Dissertation/Thesis Abstract

Cosmic-Ray Acceleration in the Cygnus OB2 Stellar Association
by Hona, Binita, Ph.D., Michigan Technological University, 2020, 183; 27742912
Abstract (Summary)

The Cygnus region of our Galaxy consists of an active star forming region and a wealth of various astrophysical sources such as pulsar wind nebulae (PWN), supernova remnants (SNRs), and massive star clusters. Massive stellar clusters and associations have been postulated as possible sources of cosmic rays (CRs) in our Galaxy. One example of a gamma-ray source associated with a stellar association lies in the Cygnus region known as the "Cygnus Cocoon". It is an extended region of gamma-ray emission in the Cygnus X region and attributed to a possible superbubble with freshly accelerated CRs which are hypothesized to produce gamma rays via interaction with the ambient gas nuclei. The emission region is an environment of lower particle density and is surrounded by ionization fronts like a carved-out cavity or a cocoon.

CRs in the Cocoon could have originated in the OB2 association and been accelerated at the interaction sites of stellar winds of massive type O stars. So far, there is no clear association at TeV energies. In the study presented in this thesis, I used data collected by the HAWC Observatory over 1038 days to disentangle the TeV gamma-ray emission from 2HWC J2031+415, a source which was previously reported in the 2nd HAWC catalog and is collocated with the Cygnus Cocoon, into two components: a pulsar wind nebula and the Cygnus superbubble. The contribution from the Cygnus superbubble is detected at a significance level of ~ 12 sigma with maximum photon energies above 100 TeV, the highest measured yet. Based on the spectrum and morphology of gamma-rays across six decades of energy, and the non-detection of radio and X-ray photons from this region, the gamma-rays are plausibly of a hadronic origin. There is a spectral softening above 1 TeV, which can be explained by two hadronic scenarios. Either there is a leakage of CRs from the superbubble resulting in a spectral break from GeV to TeV or the spectral softening is due to cutoff energy, an upper limit to the particle acceleration by the stellar winds.

Indexing (document details)
Advisor: Huentemeyer, Petra H
Commitee: Nitz, David, Fick, Brian, Sha, Qiuying
School: Michigan Technological University
Department: Physics
School Location: United States -- Michigan
Source: DAI-B 81/11(E), Dissertation Abstracts International
Subjects: Astrophysics
Keywords: Cygnus region
Publication Number: 27742912
ISBN: 9798645466718
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