Spectral State Switching in Mrk 421: Results from the AstroSat LAXPC/SXT Observations

TL;DR

This study analyzes the X-ray spectral variability of blazar Mrk 421 using AstroSat observations, revealing two distinct spectral states and flux variability patterns consistent over multiple years, suggesting fluctuations in particle acceleration conditions.

Contribution

First detailed flux-resolved X-ray spectral analysis of Mrk 421 showing two spectral states and their evolution over time using simultaneous AstroSat data.

Findings

Spectra are well described by a broken power-law model.

The source exhibits two discrete low-energy spectral states.

Flux states follow a double lognormal distribution.

Abstract

We carried a detailed time and flux resolved X-ray spectral analysis of the high-synchrotron-peaked blazar Mrk\,421 using simultaneous AstroSat and LAXPC20/SXT observations. The 100\,s binned LAXPC20 light curve obtained during 3--8 January 2017 reveals pronounced flux variability. The source exhibits a fractional variability amplitude of $F_{\mathrm{rms}} = 0.210 \pm 0.005$ in the SXT band and $F_{\mathrm{rms}} = 0.316 \pm 0.006$ in the LAXPC20 band. During this interval, the source reached a peak count rate of 122.94\,counts\,s$^{-1}$. This enabled us to carry flux resolved spectroscopy by selecting ten flux states, S1--S10 each having a width of 8\,counts\,s$^{-1}$. We noted that the spectra in these flux states are well described by a synchrotron-convolved broken power-law (BPL), which consistently provides a better fit than a log-parabola. The low-energy particle index (index before the break) is found to cluster around two discrete values across flux states indicating two spectra states in the source. The break energy consistently moves to high energy with increase in flux level in these states. Time-resolved spectroscopy (10-ks segments) confirms that the flux histogram is best modelled as a double lognormal distribution and the index histogram is bimodal. Inclusion of two additional long observations spanning 2017-2019 shows the same double-state behaviour on longer timescales. Together, the results indicate that Mrk\,421 routinely occupies two dominant spectral; in a leptonic synchrotron framework this can be explained by Gaussian-like fluctuations in acceleration conditions producing lognormal flux states.