Galactic cosmic rays from star clusters and nearby supernovae remnants

Abstract

Cosmic rays (CRs) are high-energy particles, predominantly originating from outside the Earth’s atmosphere. These particles, ranging from protons to heavier nuclei, propagate through space at nearly the speed of light, carrying immense amounts of energy. Those are believed to come from sources from our galaxy (such as supernovae explosions, young massive star clusters, etc) as well as from outside of our galaxy (such as black holes, radio galaxies, and other energetic events in the universe). It is important to understand various sources, especially particle acceleration mechanisms in different energy ranges in these systems and several phenomena associated with particle propagation. In this thesis, we explore young massive star clusters as potential cosmic ray accelerators, particularly in the TeV-PeV energy range. Based on recent gamma-ray observations from young star clusters, we use numerical simulations and phenomenological models to explain the various aspects of cosmic ray acceleration and propagation. Our results shine light on many interesting features, such as (i) recent gamma-ray observations from young star cluster Westerlund1 can be associated with underlying cosmic rays that are accelerated in this environment, (ii) the cosmic rays originating from a distribution of massive star clusters in the Galaxy can act as potential second component of Galactic cosmic rays, (iii) the effect of nearby cosmic ray sources on the observed spectra. We connect our numerical and analytical work with available cosmic-ray data, γ-ray data, and X-ray observation. We also develop numerical models that solve the propagation equation of cosmic rays considering different associated microphysics. Using this model we try to explain different observed spectral features of different cosmic ray elements We investigate the implications of cosmic ray acceleration within the massive compact star cluster Westerlund 1, following its recent detection in γ-rays (Aharonian et al., 2019; Abeysekara et al., 2021). Recent observations unveil a radial distribution of the CR energy density following a 1/r profile. We delve into whether this profile can serve as a discriminatorx between two debated scenarios: (1) continuous CR acceleration within stellar wind-driven shocks in the star cluster and (2) discrete CR acceleration within multiple supernova shocks. Utilizing idealized two-fluid simulations and exploring various acceleration sites and diffusion coefficients, we derive the CR energy density profile and luminosity to best fit the γ-ray observations. We discover significant discrepancies between the inferred CR energy density profiles from γ-ray luminosity and mass observations and the true radial profile. CR acceleration occurring either at the cluster’s core region or the wind termination shock can account for the observations, provided the diffusion coefficient is approximately κcr ∼ 1027 cm2 s−1 and around 10%−20% of the shock power/post-shock pressure is allocated to the CR component. Additionally, we explore the possibility of discrete supernova (SN) explosions driving CR acceleration and find that with an injection rate of one SN occurring approximately every 0.03 Myr, the observed γ-ray profile can be explained. This multiple SN scenario remains consistent with X-ray observations only if the thermal conductivity closely resembles the Spitzer value