Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations

TL;DR

This study uses broadband AstroSat observations to analyze the X-ray spectral curvature of blazar Mkn 421, comparing different particle energy distribution models through time-resolved spectroscopy to understand the underlying physical processes.

Contribution

It demonstrates that time-resolved spectral analysis can distinguish between degenerate models of particle energy distribution in blazars, providing insights into their physical mechanisms.

Findings

The EDD and EDA models better fit the spectra than the {} max model.

Spectral parameter correlations can differentiate between models.

Physical parameters inferred from models are often inconsistent with assumptions.

Abstract

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.