Transient quasi-periodic oscillations at gamma-rays in the TeV blazar PKS 1510-089

We present periodicity search analyses on the gamma-ray light curve of the TeV blazar PKS 1510-089 observed by the Fermi Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: A 3.6-d QPO during the outburst in 2009 that lasted five cycles (MJD 54906-54923); and a periodicity of 92 d spanning over 650 d from 2018 to 2020 (MJD 58200-58850), which lasted for seven cycles. We employed the Lomb-Scargle periodogram, Weighted Wavelet Z-transform, REDFIT, and the Monte Carlo light-curve simulation techniques to find any periodicity and the corresponding significance. The similar to 3.6-d QPO was detected at a moderate significance of 3.5 sigma, while the detection significance of the 92-d QPO was similar to 7.0 sigma. We explore a few physical models for such transient QPOs including a binary black hole system, precession of the jet, a non-axisymmetric instability rotating around the central black hole near the innermost stable circular orbit, the presence of quasi-equidistant magnetic islands inside the jet, and a geometric model involving a plasma blob moving helically inside a curved jet.