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

Abstract

PG 1553+113 is a well-known blazar exhibiting evidence of a $\sim 2.2$-yr quasi-periodic oscillation (QPO) in radio, optical, X-ray, and ${\hat{I}}^3$-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 $21.8\pm 4.7$ yr oscillation. On its own, this $\sim 22$-yr period has a modest statistical significance of $1.6\ddot{I}ƒ$ 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 $\sim 3.6\ddot{I}ƒ$. The next peak of the 22-yr oscillation is predicted to occur around July 2025. We find that such a $\sim$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 $\gtrsim 0.2$, an eccentricity $\lesssim 0.1$, and accretes from a disk with characteristic aspect ratio $\sim 0.03$. The putative binary radiates nHz gravitational waves, but the amplitude is $\sim 10-100$ times too low for detection by foreseeable pulsar timing arrays.

20 pages, 15 figures, 1 table