Dissertation
Probing AGN accretion flows with multi-wavelength time-domain observations
Doctor of Philosophy (Ph.D.), Drexel University
Jun 2023
DOI:
https://doi.org/10.17918/00001733
Abstract
At UV/optical wavelengths, the luminosity of Active Galactic Nuclei (AGNs) varies stochastically on timescales ranging from hours to years. This stochastic variability originates from AGN's accretion disk and provides unique information about its inner workings. It has recently been shown that AGN UV/optical variability exhibits different characteristics at short and long timescales. This dissertation utilizes the approach of directly modeling AGN UV/optical light curves as a noise-driven damped harmonic oscillator (DHO) process. This approach enables us to connect short and long-term variability and probe the accretion flows in AGNs. The DHO process is a second-order continuous-time autoregressive moving-average process--a statistical model commonly used to forecast the stock market. We carry out two investigations in this study: 1) we apply the DHO model to ~ 12,000 AGNs with 8-years-long ugriz light curves collected from the Sloan Digital Sky Survey (SDSS); 2) we repeat the process for ~ 24,000 AGNs with combined gri light curves from SDSS and the PanSTARRS1 (PS1) survey. The distributions of DHO fits obtained in these two investigations are broadly consistent, demonstrating that AGNs occupy a unique part of the DHO parameter space. We find that: 1) the best-fit DHO parameters correlate with the probed rest-frame wavelength ([lambda] RF ) and the fundamental properties of our modeled AGNs; 2) the modeled long-term variability at [lambda] RF > 2800A might be contaminated by the Balmer diffuse continuum external to the accretion disk; 3) the long-term characteristic variability timescale extracted by the DHO exhibits a wavelength dependence that is closest to the theoretical prediction when compared to similar timescales extracted using other methods. We demonstrate that the DHO process is a viable choice for directly modeling AGN's UV/optical variability. The upcoming all-sky time-domain survey, Rubin Observatory's Legacy Survey of Space and Time (LSST), will benefit from this approach, given LSST's superior light curves.
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Details
- Title
- Probing AGN accretion flows with multi-wavelength time-domain observations
- Creators
- Weixiang Yu
- Contributors
- Gordon T. Richards (Advisor)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Number of pages
- xiii, 81 pages
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- College of Arts and Sciences; Physics; Drexel University
- Other Identifier
- 991021120114304721