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The binary system containing the pulsar PSR 1913+16 and ultra-violet and x-radiation from accreting magnetic white dwarfs

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Title: The binary system containing the pulsar PSR 1913+16 and ultra-violet and x-radiation from accreting magnetic white dwarfs
Author(s): Masters, Albert Ronald
Doctoral Committee Chair(s): Lamb, D. Q.
Department / Program: Physics
Discipline: Physics
Degree: Ph.D.
Genre: Dissertation
Subject(s): binary system pulsar PSR ultraviolet radiation x radiation accreting magnetic white dwarfs
Abstract: Part I of the thesis deals with the binary system containing the pulsar PSR 1913+16. The system has been touted as a laboratory for testing relativistic theories of gravity, and is also. a challenge for theories of stellar evolution. However, proposed uses of the system rely on assumptions about the nature of the pulsar's unobserved companion. I discuss ways of determining the nature of the companion from observation of the pulsar. Geometrical constraints on the size of the pulsar's orbit and the observed slow rate of the orbit's precession require that the companion be a black hole, a neutron star, a white dwarf or a helium main-sequence star. Observable second-order relativistic effects mayor may not further restrict the list of candidates. The discussion summarjzes Masters and Roberts, 1975 Ap. J. (Letters), l.25.., L 107, and Roberts t Masters and Arnettt 1976 Ap. J. , gQ3" 196. Part II of the thesis treats ultra-violet and X-radiation from accreting magnetic white dwarfs. Matter from a companion star falling onto a white dwarf is shock-heated near the stellar surface and radiativaly cooled. I approximate the post-shock region by a uniform, geometrically thin slab and determine the physical conditions behind the shock and the emitted spectrum for a range of stellar masses, magnetic fields and accretion rates. At low magnetic fields and high accretion rates, bremsstrahlung is the dominant cooling mechanism and the post-shock material is a single fluid (the electrons and :ions have a common temperature). As the magnetic fit-hd increases or the accretion rate decreases, cyclotron emission becomes more important than bremsstrahlung. At slightly higher fields or lower accretion rates, the high efficiency of cyclotron cooling prevents the electrons from coming into equilibrium wHh the ions behind the shock. The matter then behaves as two fluids. At still higher fields and lower accretion rates, a variety of effects associated with rapid cyclotron cooling produce a non-hydrodynamic regime. Cyclotron emission from an optically thin plasma consists of discrete lines at harmonics of the cyclotron frequency. At mildly realistic temperatures (the case of interest) the discrete lines are broadened into a smooth continuum, which decreases rapidly with frequency above the fundamental. The post-shock plasma near the surface of a white dwarf is optically thick to cyclotron emission up to high harmonics. The cyclotron spectrum actually emitted at low harmonics is theref0re a Raylejgh-Jeans spectrum with a brightness temperature equal to that of the electrons. At an energy E ranging from a few eV to a few. tens of eV, the cyclotron spectrum becomes optically thin and falls rap:idly with increasing energy. In order to determine the cyclotron cooling rate, and therefore the temperature and spectrum of the post-shock region, one needs to know E, the energy at which the optically thin cyclotron spectrum crosses the Rayleigh-Jeans spectrum. I have therefore calculated, and derived analytic fits to, cyclotron spectra from mildly relativistic plasmas (5-100 keV) over a frequency range from one to eighty times the cyclotron frequency. Calculated spectra from accreting white dwarfs have three components; cyclotron emission and bremsstrahlung from the shock-heated plasma, and black-body radiation from the radiatively heated surface of the white dwarf. The bremsstrahlung component can be observable in hard X-rays, while the black-body component can be'observab1e in soft X-rays. The observed spectrum varies with time when the accretion rate does. Ordinarily, only a few percent of the total accretion luminosity is accessible to existing detectors. Nonetheless, observations of hard and soft X-rays can still determine the mass and magnetic field of the star, and its accretion rate. As examples, I apply the model to X-ray observations of AM Her, SS, Cyg and U Gem.
Issue Date: 1978
Genre: Dissertation / Thesis
Type: Text
Language: English
URI: http://hdl.handle.net/2142/25593
Rights Information: 1978 Albert Ronald Masters
Date Available in IDEALS: 2011-06-30
Identifier in Online Catalog: 355790
 

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