Non-thermal emission from young supernova remnants: Implications on cosmic ray acceleration

Miguel A Araya-Arguedas, Purdue University

Abstract

For a long time, supernova remnants have been thought to constitute the main source of galactic cosmic rays. Plausible mechanisms have been proposed through which these objects would be able to transfer some of their energy to charged particles. Detailed studies of SNRs, particularly allowed by the spectral and spatial resolution obtained with telescopes such as the Chandra X-Ray Observatory, have permitted us to understand some of the properties of high-energy particles within these objects and their interactions with their environment. In the first part of this work, the basic concepts of particle acceleration in SNRs are outlined, and the main observational tools available today for studying high-energy phenomena in astrophysics are mentioned briefly. In the second part, a study of non-thermal emission from the young SNR Cassiopeia A is presented. Through the use of a very deep one million-second Chandra observation of this remnant, the spectral evolution across non-thermal filaments near the forward shock was studied. A consistent hardening of the spectrum towards the exterior of the remnant was found and explained via a model developed that takes into account particle diffusion, plasma advection and radiation losses. The role of particle diffusion was studied and its effect on the photon spectral index quantified. In the model, the diffusion is included as a fraction of Bohm-type diffusion, which is consistent with the data. The model also allowed an estimation of the electron distribution, the magnetic field and its orientation, as well as the level of magnetic turbulence. In the third part, a multi-wavelength study of two young SNRs is presented. Multi-wavelength modeling of spectral energy distributions (SED) may hold the key to disentangle the nature and content of cosmic rays within these objects. The first model shown presents state of the art measurements gathered for Cassiopeia A, and the modeling is based partly on the results presented in the second part. The good quality SED shown at gamma ray energies was obtained from observations by the VERITAS array and the recently-launched Fermi telescope. It is found that the leptonic emission expected from the different electron populations observed in this remnant can account for the observed flux at TeV energies but fails to account for the SED at GeV energies where instead a hadronic population responsible for pion-zero production is proposed to explain the data, constituting evidence for ion acceleration in Cassiopeia A. A similar treatment was followed for data from Tycho's SNR and the multi-wavelength modeling of this source is presented. The radio to X-ray data can be well explained with one population of electrons, and the gamma ray fluxes observed at TeV energies are compatible with leptonic emission expected from non-thermal Bremsstrahlung and Inverse Compton scattering processes when adopting a magnetic field value which corresponds to a lower limit allowed by the data. Results indicating a possible detection of this source at GeV energies are shown. The flux measured at these energies, if confirmed to be associated with Tycho's SNR, would be compatible, similarly to the case of Cassiopeia A's SED, with hadronic emission and a higher field.

Degree

Ph.D.

Advisors

Cui, Purdue University.

Subject Area

Astrophysics

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