Study of periodic signals from blazars

TL;DR

This paper reviews the search for quasi-periodic oscillations in blazar light curves, highlighting recent findings of year-scale QPOs across multiple frequencies and discussing potential astrophysical origins such as binary black holes and jet precession.

Contribution

It provides a summary of recent observational evidence for QPOs in blazars and discusses possible physical mechanisms behind these signals, addressing challenges in their detection and significance.

Findings

Year-timescale QPOs detected in multiple blazars across different wavelengths.

Potential explanations include binary supermassive black holes, Lense-Thirring precession, and jet precession.

Highlights difficulties in confirming the significance of QPO detections due to red-noise and transient signals.

Abstract

The search for periodic signals from blazars has become an actively pursued field of research in recent years. This is because periodic signals bring us information about the processes occurring near the innermost regions of blazars, which are mostly inaccessible to our direct view. Such signals provide insights into some of the extreme conditions that take place in the vicinity of supermassive black holes that lead to the launch of the relativistic jets. In addition, studies of characteristic timescales in blazar light curves shed light on some of the challenging issues in blazar physics that includes disk-jet connection, strong gravity near fast rotating supermassive black holes and release of gravitational waves from binary supermassive black hole systems. However, a number of issues associated with the search quasi-periodic oscillations (QPOs) in blazar e.g., red-noise dominance, modest significance of the detection, periodic modulation lasting for only a couple of cycles and their transient nature, make it difficult to estimate the true significance of the detection. Consequently, it also becomes difficult to make meaningful inferences about the nature of the on-going processes. In this proceedings, results of study focused on searching QPOs in a number of blazar multi-frequency light curves are summarized. The time series analyses of long term observations of the blazars revealed presence of year-timescale QPOs in the sources including OJ 287 (optical), Mrk 501 (gamma-ray), J1043+2408 (radio) and PKS 0219-164 (radio). As likely explanations, we discuss a number of scenarios including binary supermassive black hole systems, Lense-Thirring precession, and jet precession.