Presentation #116.109 in the session Stellar/Compact Objects.
Solar and stellar flares are the most energetic events on low-mass stars. They represent the rapid conversion of magnetic energy of active regions into kinetic or thermal energies, radiating from radio to gamma-rays and ejecting high-energy nuclei and electrons. Even with substantial energy variations, the stellar flares share similar behavior and characteristics and arise from the universal magnetic reconnection mechanism. The rising phase holds crucial information on the flare geometry and heating mechanism. However, the phase lasts ~1 ksec for most flares and so has been mostly limited to duration or crude hardness ratio studies. The NICER X-ray observatory observed two powerful X-ray flares equivalent to superflares from the nearby young solar-like star, kappa1 Ceti, in 2019. The observations followed 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. 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 flare in September, 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 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 observed EMs of the cool components have several times smaller than the EMs of the simulated counterparts, suggesting a suppression of conduction cooling possibly by the expansion of the loop cross-sectional area or turbulent fluctuations. The cool component’s time lag and EM ratio provide important constraints on the flare loop geometry.