Multi-Wavelength Variability of PG 1553+113

PG 1553+113 is a high-synchrotron-peaked BL Lac object widely recognized as the best candidate for hosting a binary super-massive black hole system driven by a sustained quasi-periodic oscillation. This review synthesizes multi-wavelength and polarimetric observations of PG 1553+113 across the electromagnetic spectrum, ranging from radio to very-high-energy $\gamma$ rays. Continuous monitoring by the Fermi Gamma-ray Space Telescope over a 15-year baseline confirms a prominent ∼2.18-year periodicity in the GeV $\gamma$-ray light curve at a significance exceeding 4.0-$\sigma$, accompanied by synchronized modulations in the optical regime. Long-term multi-instrument campaigns reveal complex cross-band behavior: while optical and $\gamma$-ray fluxes are strongly correlated, X-ray and VHE emissions do not show equivalent long-term periodic patterns, favoring multi-zone emission models. High-resolution radio interferometry and multi-wavelength polarimetric observations, including recent X-ray measurements by IXPE, show higher polarization degrees in X-rays than in optical and radio bands and instances of orphan polarization swings. These features point toward energy-stratified emission zones, magnetic reconnection, or turbulence in shock-accelerated jet environments. The observed modulations are evaluated against theoretical scenarios, encompassing super-massive black hole binary orbital dynamics, Lense–Thirring disk precession, helical jet geometries, and intrinsic accretion disk instabilities.