Quasi-periodic variability in the γ-Ray blazar PKS 0426–380

Abstract

The identification of quasi-periodic oscillations (QPOs) inactive galactic nuclei (AGN) light curvescan shed light on supermassive black holes (SMBHs) and their surrounding central engines. They could signal the existence of SMBH binaries and reflect theirpreponderance through cosmic evolution. The Flat Spectrum Radio Quasar (FSRQ) at intermediate redshift (z=1.1) PKS 0426–380 was reported to possibly exhibit a quasi-periodic signal. Using the Fermi-LAT Pass 8 data, we generate an updated γ-ray light curve (LC) time series of this object from 2008 August to 2020 November. Aperture photometry γ-ray LCs were extracted at three energy ranges (E1: 200 MeV–1 GeV, E2: 1 GeV–10 GeV, and E3: 10 GeV–500 GeV). The cross-correlation function was applied to examine the variability properties within these bands. The source showed an E1-E2 high degree of correlation. Its flux distribution was noted to be closer a log-normal probability density function than a Gaussian. A search for QPOs in 100 MeV–500 GeV γ-ray LC was carried out using Lomb–Scargle periodogram, as well as the weighted wavelet Z-transform. Monte Carlo simulations were used to test the significance of the results. Furthermore, the REDFIT tool was used to precisely evaluate the power spectrum signal against the red-noise background. The results confirm the existence of QPO for the PKS 0426–380 blazar at high energy γ-rays, lasting for at least 12.2 yr with a period 3.65±0.45 yr at significance ∼3σ. They are consistent with this distant blazar being a binary system of SMBH with separation R∼0.0029pc, total mass M∼3×108M⊙, and timescale for SMBHs merging TGW∼6.9×104yr.