Time-resolved and High-resolution X-ray spectroscopy of Super-flares on active stars AB Dor and V711 Tau

We present an X-ray study of 24 super-flares detected from active stars AB Dor and V711 Tau using the observations from the XMM-Newton. Of these flares, 13 are detected in AB Dor and three in V711 Tau. The duration for these flares ranges from 0.7 - 5.8 h, with a rise time between 5 min to 1.4 h and a decay time from 12 min to 2.4 h. The flare-to-quiescent flux ratio ranges between 1.2 - 34 and 1.2 - 1.4 for AB Dor and V711 Tau, respectively.

The X-ray luminosity of the flares spans 10^(30.95-33.03) erg/s, and the total energy in the 0.3–10.0 keV band ranges between 10^(34.07-37.03) erg. The quiescent state of both stars can be modeled with a 3-T plasma, exhibiting temperatures of 3.36, 11.02, and 22.04 MK for AB Dor and 3.02, 6.96, and 12.53 MK for V711 Tau. The time-resolved spectroscopy (TRS) reveals peak flare temperatures between 31 - 89 MK, with peak emission measures (EM) of 10^(52.5-54.7) cm^-3 for AB Dor and 32 - 39 MK and 10^(53.8-54.9) cm^-3 for V711 Tau. The two to three-fold increase in global abundances during flares indicates chromospheric evaporation.

High-resolution spectroscopy using the VAPEC model shows the inverse-FIP effect in both stars, indicating an enhancement of elements with FIP > 10 eV in their coronas. The coronal loop lengths range from 10^(9.9-10.7) cm for star AB Dor and 10^(10.8-10.9) cm for star V711 Tau, providing information about the geometry of the coronal loops. The minimum magnetic field required to confine the plasma within these loops is estimated to be 200 - 700 G and 350 - 450 G, respectively.