NICER Perspective on TeV Blazar Mrk~421: X-ray Variability and Particle Acceleration

Abstract

Mrk~421 is one of the most fascinating blazars, widely studied across the electromagnetic spectrum using observations at various wavebands, from radio to the TeV gamma ray bands. We present the first detailed spectral and timing analysis of the TeV blazar Mrk~421 based on 45 X-ray observations from the \textit{NICER} X-ray telescope, collected over two years from 2022 to 2024. The source exhibits strong X-ray variability across intraday and long-term timescales. During this period, we observe a dramatic change in flux, from $\sim 50$ to $\sim 1380$~cts~s${}^{-1}$, representing a $\sim 28$-fold increase. Spectral modeling with power-law, broken power-law, and log-parabolic functions shows that the log-parabola provides the most accurate description of the X-ray spectra. The hardness ratio analysis confirms a \textit{harder-when-brighter} trend, consistent with the anticorrelation between flux and photon index($\Gamma$). Correlation studies reveal a positive relation between the photon index ($\alpha$) and the curvature parameter ($\beta$) of the log-parabola model, a negative correlation between $\beta$ and synchrotron peak energy ($E_{\mathrm{p}}$), and a positive correlation between $E_{\mathrm{p}}$ and flux. In addition, the observed rapid variability indicates that the X-ray emission originates from a compact region located close to the central engine. Furthermore, using a log-parabolic electron energy distribution within the synchrotron jet scenario, we simulate the observed anti-correlation between the $E_{\mathrm{p}}$ and $\beta$. These features can be interpreted within the framework of energy-dependent particle acceleration in blazar jets, which are often associated with turbulence, strong magnetic fields, and relativistic outflows.

20 pages, 8 figures. Accepted for publication in The Astrophysical Journal (ApJ)

References in corpus (57)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
• On the Absorption of X-rays in the Interstellar Medium
• A Unifying View of the Spectral Energy Distributions of Blazars
• On characterising the variability properties of X-ray light curves from active galaxies
• The Spectral Energy Distribution of Fermi bright blazars
• A theoretical unifying scheme for gamma-ray bright blazars
• A low level of extragalactic background light as revealed by gamma-rays from blazars
• Non-linear X-ray variability in X-ray binaries and active galaxies
• X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
• Comptonization of Infrared Radiation from Hot Dust by Relativistic Jets in Quasars
• Log-parabolic spectra and particle acceleration in the BL Lac object Mkn 421: spectral analysis of the complete BeppoSAX wide band X-ray data set
• The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies
• Modeling the Emission Processes in Blazars
• Swift observations of the very intense flaring activity of Mrk 421 during 2006: I. Phenomenological picture of electron acceleration and predictions for the MeV/GeV emission
• Stochastic Acceleration and the Evolution of Spectral Distributions in SSC Sources: A Self Consistent Modeling of Blazars' Flares
• The First LHAASO Catalog of Gamma-Ray Sources
• Log-parabolic spectra and particle acceleration in blazars. III: SSC emission in the TeV band from Mkn 501
• A Multi-wavelength View of the TeV Blazar Markarian 421: Correlated Variability, Flaring, and Spectral Evolution
• New limits on the density of the extragalactic background light in the optical to the far-infrared from the spectra of all known TeV blazars
• Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013
• An Optical View of BL Lacertae Objects
• X-ray spectral evolution of TeV BL Lac objects: eleven years of observations with BeppoSAX, XMM-Newton and SWIFT satellites
• The 2009 multiwavelength campaign on Mrk 421: Variability and correlation studies
• Log-parabolic spectra and particle acceleration in blazars - II: The BeppoSAX wide band X-ray spectra of Mkn 501
• Long Term Variability of Cyg X-1, II. The rms-Flux Relation
• Signatures of synchrotron emission and of electron acceleration in the X-ray spectra of Mrk 421
• The O VII X-ray forest toward Markarian 421: Consistency between XMM-Newton and Chandra
• Detection of Possible Quasi-periodic Oscillations in the Long-term Optical Light Curve of the BL Lac Object OJ 287
• Stochastic Modeling of the Fermi/LAT Gamma-ray Blazar Variability
• Stochastic modeling of multiwavelength variability of the classical BL Lac object OJ 287 on timescales ranging from decades to hours
• X-ray Emission of Mkn 421: New Clues From Its Spectral Evolution. I. Temporal Analysis
• Coordinated Multiwavelength Observations of BL Lacertae in 2000
• The Ubiquity of the rms-flux relation in Black Hole X-ray Binaries
• Kepler observations of rapid optical variability in the BL Lac object W2R1926+42
• X-ray Intraday Variability of Five TeV Blazars with NuSTAR
• Hard X-ray properties of NuSTAR blazars
• Evolution of the Synchrotron Spectrum in Mrk421 during the 1998 Campaign
• Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017
• Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
• The Great Markarian 421 Flare of February 2010: Multiwavelength variability and correlation studies
• Long-term optical polarization variability and multiwavelength analysis of Blazar Mrk 421
• XMM - Newton observations of Markarian 421
• Magnetic Field Properties inside the Jet of Mrk 421: Multiwavelength Polarimetry Including the Imaging X-ray Polarimetry Explorer
• NICER Discovery of Millisecond X-ray Pulsations and an Ultracompact Orbit in IGR J17494-3030
• Blazar Variability: A Study of Non-stationarity and the Flux-RMS Relation
• X-ray Intraday Variability of the TeV Blazar Mrk 421 with {\it Chandra}
• X-ray timing and spectral variability properties of blazars S5 0716+714, OJ 287, Mrk 501, and RBS 2070
• X-ray intraday variability of the TeV blazar Mrk 421 with {\it XMM-Newton}
• Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations
• Rapid X-ray Variability in Mkn 421 during a Multiwavelength Campaign
• Optical polarization properties of February 2010 outburst of the blazar Mrk 421
• RMS-Flux Relation and Disc-Jet Connection in Blazars in the Context of the Internal Shocks Model
• X-ray Studies of Blazar 1ES 1959+650 Using SWIFT & XMM-NEWTON Satellite
• Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data
• Sub-Day Timescale X-ray Spectral Variability of the TeV Blazars Mrk 421 and 1ES 1959+650
• Transitional Spectral Behavior in Blazar OJ 287: A Comprehensive Multi-Epoch SED Study
• Study of Intra-Day Flux Distributions of Blazars Using XMM-Newton Satellite