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Identifier

000465401

Title

Study of the optical/UV time lags in quasars

Alternative Title

Μελέτη καθυστερήσεων στις μεταβολές στα υπεριώδη και οπτικά μήκη κύματος σε ενεργούς γαλαξίες

Author

Λάγγης, Δημήτριος-Αλκίνοος Α.

Thesis advisor

Παπαδάκης, Ιωσήφ

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Ζέζας, Ανδρέας
Κυλάφης, Νικόλαος

Abstract

Extensive, multi-wavelength monitoring campaigns of nearby and higher redshift Active Galactic Nuclei (AGN) have shown that the UV/optical variations are well correlated with time delays which increase with increasing wavelength. Such behaviour is expected in the context of the X-ray thermal reverberation of the accretion disc in AGN. Our main objective is to measure the mean time-lags spectrum of quasars, which are more distant, more luminous and have larger black hole (BH) mass than the nearby AGN, using data from various surveys, and fit it with sophisticated X-ray reverberation time-lags models. We can investigate in this way whether X-ray reverberation can indeed explain the observed continuum time-lags, and we can also determine physical parameters such as the X-ray corona height and the spin of the black hole in these systems. We use archival time-lag measurements for quasars from different surveys, and we compute their rest frame, mean time-lags spectrum. We fit the data with analytical X-ray reverberation models, using χ^2 statistics, and fitting for both maximal and non spinning BHs, for various color correction values and X-ray corona heights. We found that X-ray reverberation can explain very well the observed time-lags, assuming the measured BH mass, accretion rate and X-ray luminosity of the quasars in the sample. This is the first time that X-ray reverberation is shown to be in agreement with the observed continuum time lags in quasars. The model agrees well with the data both for non-rotating and maximally rotating black holes, as long as the corona height is larger than $~40 gravitational radii. This is in agreement with previous results which showed that X-ray reverberation can also explain the disc radius in micro-lensed quasars, for the same corona heights. The corona height we measure depends on the model assumption of a perfectly flat disc. More realistic disc models may result in lower heights for the X-ray corona.

Language

English

Subject

Active Galaxies

Quasars

Αccretion disc

Δίσκος προσαύξησης

Ενεργοί γαλαξίες

Κβάζαρ

Issue date

2024-07-11

Collection