Supermassive black holes are usually found at the centers of galaxies behind thick layers of gas and dust. These active galactic nuclei (active meaning actively accreting, galactic nuclei meaning centers of galaxies; AGN) grow behind thick obscuring layers of gas and dust. The most obscured AGN become optically-thick to Compton scattering and are known as “Compton-thick.” The Cosmic X-ray Background predicts up to half of all AGN should be Compton-thick, but a much lower fraction have actually been observed to be and few examples are known.
This past summer, I was a Summer Undergraduate Research Fellow with the High Energy Astrophysics Group at Caltech, where I worked with Professor. Fiona Harrison and Dr. Peter Boorman on confirming a new Compton-thick AGN. For this project, we contructed a broadband X-ray spectrum using archival observations from XMM-Newton and NuSTAR. A contaminating source prevented us from taking the spectrum of our target AGN in the hard X-ray (i.e. the more energetic observation from NuSTAR), so we extracted the combined spectrum and strategically fit models to disentagle them. We conducted follow-up optical spectroscopy on the contaminant and determined it to be a new quasar. We used a nested-smapling algorithm to fit a phenomonological model and three physically-motivated obscuration models and found the AGN to be unambiguously Compton-thick. Excitingly, we plotted the sensitivity of the instruments to this object and found it to be at the very limit of detectability, making this a useful edge-case and shedding light on why so few Compton-thick sources have been found.
My iPoster from the 243rd AAS Meeting can be viewed here.
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