Speaker
Description
Blazars represent the most extreme subclass of active galactic nuclei, characterized by relativistic jets aligned close to the line of sight and exhibiting strong variability across the electromagnetic spectrum. Their non-thermal emission is dominated by relativistically beamed radiation from the jet, making them ideal laboratories for studying particle acceleration and energy dissipation under extreme physical conditions. X-ray observations provide a powerful probe of the particle acceleration and radiative processes operating within these jets. We present the first detailed spectral and timing study of the TeV blazar Mrk 421 based on archival observations from the NICER mission.
The source exhibits pronounced X-ray variability on both intraday and long-term timescales. Spectral modeling using power-law, broken power-law, and log-parabolic functions indicates that the log-parabolic model provides the best description of the X-ray spectra. Hardness ratio analysis reveals a clear harder-when-brighter behavior, implying enhanced particle acceleration during high-flux states. We also find significant correlations among the spectral parameters, including a positive relation between the spectral index ($\alpha$) and curvature parameter ($\beta$), as well as an anticorrelation between the synchrotron peak energy ($E_{\rm p}$) and spectral curvature.
Using a log-parabolic electron energy distribution within a synchrotron jet framework, we reproduce the observed anticorrelation between $E_{\rm p}$ and $\beta$, consistent with energy-dependent particle acceleration processes in turbulent relativistic jets. The observed spectral evolution, together with the rapid variability, suggests that the X-ray emission originates from a compact region within the jet where particle acceleration and radiative cooling compete on short timescales. These results provide new insights into the physical mechanisms governing the high-energy emission of blazars and demonstrate the capability of NICER observations to constrain particle acceleration processes in relativistic jets.