Quasi-periodicities of BL Lac Objects

TL;DR

This paper reviews potential year-long quasi-periodicities in BL Lac objects across gamma-ray and optical bands, analyzes specific light curves, and discusses implications for supermassive binary black hole models and gravitational wave constraints.

Contribution

It provides a homogeneous analysis of BL Lac light curves, estimates the fraction of quasi-periodic objects, and discusses the implications for black hole models and cosmic density.

Findings

Quasi-periodicities are more common in BL Lac objects than quasars at z≤1.

The observed quasi-periodicities challenge the supermassive binary black hole scenario.

There is a potential tension with gravitational wave background limits from pulsar timing arrays.

Abstract

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.