Constraining the PG 1553+113 binary hypothesis: interpreting hints of a new, 22-year period

PG 1553+113 is a well-known blazar exhibiting evidence of a ∼ 2.2-yr quasi-periodic oscillation (QPO) in radio, optical, X-ray, and γ-ray bands. Since QPO mechanisms often predict multiple QPOs, we search for a second QPO in its historical optical light curve covering a century of observations. Despite challenging data quality issues, we find hints of a 21.8 ± 4.7 yr oscillation. On its own, this ∼ 22-yr period has a modest statistical significance of 1.6σ when accounting for the look-elsewhere effect. However, the joint significance of both the 2.2- and 22-yr periods arising from colored noise alone is ∼ 3.6σ. The next peak of the 22-yr oscillation is predicted to occur around July 2025. We find that such a ∼10:1 relation between two periods can arise in the gas dynamics of a plausible supermassive black hole binary model of PG 1553+113. While the 22-yr QPO is preliminary, an interpretation of PG 1553+113's two QPOs in this binary model suggests that the binary engine has a mass ratio ≳ 0.2, an eccentricity ≲ 0.1, and accretes from a disk with characteristic aspect ratio ∼ 0.03. The putative binary radiates nHz gravitational waves, but the amplitude is ∼10-100 times too low for detection by foreseeable pulsar timing arrays.