Jet Precession Driven by a Supermassive Black Hole Binary System in the BL Lac Object PG 1553+113

Abstract

The recent discovery of a roughly simultaneous periodic variability in the light curves of the BL Lac object PG 1553+113 at several electromagnetic bands represents the first case of such odd behavior reported in the literature. Motivated by this, we analyzed 15 GHz interferometric maps of the parsec-scale radio jet of PG 1553+113 to verify the presence of a possible counterpart of this periodic variability. We used the Cross-Entropy statistical technique to obtain the structural parameters of the Gaussian components present in the radio maps of this source. We kinematically identified seven jet components formed coincidentally with flare-like features seen in the γ-ray light curve. From the derived jet component positions in the sky plane and their kinematics (ejection epochs, proper motions, and sky position angles), we modeled their temporal changes in terms of a relativistic jet that is steadily precessing in time. Our results indicate a precession period in the observer's reference frame of 2.24 ± 0.03 years, compatible with the periodicity detected in the light curves of PG 1553+113. However, the maxima of the jet Doppler boosting factor are systematically delayed relative to the peaks of the main γ-ray flares. We propose two scenarios that could explain this delay, both based on the existence of a supermassive black hole binary system in PG 1553+113. We estimated the characteristics of this putative binary system that also would be responsible for driving the inferred jet precession.

Gamma-ray quasi-periodicities of blazars. A cautious approach

“…This scenario has indeed been recently invoked and discussed by several authors (e.g. Sandrinelli et al 2014;Ackermann et al 2015;Sandrinelli et al 2016a;Sobacchi et al 2017;Cavaliere et al 2017;Caproni et al 2017;Tavani et al 2018;Yan et al 2018) for some of the blazars considered in this study. A direct consequence of a relatively large population of binary supermassive black holes could be the emission of gravitational waves producing a background important in the frequency range covered by the Pulsar Timing Array (Hobbs & Dai 2017).…”

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

“…Additional physical mechanisms are required. Within the framework of a binary SMBH system, one plausible scenario is that the secondary black hole crosses and perturbs the accretion disc of the primary black hole, inducing a temporary enhancement of the accretion rate, which in turn leads to increased jet emission (Sillanpaa et al 1988;Valtaoja et al 2000;Caproni et al 2017;Britzen et al 2018). Another mechanism that could be responsible for the jet wobbling is the possible shuttle of the core closer to the jet apex, as reported in several studies (e.g.…”

Spectral energy distributions of candidate periodically variable quasars: testing the binary black hole hypothesis

“…The tentatively higher blazar fraction found in candidate periodic quasars ( §4.2) may also be naturally explained as being driven by a variability-selection bias, given that blazars are also more variable in the optical (e.g., Ruan et al 2012). On the other hand, if most candidate periodic quasars are robust, the higher blazar fraction could imply significant optical contamination from precessing radio jets (e.g., Kudryavtseva et al 2011;Ackermann et al 2015;Caproni et al 2017).…”

“…Furthermore, even if the suggested periodicity were true, the physical mechanism driving the periodicity is still uncertain (e.g., Graham et al 2015a;Charisi et al 2018). In addition to BSBHs, alternative scenarios may be responsible for driving optical periodicity, including warped accretion disks (e.g., Tremaine & Davis 2014), radio jet procession (e.g., Kudryavtseva et al 2011;Caproni et al 2017;Sobacchi et al 2017), quasi-periodic oscillations (QPOs) from e.g., Lens-Thirring procession (e.g., Stella & Vietri 1998;Ingram & Done 2011), and resonant accretion of magnetic field lines (i.e., "magnetic breathing" of the accretion disk; e.g., Villforth et al 2010). Complementary tests are needed to verify any candidate periodicity and to sort out alternative scenarios for its physical origin.…”