Broadband Variability Analysis of FSRQ PKS\,0402-362 with Indications of Quasi-Periodic Modulation
Zeeshan Nazir, Sikandar Akbar, Zahir Shah, Athar A. Dar, Zahoor Malik
TL;DR
This study analyzes PKS 0402-362 with Fermi-LAT and Swift data spanning 2010–2024 to characterize long-term γ-ray variability, spectral behavior, and multiwavelength emission. The authors find that γ-ray flux and photon index distributions are better described by a double log-normal profile, indicating two distinct flux states, and report a candidate quasi-periodic modulation at ∼413 days, though the result is cautious due to limited cycles. Broadband SEDs for six flux states are well reproduced by a one-zone leptonic emission model with synchrotron, SSC, and EC components, with the external IR photon field dominating the EC process and the emission region located outside the BLR; high-flux states feature harder electron spectra and lower magnetic fields, consistent with harder-when-brighter behavior. The analysis shows the jet becomes more particle- or kinetic-energy–dominated during flares, and the modeling provides physical constraints on magnetic field strength, break energies, and jet power, offering insights into acceleration and cooling processes in powerful FSRQs.
Abstract
We present a comprehensive temporal and spectral study of the flat-spectrum radio quasar PKS~0402$-$362 using \textit{Fermi}-LAT/Swift-XRT/UVOT observations spanning from MJD 54686-60321. The $γ$-ray light curve exhibits multiple phases of enhanced activity, with the fractional variability parameter ($F_{\mathrm{var}}$) showing larger amplitudes at longer timescales, consistent with variability trends observed in other FSRQs. Statistical analysis of the flux and spectral index distributions using the Anderson--Darling test and histogram fitting reveals that both distributions deviate from a single log-normal form and are better represented by a double log-normal profile, indicating two distinct flux states. A search for quasi-periodic oscillations in the $γ$-ray emission using the Lomb--Scargle periodogram identified a significant periodic signal at $\sim$413~days with a confidence level exceeding $3σ$. However the proximity of the timescale to one year and limited number of observed cycles prevents a definitive interpretation. Broadband spectral energy distributions for six flux states were modeled using a one-zone leptonic framework incorporating synchrotron, synchrotron self-Compton (SSC), and external Compton (EC) components. The SEDs are well reproduced with physically reasonable parameters: high-flux states exhibit harder electron spectra and lower magnetic field strengths ($B \sim 0.2--0.6\,\mathrm{G}$), while low-flux states show softer spectra and stronger magnetic fields ($B \sim 1.3\,\mathrm{G}$). The fitted break energy decreases during high-flux states, suggesting enhanced radiative cooling and a transition toward a particle- or kinetic-energy-dominated jet. These trends are consistent with the ``harder-when-brighter'' behavior commonly observed in blazars.