Quasi-periodic oscillations in the gamma-ray light curves of bright active galactic nuclei

Abstract

The detection of quasi-periodic oscillations (QPOs) in the emission of active galactic nuclei (AGNs) can provide insights into the physics of the super-massive black-holes (SMBHs) powering these systems, and could represent a signature of the existence of SMBH binaries. The identification of long term QPOs, characterized by periods on the order of several months to years, is particularly challenging and can only be achieved via all-sky monitoring instruments like the Fermi-LAT satellite. We aim to identify QPOs in the $\gamma$-ray light-curves of the thirty-five brightest AGNs within the Fermi-LAT catalog, including data from the beginning of the Fermi mission (August 2008) to April 2021, and energies from 100 MeV to 300 GeV. Two time binnings are investigated, 7 and 30 days. The search for quasi-periodic features is then performed using the continuous wavelet transform. The significance of the result is tested via Monte Carlo simulations of artificial light curves with the same power spectral density and probability distribution function as the original light curves. We identify twenty-four quasars with candidate QPOs. Several of our candidates coincide with previous claims in the literature: PKS~0537-441, S5~0716+714, Mrk~421, B2~1520+31, and PKS 2247-131. All our candidates are transient. The most significant multi-year QPO, with a period of about 1100 days, is observed in the quasar S5~1044+71, and is reported here for the first time.