Understanding the X-ray spectral curvature of Mkn 421 using broadband
  AstroSat observations
  We present a time-resolved X-ray spectral study of the high energy peaked
blazar Mkn 421 using simultaneous broadband observations from the LAXPC and SXT
instruments on-board AstroSat. The ~ 400 ksec long observation taken during 3-8
January, 2017 was divided into segments of 10 ksecs. Each segment was fitted
using synchrotron emission from particles whose energy distribution was
represented by a log-parabola model. We also considered particle energy
distribution models where (i) the radiative cooling leads to a maximum energy
({\xi} max model), (ii) the system has energy dependent diffusion (EDD) and
(iii) has energy dependent acceleration (EDA). We found that all these models
describe the spectra, although the EDD and EDA models were marginally better.
Time resolved spectral analysis allowed for studying the correlation between
the spectral parameters for different models. In the simplest and direct
approach, the observed correlations are not compatible with the predictions of
the {\xi} max model. While the EDD and EDA models do predict the correlations,
the values of the inferred physical parameters are not compatible with the
model assumptions. Thus, we show that spectrally degenerate models, can be
distinguished based on spectral parameter correlations (especially those
between the model normalization and spectral shape ones) making time-resolved
spectroscopy a powerful tool to probe the nature of these systems.
