Delayed Development of Cool Plasmas in X-Ray Flares from the Young Sun-like Star κ¹ Ceti
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Author/Creator ORCID
Date
2023-02-22
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Citation of Original Publication
Hamaguchi, Kenji et al. "Delayed Development of Cool Plasmas in X-Ray Flares from the Young Sun-like Star κ¹ Ceti." The Astrophysical Journal 944, No. 2 (2023 February 22). https://doi.org/10.3847/1538-4357/acae8b.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
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 κ¹ Ceti in 2019. NICER follows each flare from the
onset through the early decay, collecting over 30 counts s⁻¹ near the peak, enabling a detailed spectral variation
study of the flare rise. The flare in September varies quickly in ∼800 s, 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 and
∼2–4 keV. Both components vary similarly, but the cool component lags by ∼200 s with a four to six 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 that
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 a 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 EM ratio provide important constraints on the flare loop
geometry.