2023ApJ...950..174S

We report a 325(-7, +8) day quasi-periodic oscillation (QPO) in the X-ray emission of the blazar Mkn 421 based on data obtained with the Rossi X-ray Timing Explorer satellite. The QPO is seen prominently in the ASM data (at least 15 cycles) due to the fact that it has had near-continuous sampling for more than a decade. The PCA data, where the sampling is not uniform and shows many large gaps, provide supporting evidence at lower significance. The QPOs are an important observable in accretion disks, can be modulated by various orbital timescales, and may be generated by a number of mechanisms. They have been studied extensively in X-ray binaries and should be present in active galactic nuclei (AGN) if they are governed by a common set of physical principles. In jetted sources, QPOs can probe jet-disk interactions or helical oscillations. This QPO has previously been claimed intermittently in X-ray, radio, and gamma-ray data, but the continuous, 15 yr extent (1996-2011) of the ASM observations (in which Mkn 421 is the brightest AGN observed) provides a unique window. The QPO appears to be present for nearly the entire extent of the ASM observations. We explore various physical origins and modulating mechanisms, particularly interpretations of the QPO as a result of disk-jet interactions, due to either an accretion disk limit cycle, jet instabilities, or helical motions. Limit cycle oscillations would not interact with either Keplerian or Lense-Thirring modulated oscillations; however, those associated with jet instabilities or helical motions in the jet would likely be modulated by Lense-Thirring precession.