Gamma-ray Analysis of the Most Energetic Blazars to Probe the Cosmos

The extragalactic background light (EBL) is all the accumulated infrared and optical radiation in the universe produced by stars, dust, and active galactic nuclei (AGNs). Studying EBL is important for understanding the energetics of the universe and its evolution. Directly measuring EBL is difficult due to the contribution of zodiacal light that outshines the EBL in the foreground. We present a method of indirect EBL measurement using gamma-ray emissions of the brightest AGNs, blazars, to probe the EBL density along the line of sight. This project utilizes ten years of LAT data, the largest dataset ever used for an EBL study. Gamma-ray photons are a viable probe for EBL due to their interactions: when a gamma-ray photon collides with another photon, they annihilate in a process called pair production, forming a positron and electron pair and causing absorption that alters the observed spectrum. For certain blazars, the observed spectrum did not validate any existing EBL models. This warrants additional analysis as it might indicate local EBL anisotropy. To constrain the parameters of the EBL model, we present a complex multi-step likelihood analysis of the deviant sources to produce gamma-ray spectra, leveraging the Fermi Science Tools created by NASA for the Fermi-LAT mission. The likelihood analysis attributes photons to known sources from the FL8Y catalog, a recently released database of all known gamma-ray sources. The developed method is applicable to hundreds of LAT gamma-ray sources and we hope that it will provide the best gamma-ray based EBL measurement to date.