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NICER and Swift observations of X-ray Superflares on active RS CVn-type binary UX Ari

Presentation #308.02 in the session Binaries at Different Evolutionary Stages.

Published onJun 29, 2022
NICER and Swift observations of X-ray Superflares on active RS CVn-type binary UX Ari

Flares are the most extreme evidence of the magnetic activities of the Sun and the stars. Observations of the flares are essential to provide valuable constraints to the existing dynamo theory. We present the analysis of thirteen X-ray flares on a 6.44-day period active RS Canum Venaticorum-type tidally locked binary UX Ari. Twelve X-ray flares were observed with NASA’s Neutron star Interior Composition Explorer (NICER) between 2017–2020. Whereas a huge flare triggered the Burst Alert Telescope of the Neil Gehrels Swift Observatory (Swift) on 15 July 2014. The peak X-ray luminosity of the flare observed with the Swift satellite was estimated to be 3.3 × 1033 erg s-1 in 0.3–10 keV band, and this flare is identified as the largest X-ray flare ever observed on UX Ari. NICER observations of UX Ari show several smaller flares having the peak luminosity of the order of 1032–33 erg s-1. The largest NICER flare was observed on 17 August 2020 with the peak X-ray luminosity of 2.1 × 1033 erg s-1. We performed detailed time-resolved X-ray spectroscopy of all the flares using the Astrophysical Plasma Emission Code. We found evidence of three temperature plasma, where two cooler temperatures of 3 and 15 MK were found to be associated with the quiescent corona. The hottest temperature was found to vary with the flare and was identified as the flare temperature. The emission measure was found to follow the flare light curves and peaked at the order of 1054 cm-3. Using the hydrodynamic loop modeling, we derived the loop lengths of the order of 1011 cm. The total magnetic field produced at the coronal height is found to be of the order of kilo-Gauss. Our analysis indicates that the large magnetic field at the coronal height is due to the presence of an extended convection zone of the sub-giant and the high orbital velocity.

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