Date of Award
2024
Document Type
Open Access Dissertation
Degree Name
Doctor of Philosophy in Physics (PhD)
Administrative Home Department
Department of Physics
Advisor 1
Petra Huentemeyer
Committee Member 1
Brian Fick
Committee Member 2
Elena Giusarma
Committee Member 3
Quiying Sha
Abstract
In this thesis, I present an analysis pipeline that allows one to search for and identify sources of very-high-energy (VHE) γ-ray emission in sky maps created from data collected with the High Altitude Water-Cherenkov (HAWC) Observatory. The HAWC Observatory is located at about 4100 m elevation just below Pico de Orizaba, the highest mountain in Mexico also known as Citlaltepetl (from the Nahuatl words citlal(in) = star, and tepetl = mountain). The observatory is an array of several hundred detector stations that measure signals from particle showers, so-called extensive air showers (EAS), that are initiated in interactions of primary cosmic rays and gamma rays when they strike the Earth’s atmosphere. Approaching its 10th anniversary, the HAWC Observatory has so far collected a total of about 10 PB of raw data, which are processed and condensed down to a few GB of high-level gamma-ray enhanced sky maps that cover approximately two-thirds of the sky. These maps contain the directional information of signal and background EAS events. These events are binned by the fraction of sensors that are triggered by the particles of an EAS event, an observable that is correlated with the primary particle energy. These parameters are used as inputs of the analysis pipeline and analyzed using a likelihood ratio approach. In the first step, the maps are fit with models of an increasing number of point-like sources on top of a template of the predicted diffuse γ-ray flux. Once the addition of another point source model no longer improves the modeling by a specified Test Statistics (TS) threshold, in a second step the source excesses thus far identified are tested for angular extent, usually leading to a reduction of the number of sources as statistical fluctuations will be absorbed by the extended source models. In a final step, the remaining VHE sources are tested for curvature in their spectral energy distribution, i.e. a cut-off or suppression of emission at the highest detectable γ-ray energies. This automated approach enhances the reproducibility of analyses undertaken on HAWC sky maps. sources are identified in sky maps representing 2967 days of HAWC observations. A total of 12 TeV sources in this list have no identified counterparts in other VHE catalogs and will be discussed individually. Among the 85 identified sources 57 are associated within 1° with pulsars, 12 of which are TeV Halo candidates, 19 are associated with supernova remnants, a total of 7 sources are associated closely with low mass x-ray binaries, 4 offset lobes, and 3 with the binary directly within 0.2°. A comparison with LHAASO observations shows that of the 85 4HWC sources 22 have no counterpart in the 1LHAASO catalog. A total of 38 4HWC sources are associated with sources that have been detected by Imaging Atmospheric Cherenkov Telescopes (IACTs). Computing the position offset between 4HWC and these two sets of associated sources shows very good agreement between HAWC and LHAASO (mean declination offset of 0.01°) and IACTs (mean declination offset of 0.02°). This represents a significant improvement in data reconstruction compared to the 3HWC catalog which had a significant offset with IACT components of over 0.12° worsening with declination. Altogether the constructed 4HWC represents the largest and most sensitive VHE source catalog produced by HAWC to date.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Recommended Citation
Groetsch, Samuel J., "CONSTRUCTING THE 4TH HAWC CATALOG OF VERY-HIGH-ENERGY SOURCES USING AN AUTOMATED LIKELIHOOD PIPELINE SEARCH", Open Access Dissertation, Michigan Technological University, 2024.
