2023MNRAS.524.3797D

We study simultaneous soft (0.7-7 keV) and hard (7-20 keV) X-ray light curves at a total of eight epochs during 2016-2019 of two TeV blazars Mrk 421 and 1ES 1959+650 observed by the SXT and LAXPC instruments on-board AstroSat. The light curves are 45-450 ks long and may be sampled with time bins as short as 600-800 s with high signal-to-noise ratio. The blazars show a harder when brighter trend at all epochs. Discrete cross-correlation functions indicate that the hard and soft X-ray variability are strongly correlated. The time lag is consistent with zero in some epochs, and indicates hard or soft lag of a few hours in the rest. In the leptonic model of blazar emission, soft lag may be due to slower radiative cooling of lower energy electrons while hard lag may be caused by gradual acceleration of the high energy electrons emitting at the hard X-ray band. Assuming the above scenario and the value of the Doppler factor (δ) to be 10-20, the hard and soft lags may be used to estimate the magnetic field to be ∼0.1 Gauss and the acceleration parameter to be ∼10⁴ in the emission region. Due to the availability of the high time resolution (∼ minutes to hours) light curves from AstroSat, the value of the illusive acceleration parameter could be estimated, which provides a stringent constraint on the theories of particle acceleration in blazar jets.