The Transport of High Energy Cosmic Rays in a TeV Gamma Ray Pulsar Wind Nebula

Abstract

Pulsars are dense, rapidly spinning remnants of massive stars. Charged particles are accelerated beyond TeV energies by the extreme environment around the pulsar and emit radiation to form the Pulsar Wind Nebula (PWN). These particles escape into the interstellar medium (ISM) and interact with the ISM or soft photon fields to produce gamma rays or with magnetic fields to produce radio to X-ray emission. One of the major mysteries in modern-day astrophysics is how protons and electrons propagate (e.g. di↵usion or advection) within PWN environments. HESS J1825-137 is a bright, extended TeV PWN, making it an ideal laboratory to study particle transport in PWN. Both the HAWC and LHAASO observatories have observed gamma-ray emission from HESS J1825-137 greater than 50TeV, indicative of a PeVatron; a source capable of accelerating electrons up to energies greater than 1015 eV. This thesis focuses on understanding the origin of the X-ray to gamma-ray emission towards HESS J1825-137. Fermi-LAT observations revealed extended GeV emission to the Galactic south of HESS J1825-137. The first portion of this thesis investigated whether this GeV emission originated from the PWN associated with HESS J1825-137, its progenitor supernova remnant (SNR) or a source linked to the nearby X-ray binary LS 5039. ISM gas analysis was first conducted towards this region to constrain the multi-wavelength emission. The analysis highlighted a dense cloud of CO(1-0) gas lying towards the GeV region at the same distance as LS 5039 that is coincident with a HU SNR rim associated with HESS J1825-137. The results of the gas analysis was combined with spectral energy distribution (SED) modelling to show that neither a source associated with HESS J1825-137 or LS 5039 is likely to be the sole origin of high-energy protons or electrons towards the GeV region. A combination of both sources could result in the gamma-ray emission. This study emphasised the complexity of the region towards HESS J1825-137. The second part of this thesis investigated the multi-wavelength SED and gamma-ray morphology towards HESS J1825-137 to disentangle the transport mechanisms of electrons from the pulsar. Electrons escaping the PWN propagate di↵usively (where particles scatter o↵ turbulence, resulting in ’random-walk’) and/or via advection (the bulk motion of particles). The region towards HESS J1825-137 was divided into a 3D grid of spatially-dependent number density and magnetic field. The transport of electrons from the pulsar wind nebula was then modelled using a numerical solution of the transport equation to reproduce the multi-wavelength SED and gamma-ray morphology seen towards HESS J1825-137. A di↵usive model with an advective velocity of 0.0022 (2 is the speed of light) towards lower Galactic longitudes can broadly explain the observations. Additionally, a turbulent region of gas with a magnetic field between 20−60 μG is required to prevent significant gamma-ray contamination towards the nearby northern TeV source, HESS J1826-136. The modelling conducted in this thesis is not constrained to HESS J1825-137 and can be applied to other TeV PWN or other gamma-ray sources to develop understanding of how cosmic-ray sources evolve in their respective environments.