Quasi-Periodic Signals from Active Galactic Nuclei

Dear Colleagues,

The search for periodic signals from active galactic nuclei (AGN, especially radio-loud ones) has become a widely discussed topic in recent years. Such periodic changes, most likely originating from the innermost regions of the sources, bear imprints of the processes occurring near the central engine, which is mostly inaccessible to our direct view. For this reason, periodic signals could provide unique information on various aspects of quasar/blazar studies, such as disk–jet connection, magnetic field configuration, and, more importantly, strong gravity near supermassive black holes and the release of gravitational waves from binary supermassive black hole systems. However, a number of hurdles associated with the search for quasi-periodic oscillations (QPOs) in AGN, including red-noise dominance, the modest significance of the detection, and periodic modulation that only lasts for a couple of cycles, make it difficult to estimate the detection’s true significance. Moreover, there seems to be no consensus on the methods that are to be used to estimate the significance of QPO features against spurious detection. As a result, it is hard to make meaningful inferences about the nature of the processes thought to be responsible for QPOs.

In this Special Issue, I invite you to contribute to a discussion that aims to resolve these issues and pave the way for the further advancement of the field. We will also cover a broader range of topics related to the quasi-periodic oscillations in AGN:

• AGN and relativistic jet physics;
• supermassive binary black holes;
• high-energy emissions from extra-galactic periodic systems;
• accretions processed around supermassive black holes under extreme gravity;
• instabilities at the accretion disk and relativistic jet interface; and
• methods of time series analysis and estimation of significance.

Dr. Gopal Bhatta
Guest Editor

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