Quasar physical pairs.

Osservatorio di Brera, Merate - Osservatorio astronomico di Padova, Padova.openopenAstronomia e AstrofisicaTREVES, ALDOSandrinelli, AngelaSandrinelli, Angel

Quasar physical pairs.

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Pulsar Timing Constraints on the Fermi Massive Black-Hole Binary Blazar Population

Blazars are a sub-population of quasars whose jets are nearly aligned with the line-of-sight, which tend to exhibit multi-wavelength variability on a variety of timescales. Quasi-periodic variability on year-like timescales has been detected in a number of bright sources, and has been connected to the orbital motion of a putative massive black hole binary. If this were indeed the case, those blazar binaries would contribute to the nanohertz gravitational-wave stochastic background. We test the binary hypothesis for the blazar population observed by the \textit{Fermi} Gamma-Ray Space Telescope, which consists of BL Lacertae objects and flat-spectrum radio quasars. Using mock populations informed by the luminosity functions for BL Lacertae objects and flat-spectrum radio quasars with redshifts $z \le 2$, we calculate the expected gravitational wave background and compare it to recent pulsar timing array upper limits. The two are consistent only if a fraction $\lesssim 10^{-3}$ of blazars hosts a binary with orbital periods $<5$ years. We therefore conclude that binarity cannot significantly explain year-like quasi-periodicity in blazars.Comment: 5 pages, 4 figures, accepted by MNRAS Letter

Quasi-periodicities of BL Lac Objects

We review the reports of possible year-long quasi-periodicities of BL Lac objects in the $\gamma$-ray and optical bands, and present a homogeneous time analysis of the light curves of PKS2155$-$304, PG1553+113, and BL Lac. Based on results from a survey covering the entire Fermi $\gamma$-ray sky we have estimated the fraction of possible quasi-periodic BL Lac objects. We compared the cyclical behaviour in BL Lac objects with that derived from the search of possible optical periodicities in quasars, and find that at z$\lesssim$1 the cosmic density of quasi-periodic BL Lac objects is larger than that of quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a supermassive binary black hole (SBBH) scenario, there could be a tension with the upper limits on the gravitational wave background measured by the pulsar timing array. The argument clearly indicates the difficulties of generally associating quasi-periodicities of BL Lac objects with SBBHs.Comment: In publication on A&A, 6 pages, 4 figure (11 plots). Minor corrections adde

Quasi-Periodicities at Year Time Scales in Blazars

We examine the 2008-2016 gamma-ray and optical light curves of a number of bright Fermi blazars. In a fraction of them, the periodograms show possible evidence of quasi-periodicities related in the two bands. This coincidence strengthens their physical meaning. Comparing with results from the periodicity search of quasars, the presence of quasi-periodicities in blazars suggests that the basic condition for its observability is related to the relativistic jet in the observer direction, but the overall picture remains uncertain