X-ray and optical studies of binary x-ray pulsars
Creators: name: Watts, DJ
Publisher: name: University Of Tasmania
Year: 2023

description: This dissertation is divided into two main areas. Part (i) in Chapters 2 to 4 deals with the University of Tasmania contribution to the University of Tasmania/Imperial College, London (UTIC) hard X-ray balloon-borne platform and the balloon flight of 1981 December 2. Part (ii) presented in Chapters 5 to 8 discusses the optical and soft X-ray observations of cataclysmic variables and miscellaneous X-ray sources. The introduction to both parts is in Chapter 1. (i) The UTIC observatory consists of two large area detectors mounted in an alt-az configuration. The UT detector, with a sensitive area of 5200 cm\\(^2\\), is a xenon-filled proportional counter sensitive in the energy range 20 to 100 keV. The IC instrument consists of 12 phoswich detectors with a combined area of 1680 cm\\(^2\\) and an energy range of 20 to 200 keV. Both detectors use 2¬¨‚àû FWHM collimators. The platform and UT detector are fully described in Chapter 2. An onboard microprocessor continuously updates and drives to the altitude and azimuth of the object under observation. Both detectors are driven in altitude via a common shaft, and azimuth is controlled by reference to the earth's magnetic field with an accuracy of ~8 arcmin rms. A sunsensor is used to check the platform azimuth. Data from each detector are independently encoded into a PCM bit stream and telemetered on separate subcarriers. The data are treated separately at the ground station with independent UT and IC computers which provide output on VDU and teleprinter of housekeeping, aspect and quick-look X-ray data. The platform was flown on 1981 December 2 for a duration at float of 6.5 hours. A mechanical fault of the altitude drive limited observations to two objects, the extreme Seyfert 1 galaxy IC4329A and the pulsar GX1+4. The upper limits on IC4329A are consistent with the soft X-ray flux measured by satellites. GX1+4 was in a hard low flux state. Due to this low intensity, no periodic structure could be established. Comparison of the spectrum with previous observations shows GX1+4 exhibits spectral variability similar to other luminous galactic X-ray sources including Cyg X-1. The source of the hard X-ray photons is suggested to be soft photons that have been Compton scattered from a region or cloud of hot electrons. Changes in the size of this scattering region can explain the observed hardness and luminosity variations provided the total number of electrons in the scattering region are conserved. (ii) The white dwarf equivalent of the binary neutron star pulsars are the recently discovered class of intermediate polars which are a subset of the cataclysmic variables. Optical observations of the intermediate polars 2A0526-328 and Vl223 Sgr and soft X-ray observations of 2A0526-328 and the nova-like object VY Scl are presented in Chapters 5 and 6. Spectroscopic observations of 2A0526-328 confirm that the emission line radial velocity period (5.48 hour) is significantly different from the photometric period (5.19 hour). Changes in the emission line velocity and profiles suggest that the accretion process is controlled by the white dwarf's magnetic field. Einstein X-ray observations show it is a hard source with a thermal spectrum of kT - 19 keV. There is no evidence of any strong soft X-ray component. Quasi-periodic variations on a timescale of ~700 sec are observed in, the X-ray light curve. Similar fluctuations have been reported in optical photometry. A model for the object is proposed in which a rotating white dwarf revolves with a 4-day period in a retrograde orbit with a subluminous red giant. Predictions on the nature of the secondary are given and other observational tests are proposed. Optical and IR light curves of Vl223 Sgr confirm the photometric period of 0.14 day. The source of the 794 sec pulsations cannot be ascertained from the present data due to the failure to find a unique orbital beating period. The spectrum from UV to IR is consistent with a disc model with no IR excess from the secondary being apparent. Spectroscopic observations over 2 hours showed a small radial velocity change accompanied by complex line profile changes. The low mass function derived from the radial velocity movement implies a low inclination angle for reasonable component masses. Upper limits to pulsed polarization at any of the expected white dwarf spin periods are ¬¨¬±3% at V, ¬¨¬±1% at R and ¬¨¬±10% at K. A simple light curve model of the intermediate polars is developed and compared to the observed photometric data of 2A0526-328, vi223 Sgr and H2252-035. Einstein observations of VY Scl indicate the spectrum is soft (kT = 1.2 keV) with little absorption (n\\(_H\\) = 4 x 10\\(^{20}\\)cm\\(^{-2}\\)). The low absorption may be a consequence of the system being pole-on with little obscuring material in the line of sight from the accretion disc. Finally, summaries of observations of candidate stars in HEA0-1 and Einstein X-ray fields are presented in Chapter 8.