Probing Spectral Evolution and Intrinsic Variability of Mkn\,421: A Multi-Epoch AstroSat Study of X-ray Spectra

TL;DR

This multi-epoch AstroSat study of Mkn 421 analyzes X-ray spectral variability, revealing that spectral changes are driven by particle acceleration and escape timescales rather than magnetic or Doppler factors, with consistent pivot energies across observations.

Contribution

The paper introduces a detailed time-resolved spectral analysis of Mkn 421 across multiple epochs, demonstrating model-independent pivot energies and linking variability to particle acceleration processes.

Findings

Inverse correlation between particle density and spectral index

Constant pivot energies across different models and epochs

Variability driven by particle acceleration and escape timescales

Abstract

Our study presents a time-resolved X-ray spectral analysis of Mkn 421, using AstroSat observations taken during different epochs between 2016 and 2019. The variability of the source in X-rays is utilized to investigate the evolution of its spectral properties. Each observation period was divided into segments of about 10 ks, and we employed three forms of particle distributions: broken-power law (BPL), log-parabola (LP), and power-law with maximum electron energy (xi-max model) undergoing synchrotron losses to fit the broad X-ray spectrum in each segment. We observed that all of these models provided good fits to the spectra. In the case of the broken-power law model, we investigated the relationship between normalized particle density at an energy less than the break energy and the index before the break. The results revealed an inverse correlation between the index and particle density with no time delay. Additionally, correlations between spectral parameters were used to determine the pivot energy. We observed that the pivot energy remained the same across the observations. For xi-max and LP models, we define analogous pivot energies and show that they also do not vary, indicating the model-independent nature of the result. The constant pivot energy suggests that the source's variability arises from index variations and not due to changes in the normalization. Consequently, parameters such as magnetic field strength, Doppler factor, etc., do not contribute to the source's variability. Instead, variations are primarily associated with the acceleration or escape timescales of emitted particles within the source.