Thermal and Non-Thermal Contributions to the X-ray Spectrum of Large Magellanic Cloud Supernova Remnants

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Title: Thermal and Non-Thermal Contributions to the X-ray Spectrum of Large Magellanic Cloud Supernova Remnants
Author: Hendrick, Sean Patrick
Advisors: Kazimierz J. Borkowski, Committee Member
Stephen P. Reynolds, Committee Chair
Thomas Drake, Committee Member
Chueng R. Ji, Committee Member
Abstract: The study of supernovae and the expanding remnants that form after the explosion is important to understanding star formation and the distribution of elements in the interstellar medium (ISM). In supernova explosions all elements beyond iron are synthesized and then distributed into the ISM, where new star formation is begun in the wake of the explosion. Examining the galactic sample of supernova remnants (SNRs) is limited by absorption and distance uncertainties. The Large Magellanic Cloud (LMC) contains over thirty SNRs at a known distance with little absorption. By examining the X-ray spectrum of a large sample of supernova remnants we can learn how they evolve and how the elements are distributed. This work examines the X-ray spectrum of eleven archival ASCA observations and three new Chandra observations of LMC remnants. The plasma that is responsible for the X-ray emission is heated by a shock wave created by the supernova explosion that expands into the ISM. The picture is further complicated by a reverse shock that is created at the interface between the ISM and the material ejected from the star by the explosion. The ejecta is comprised of elements made by stellar fusion, while the ISM is mostly hydrogen and helium. After the shock passes, the ions contain most of the thermal energy while the electrons are still cold. Coulomb interaction between the ions and electrons behind the shock heat the electrons and continue to ionize the heavy elements, until equilibrium is reached. Non-equilibrium ionization (NEI) models are used to define the thermal contributions to the spectrum. Existing models assume that hydrogen and helium are the sole source of electrons in the plasma. A new NEI model is introduced which accounts for the overabundance of heavy elements in the remnant ejecta and their contribution to the electron density. At each step in the ionization history of the plasma, the ionization fractions of ten elements are determined. This is a plane-parallel shock model that can account for non-Coulomb heating at the shock front and uses shock velocity as a parameter rather than shock temperature. The heavy element model is tested against the current models and used to fit the Chandra observations. Hard, non-thermal tails that are well described by a power law have been observed in the X-ray spectrum of several remnants. Synchrotron radiation observed in the radio regime for low energy electrons can be extrapolated to X-ray energies to account for high energy(> 1 TeV) electrons. By combining X-ray data with radio observation parameters we can constrain the maximum energies of the shock-accelerated electrons in the supernovae above which the electron spectrum must steepen. None of the remnants in this study have a maximum energy above 100 TeV. The heavy element NEI model has proven a valuable tool that will be released to the community. It determined the forward shock velocities of the Chandra observations to be 1018 km/s (0534-69.9), 1080 km/s (0548-70.4) and 1170 km/s (0453-69.5). SNR 0534-69.9 and SNR 0548-70.4 spectra show middle-aged remnants, type II and type Ia explosions respectively, with significant contributions from the ejecta inside the outer shell. The SNR 0453-68.5 observation indicates the presence of a pulsar wind nebula.
Date: 2003-04-15
Degree: PhD
Discipline: Physics

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