Delayed Development of Cool Plasmas in X-ray Flares from kappa1 Ceti

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, 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.

21 pages, 12 figures, to be published in the Astrophysical Journal

References in corpus (19)

• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• X-Ray Spectroscopy of Stars
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Magnetic Properties of Solar Active Regions that Govern Large Solar Flares and Eruptions
• Global Energetics of Solar Flares: V. Energy Closure in Flares and Coronal Mass Ejections
• Diagnostics of stellar flares from X-ray observations: from the decay to the rise phase
• Large X-ray Flares on Stars Detected with MAXI/GSC: A Universal Correlation between the Duration of a Flare and its X-ray Luminosity
• Simultaneous Multiwavelength Flare Observations of EV Lacertae
• What determines the X-ray intensity and duration of a solar flare?
• The RS CVn type star GT Mus shows most energetic X-ray flares throughout the 2010s
• The direct relation between the duration of magnetic reconnection and the evolution of GOES light curves in solar flares
• One Year in the Life of Young Suns: Data Constrained Corona-Wind Model of kappa1 Ceti
• Universal Scaling Laws for Solar and Stellar Atmospheric Heating
• Geometric Assumptions in Hydrodynamic Modeling of Coronal and Flaring Loops
• On the Area Expansion of Magnetic Flux-Tubes in Solar Active Regions
• Solar Flare Irradiance: Observations and Physical Modeling
• Evolution of Elemental Abundances During B-Class Solar Flares: Soft X-ray Spectral Measurements with Chandrayaan-2 XSM
• X-ray Super-Flares From Pre-Main Sequence Stars: Flare Modeling
• Solar Flare Heating with Turbulent Suppression of Thermal Conduction

Cited by in corpus (3)

• APO & SMARTS flare star campaign observations I. Blue wing asymmetries in chromospheric lines during mid M dwarf flares from simultaneous spectroscopic and photometric observation data
• Do Young Suns Produce Frequent, Massive CMEs? Results from Five-year Dedicated Optical Observations of EK Draconis and V889 Hercules
• Superflare on a rapidly-rotating solar-type star captured in X-rays