Discovering and Exploring Extreme Black Holes and their Environments in the Transient Universe
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2023
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Supermassive black holes (SMBHs) at the centers of galaxies are some of the most luminous objects in the universe when feeding on surrounding material. This occurs when the SMBH is continuously accreting, called an active galactic nuclei (AGN). Or, a star can wander too close to the SMBH and becomes destroyed by the tidal forces, called a tidal disruption event (TDE), and the SMBH feeds on the stellar debris. The current landscape of astrophysics is rich with missions and telescopes ideal for exploring the transient universe, including AGNs and TDEs. Surveys, which image the entire night sky on various timescales ranging from minutes to days, provide crucial data to discover and characterize these nuclear transients. After developing a large sample of highly variable AGN observed by the ASAS-SN survey, I discovered a new, unique nuclear transient that undergoes periodic flares called ASASSN-14ko. One flare in 2014 was mistakenly classified as a supernova and the periodic behavior went undetected until my discovery. ASASSN-14ko is located within AGN ESO 253-G003 with optical flares present in the ASAS-SN light curve since 2014, and after modeling the peak times I find their timings are well-modeled with a period of 115.2 +1.3 -1.2 days and period derivative of -0.0026 +/- 0.0006. I organized, led, carried out, and analyzed data in an intensive multi-wavelength observing campaign to understand this unique object, and obtained X-ray, UV, and optical photometry and spectroscopy with numerous space- and ground-based facilities since 2020. All flares are characterized by a concurrent brightening in the UV and optical. The X-ray luminosities consistently decrease and the spectra become harder during the UV/optical rise but apparently without changes in absorption. The flares' photometric and spectroscopic signatures evolve rapidly. With the current suite of data, ASASSN-14ko is observationally consistent with a repeating partial tidal disruption event. This is the only such object ever discovered that has strongly periodic flares across X-ray, UV, and optical wavelengths, and provides a new laboratory to model TDEs and refine their theoretical frameworks.
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Astrophysics, Astronomy
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215 pages
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