2019MNRAS.487.3990B

Quasi-periodic oscillations (QPO) originating from innermost regions of blazars can provide unique perspective of some of the burning issues in blazar studies including disk-jet connection, launch of relativistic jets from the central engine, and other extreme conditions near the fast rotating supermassive black holes. However, a number of hurdles associated with searching QPOs in blazars e.g. red-noise dominance, modest significance of the detection, and periodic modulation lasting for only a couple of cycles, make it difficult to estimate the true significance of the detection. In this work, we report a ∼ 330-d QPO in the Fermi/LAT observations of the blazar Mrk 501 spanning nearly a decade. To establish consistency of the result, we adopted multiple approaches to the time-series analysis and employed four widely known methods. Among these, the Lomb-Scargle periodogram and weighted wavelet z-transform represent frequency-domain-based methods whereas epoch folding and z-transformed discrete auto-correlation function are time-domain-based analyses. The power spectrum response method was followed to properly account for the red-noise, largely inherent in blazar light curves. Both local and global significance of the signal were found to be above 99 per cent over possible spurious detection. In the context where not many γ-ray QPOs have been reported to last more than 5 cycles, this might be one of the few instances where we witness a sub-year time-scale γ-ray QPO persisting nearly 7 cycles. A number of possible scenarios linked with binary supermassive black hole, relativistic jets, and accretion discs can be invoked to explain the transient QPO.