. The cool component has several times smaller EM than its simulated
counterpart, suggesting a suppression of conductive cooling possibly by the
expansion of the loop cross-sectional area or turbulent fluctuations. The cool
component's time lag and small EM ratio provide important constraints on the
flare loop geometry.
