2019ApJ...875L..22C

The search in active galactic nuclei (AGNs) for supermassive binary systems with total mass M ∼ 10⁸-10⁹ M_☉ is intertwined with the observational efforts aimed at their contributions to the stochastic background of gravitational waves (GWs). Here we discuss and compare two classes of AGNs that are outstanding in electromagnetic bands: bright, relatively distant and massive quasars, and closer jetted and less massive BL Lac objects (BL Lacs). Among the latter, we focus on the source PG 1553+113 at redshift z ≃ 0.5, one of the best periodic candidates so far, that features a five-fold repetitive pattern of period P = 2.2 yr recurring in its gamma-ray light curve, as continuously monitored by Fermi-LAT for over 9 yr and updated here to 2018 March 15. We discuss this object as an interesting candidate for hosting a supermassive binary source of GWs, and explore the possible contributions to the GW stochastic background from it and similar sources in the BL Lac class. We find the possible contributions of such sources to lie below the limits set by the current Pulsar Timing Arrays, but well within the reach of the upcoming Square Kilometer Array.