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

TL;DR

This study analyzes Mrk 421 with simultaneous AstroSat SXT and LAXPC20 data, performing flux-resolved and time-resolved X-ray spectroscopy to investigate spectral variability. The spectra are best described by a synchrotron-convolved broken power-law, revealing two discrete low-energy indices and a break energy that increases with flux, indicating two dominant spectral states. Statistical analyses show that both flux and spectral index distributions favor bimodal (double) models across a single epoch and persist across combined 2017–2019 observations, consistent with multiplicative variability in particle acceleration. The results support a leptonic jet picture in which two emission states arise from fluctuations in acceleration/cooling conditions, with implications for two-zone or two-population jet scenarios and the understanding of rapid blazar variability.

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.

Figures (7)

• Figure 1: One-hour binned SXT and LAXPC light curves of Mrk 421 obtained in the energy ranges 0.8--7.0 keV and 3.0--30.0 keV, respectively, corresponding to the observation ID "A02_005T01_9000000948".
• Figure 2: Comparison of the reduced chi-square ($\chi^2_\nu$) values obtained from the broken power-law and log-parabola fits for different spectral states of Mrk 421.
• Figure 3: Variation of the broken power-law spectral parameters of Mrk 421 with flux state (S1--S10).
• Figure 4: Correlations among the spectral parameters of the broken power-law model for Mrk 421.
• Figure 5: Variation of the break energy with flux state (S1--S10) obtained from flux resolved spectroscopy.