Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt.........7c&link_type=abstract
Thesis (PhD). UNIVERSITY OF CALIFORNIA, SAN DIEGO, Source DAI-B 62/06, p. 2756, Dec 2001, 252 pages.
Physics
10
Scientific paper
Pulsars are known to be rotating neutron stars that appear to emit regular flashes or radiation. For accretion powered pulsars, the emission is powered by the accretion of material from a normal stellar companion onto the magnetic poles of the neutron star. The conditions in these polar regions, which exhibit extremes in gravitation, temperature, and magnetic field strength, are impossible to recreate in terrestrial laboratories and are possibly unique in nature. Despite two decades of work, no compelling models exist explaining how the infalling material distributes itself across the polar caps, or how the observed X-ray continuum is formed. More fundamentally, these are unanswered questions of how matter acts and reacts in this extreme environment. By studying the X-ray spectra of these sources, we can hope to elucidate some of these questions. Some accreting pulsars exhibit absorption-like X-ray features, or cyclotron lines. The energies of these lines are the only direct measure of the magnetic field of a neutron star, and their detailed line profiles are sensitive to the physical parameters in the formation region. In this work I have used data from NASA's Rossi X-ray Timing Explorer to study the geometry, physical conditions, and dynamical behavior of phenomena in the polar regions of these rotating neutron stars. I present two new cyclotron lines I discovered during the course of the research in the spectra of 4U 0352+309 and XTE J1946+274. I outline a new method for using cyclotron line shapes as a function of neutron star rotation, along with the temporal structure of the X-ray pulses, to self consistently describe the geometry of the emission regions. This type of analysis is a powerful tool for studying the accretion structures that form at the pulsar magnetic poles. I apply the method qualitatively to three sources, and discuss prospects for future work. I find that the characteristic spectral break energy in X-ray continua is correlated with the magnetic field. This has important implications for theoretical work on the formation of the continuum. I also find two correlations among the line shape parameters, and discuss the possibility that this might imply that accretion seeks to align the dipole and spin axes. Finally, I detail a prescription for finding upper limits on cyclotron lines in accreting pulsar spectra, and apply it to twelve sources.
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