Study of the energetic X-ray superflares from the active fast rotator AB doradus

Abstract

We present the analyses of intense X-ray flares detected on the active fast rotator AB Dor using observations from the XMM– Newton. A total of 21 flares are detected, and 13 flares are analysed in detail. The total X-ray energy of these flares is found to be in the range of 1034−36 erg, in which the peak flare flux increased up to 34 times from the pre-/post-flaring states for the strongest observed flare. The duration of these flaring events is found to be 0.7 to 5.8 h. The quiescent state X-ray spectra are found to be explained by a three-temperature plasma with average temperatures of 0.29, 0.95, and 1.9 keV, respectively. The temperatures, emission measures, and abundances are found to be varying during the flares. The peak flare temperature was found in the 31–89 MK range, whereas the peak emission measure was 1052.5–54.7 cm−3 . The abundances vary during the flares and increase by a factor of ∼3 from the quiescent value for the strongest detected flare. The variation in individual abundances follows the inverse-FIP effect in quiescent and flare phases. The X-ray light curves of AB Dor are found to exhibit rotational modulation. The semi-loop lengths of the flaring events are derived in the range of 109.9−10.7 cm, whereas the minimum magnetic field to confine the plasma in the flaring loop is estimated between 200 and 700 G.