AstroSat observations of long-duration X-ray superflares on active M-dwarf binary EQ Peg

Abstract

ABSTRACT We present a comprehensive study of three large long-duration flares detected on an active M-dwarf binary EQ Peg using the Soft X-Ray Telescope of the AstroSat observatory. The peak X-ray luminosities of the flares in the 0.3–7-keV band are found to be within ∼5–10 ×  1030$\rm {erg}~\rm {s}^{-1}$. The e-folding rise- and decay-times of the flares are derived to be in the range of 3.4–11 and 1.6–24 ks, respectively. Spectral analysis indicates the presence of three temperature corona with the first two plasma temperatures remain constant during all the flares and the post-flare observation at ∼3 and ∼9 MK. The flare temperature peaked at 26, 16, and 17 MK, which are 2, 1.3, and 1.4 times more than the minimum value, respectively. The peak emission measures are found to be 3.9–7.1 ×  1053 cm−3, whereas the abundances peaked at 0.16–0.26 times the solar abundances. Using quasi-static loop modelling, we derive loop lengths for all the flares as 2.5 ± 0.5 ×  1011, 2.0 ± 0.5 ×  1011, and 2.5 ± 0.9 ×  1011 cm, respectively. The density of the flaring plasma is estimated to be 4.2 ± 0.8 ×  1010, 3.0 ± 0.7 ×  1010, 2.2 ± 0.8 ×  1010 cm−3 for flares F1, F2, and F3, respectively. Whereas the magnetic field for all three flares is estimated to be <100 G. The estimated energies of all three flares are ≳ 1034–1035erg, putting them in a category of superflare. All three superflares are also found to be the longest duration flares ever observed on EQ Peg.