Date of Award
Master of Science (MS)
Physics and Astronomy
Studies of the non-thermal Galactic source population are essential to understand how and where the bulk of cosmic rays are being accelerated and to understand the mechanisms underlying very high energy (VHE, E>50 GeV) emitters [39, 51]. The plane of the Milky Way is rich with supernova remnants (SNRs) and pulsar wind nebulae (PWNe) which are eﬃcient accelerators of cosmic rays (CRs) - whose interaction with the surrounding photon ﬁelds produces energetic γ-rays and neutrinos. SNRs and PWNe are some of the most powerful objects in our Galaxy and because they emit at very high energies (VHE, E>50 GeV), γ-rays represent an excellent probe of the non-thermal astrophysical processes in these objects.
Relativistic electrons (i.e. leptons) can produce γ-rays by non-thermal bremsstrahlung or by inverse Compton scattering (IC) on ambient photon ﬁelds, whereas protons and heavier nuclei (i.e. hadrons) can generate γ-rays by the process of pion decay, produced in collisions between relativistic hadrons and ambient material. Understanding the particle population responsible for the observed γ-ray emission can provide clues to the potential of CR acceleration as most cosmic rays are made of protons or heavier nuclei (∼10% of all cosmic rays are leptons) so, if it can be established that the γ-ray emission is hadronic in origin, then we can better understand the likelihood for hadron CR acceleration in VHE objects.
In this thesis, we report on the investigation of a very high energy (VHE), Galactic γ-ray source recently discovered at >50 GeV using the Large Area Telescope (LAT) on board Fermi. This object, 2FHL J0826.1−4500, displays one of the hardest >50 GeV spectra (Γγ ∼ 1.6) in the 2FHL sample, and a follow-up observation with XMM-Newton has uncovered diﬀuse, soft thermal emission at the position of the γ-ray source. A detailed analysis of the available multi-wavelength data shows that this source is located on the Western edge of the Vela supernova remnant: the observations and the spectral energy distribution modeling support a scenario where this γ-ray source is the byproduct of the interaction between the SNR shock and a neutral Hydrogen cloud. If conﬁrmed, this shock-cloud interaction would make 2FHL J0826.1−4500 a promising candidate for eﬃcient particle acceleration. This work has been recently published in the Astrophysical Journal .
In chapter 1, the objective of this thesis is introduced. In chapter 2, SNRs and PWNe are explained in detail with a focus on the Vela SNR - the closest composite SNR to Earth. In chapter 3, we discuss the main instruments used to obtain the γ-ray and X-ray data, namely XMM-Newton and the Fermi-LAT. Chapter 4 describes the data reduction process and spectral analysis and a multi- wavelength description of 2FHL J0826.1−4500 is presented in chapter 5. Chapter 6 tests the spectral energy distribution (SED) of the source, attempting to determine the dominant parent particle population to better understand its emission mechanisms. In chapter 7 we report our conclusions on 2FHL J0826.1−4500 and emphasize important properties that still need to be probed in order to best answer the underlying question: if 2FHL J0826.1−4500 is an eﬃcient particle accelerator, can we safely establish if this is a site generating fresh CRs or does the energetic environment favor a scenario where pre-existing CRs are being re-accelerated here?
Eagle, Jordan Lynn, "Investigation of a Candidate for Cosmic Ray Acceleration" (2019). All Theses. 3195.