Date of Award
8-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Physics and Astronomy
Committee Chair/Advisor
Marco Ajello
Committee Member
Dieter Hartmann
Committee Member
Steve Kaeppler
Committee Member
Mark Leising
Abstract
Blazars are active galactic nuclei (AGNs) with their jets aligned towards Earth. Gamma-ray bursts (GRBs) can be classified into long and short types. Long GRBs are produced by the collapse of massive stars, while short GRBs are formed by binary mergers (neutron star - neutron star or neutron star - black hole mergers). Both objects are the key probes of extreme physics processes and cosmic evolution, providing us an unique window to understand the Universe. My research aims to measure the redshift of blazars and localize GRBs for proposed and upcoming gamma-ray telescopes. BL Lac objects (BL Lacs) are a subclass of blazars whose optical spectra exhibit no or weak emission lines, posing significant challenges for redshift measurement using traditional spectroscopic methods. As a consequence, the known population of high-redshift ($z \gtrsim 1.3$) BL Lacs remains small, with only 13 BL Lacs reported in the Third Fermi-LAT AGN Catalog (3LAC). To address this limitation, an alternative method known as the photometric approach is employed to estimate photometric redshifts (photo-$z$) for BL Lacs. In my research, I have performed observations with the Southeastern Association for Research in Astronomy (SARA) ground-based telescopes, obtaining photometric data from near-infrared (nIR) to optical bands. Additionally, I requested ultraviolet (UV) observations from the {\it Swift} observatory. The photometric data are fitted to a set of spectral energy distribution (SED) templates to determine the photometric redshifts or upper limits. The photo-$z$ campaign has discovered 23 new high-redshift BL Lacs, seven of which were discovered through my work. The results demonstrate that the photometric approach is highly efficient for identifying the previously missing population of high-redshift BL Lacs. Besides the nIR to UV observations, my research also extends to high-energy gamma-ray regime. Specifically, I contributed to evaluating the GRB localization capability for the All-sky Medium-Energy Gamma-ray Observatory eXplorer (AMEGO-X) and developing localization methods for the Compton Spectrometer and Imager (COSI). AMEGO-X is a proposed space telescope designed to probe the underexplored medium-energy gamma-ray (MeV) band. To assess its GRB localization performance, I employed the Medium-Energy Gamma-ray Astronomy library (MEGAlib) and developed a Python-based pipeline to streamline the localization simulations. The results indicate that AMEGO-X can localize short GRBs with an accuracy ranging from 0.5 to 2.1 degrees, which is included in the AMEGO-X mission proposal submitted to NASA. In addition, COSI is an upcoming gamma-ray telescope that also aims to probe the MeV band. I contributed in developing COSI's data analysis package, \texttt{cosipy}, focusing on the GRB localization algorithm for GRBs. I employed a likelihood-based approach using Poisson log-likelihood ratio (LLR) test statistics (TS) to fit COSI data to models in Compton data space. Parallel computing techniques and a multi-resolution TS map were implemented to accelerate the fitting process, enabling COSI to localize GRBs within an hour of detection. Moreover, I developed the \texttt{SpacecraftFile} and \texttt{SourceInjector} modules, responsible for coordinate conversions and generating mock COSI data, respectively. Overall, my photo-$z$ research enhances our understanding of the blazar population by discovering the missing high-$z$ BL Lacs, while localization work develops the method to accurately localize GRBs. Both work improves our knowledge of extreme astrophysical phenomena and optimizes scientific returns for proposed and upcoming gamma-ray telescopes.
Recommended Citation
Sheng, Yong, "Discovering High-redshift BL Lacs and Localizing GRBs for MeV missions" (2025). All Dissertations. 4027.
https://open.clemson.edu/all_dissertations/4027
Author ORCID Identifier
0000-0002-3833-1054