Probing the Corona in Active Galactic Nuclei Using Broadband X-Ray Spectroscopy

Citation

Kamraj, Nikita (2022) Probing the Corona in Active Galactic Nuclei Using Broadband X-Ray Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cag4-k322. https://resolver.caltech.edu/CaltechTHESIS:12142021-081118617

Abstract

Active Galactic Nuclei (AGN) are some of the most luminous sources in our known universe, powered by large-scale accretion onto a supermassive black hole. Supermassive black holes are known to co-evolve with their host galaxies, with AGN playing an important role in regulating feedback in galaxies by depositing large amounts of energy through outflows and jets. While studies of AGN have dated back to the 1960's when the first quasar was discovered, much is still unknown about the key source powering the X-ray emission in AGN, i.e. the corona. In my thesis, I present detailed investigations of the properties of AGN coronae using broadband X-ray spectroscopic techniques. This work utilizes spectroscopic data taken with the NuSTAR telescope, which has revolutionized studies of the corona. Being the first focusing high energy X-ray telescope in orbit, NuSTAR's high sensitivity at hard X-ray energies (> 10 keV) has enabled robust measurements of fundamental properties of the corona, such as its temperature, from single epoch observations of AGN for the first time. In the first study presented in this thesis, I performed measurements of coronal temperature in a sample of 46 NuSTAR-observed AGN through fitting X-ray spectral models for each object. My analysis showed the temperature of the corona to be regulated by electron-positron pair production and annihilation processes. From this sample, I identified an AGN with an unusually low coronal temperature, 2MASX J19301380+3410495. I modeled the broadband X-ray spectrum of this object in detail using multi-epoch X-ray observations taken with the Swift, XMM-Newton, and NuSTAR telescopes, and found the object to also belong to a rare class of X-ray obscured but optically unobscured AGN. Using multi-wavelength information, I elucidated the nature of the complex obscuration present in 2MASX J19301380+3410495. In recent work presented in this thesis, I compiled one of the largest samples of unobscured AGN with high quality NuSTAR X-ray spectra in order to characterize how the physical properties of the corona relate to fundamental accretion parameters in AGN, such as the Eddington ratio and mass of the supermassive black hole. Finally, I discuss possible future work directed at investigating other enigmatic AGN similar to 2MASX J19301380+3410495 that have conflicting optical and X-ray classifications. Using techniques such as multi-wavelength spectropolarimetry in addition to X-ray spectroscopy, it may be possible to unveil the mechanisms of obscuration within these exotic sources that challenge classical pictures of AGN structure.

Thesis Files