Unit:
Κατεύθυνση ΑστροφυσικήLibrary of the School of Science
Author:
Vrontaki Konstantina
Supervisors info:
Κοσμάς Γαζέας, Λέκτορας, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Μαρία Πετροπούλου, Επίκουρη Καθηγήτρια, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Stanislaw Zola, Professor, Astronomical Observatory, Jagiellonian University
Original Title:
Multiwavelength study of QPOs in Active Galactic Nuclei
Translated title:
Multiwavelength study of QPOs in Active Galactic Nuclei
Summary:
Active galaxies have greater luminosity than anticipated based on their stellar population. Their luminosity is due to non-thermal phenomena that occur in a compact region in the center of the galaxy, known as Active Galactic Nucleus (AGN). A supermassive black hole (SMBH) exists in the center of the AGN, surrounded by hot magnetized plasma that creates an accretion disk. AGNs are classified in different subclasses based on their characteristics. The most energetic subclass of AGNs is blazars. Their jets are aligned with the line of sight and they display variability of different timescales across the electromagnetic spectrum. Although their signal seems non-periodic, quasi-periodic oscillations have been reported in the recent years.
In this study, that was carried out in the frame of BOSS Project, Quasi-Periodic Oscillations (QPO) of intra-day (IDV), short-term (STV) and long-term (LTV) timescales were sought in the light curves of blazars. Time-domain analysis of multi-wavelength light curves of AGNs was conducted by combining radio and optical wavelengths, and X-rays, in order to compare the results and find the correlation between the different wavelengths. A total of 21 targets were studied in 5 energy bands between 2013 and 2021. The data in optical wavelengths are from the University of Athens Observatory (UOAO), with observations conducted in the frame of BOSS Project. In radio wavelengths data were provided by Metsähovi Radio Observatory (37 GHz), Owens Valley Radio Observatory (OVRO) (15 GHz), and Xinjiang Astronomical Observatory by the Nanshan Radio Telescope (NSRT) (4.8 GHz). Data in X-rays were obtained from the Swift Space Telescope (Swift-XRT). The data were analyzed using the Discrete Fourier Transform (DFT) and the Lomb-Scargle Periodogram (LSP) in order to find the timescales of QPOs.
The detected timescales of QPOs can provide information about the inner regions of the AGN, which are difficult to observe directly with our current instruments. For example, the mass of the SMBH of AGNs is usually calculated using spectroscopic methods, but due to the featureless continuum of blazars, such methods have several constraints. On the other hand, the detected QPOs are an important tool in the estimation of the mass of the SMBH of blazars and AGNs in general, setting a limit to the value. The detected timescales also place constraints on other parameters of AGNs such as the Eddington luminosity and the accretion rate.
The comparison of the detected timescales of QPOs in different wavelengths provides insights about the region of origin of the quasi-periodic signals of the AGNs, giving information on whether the signals originate from the same region or not. This study indicates that the QPOs arise from the same region regardless the energy band that were detected. This confirms the model that suggests that quasi-periodic signals arise from instabilities in the inner region of the AGN that propagate along the jet.
Multi-wavelength time series analysis and the detection of QPOs can shed light in the mysterious nature of AGNs, the processes that occur in their inner region, where direct observations are difficult to be made, and the mechanisms that lead to quasi-periodic signals.
Main subject category:
Science
Keywords:
BOSS Project, Active Galactic Nuclei, Blazars, Quasi-periodic Oscillations, photometry, time-series analysis, Discrete Fourier Transform, Lomb-Scargle Periodogram, Multi-wavelength Astronomy
Number of references:
192
