Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

Committee Chair/Advisor

Dr. Marco Ajello

Committee Member

Dr. Markus Ackermann

Committee Member

Dr. Dieter Hartmann

Committee Member

Dr. Bradley Meyer

Committee Member

Dr. Joan Marler


The entirety of the γ-ray radiation permeating our Universe is encoded in the extragalactic γ-ray background. This is a superposition of resolved sources, mostly powerful relativistic jets powered by supermassive black holes, i.e., blazars, and an unresolved isotropic component, aka, the diffuse isotropic gamma-ray background (IGRB). Studying the IGRB can help unveil its composition, as well as unearth multi-messenger relationships between the intensities of PeV neutrinos, ultra high energy cosmic rays (> 1018 eV), and sub-TeV γ-rays. The comparable energy budgets of these three phenomena (neutrinos, UHECR, and γ-rays) indicates a physical connection or a common source amongst them. On the other hand, in the resolved regime, open questions remain about the distribution of blazars through cosmic time. Importantly, determining the properties of BL Lac sources has been challenging as their lack of emission lines hampers the determination of their redshift, and thus distance. Furthermore, some of these sources can accelerate particles up to TeV energies and produce cosmic rays and neutrinos. Determining their redshifts is therefore paramount to establishing constraints on the most powerful particle accelerators in the universe.

In my thesis, I addressed the lack of redshift of BL Lacs in the Fermi-LAT catalogs and found new high-redshift sources through the photometric technique (Rajagopal et al., 2020a), as well as through optical spectroscopy (Rajagopal et al., 2021). I also addressed the new source population of “masquerading BL Lacs” by developing a model for one of the high-z sources (Rajagopal et al., 2020b), hence pushing the understanding of this source population further. I also worked on finding and establishing a new measurement of the IGRB that can help uncover new physics and exotic processes in the universe, and the studies of multimessenger astrophysics.

Author ORCID Identifier




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