Neutron stars in binaries and in isolation

This thesis is a study of neutron stars in three distinct classes. After a brief overview of neutron stars in Chapter 1, the three systems are discussed in order of decreasing luminosity. In Chapter 2, we present a new model for the normal branch of a class of low mass X-ray binaries which show quasiperiodic oscillations: a quasi-periodic modulation in the intensity of their X-ray signal. Chapter 3 discusses a particular radio pulsar which we observed in X-rays with the ROSAT PSPC. Chapter 4 rounds out the thesis with a discussion of a class of neutron stars which have not, to date, been definitively shown to exist. We describe a search for these isolated old neutron stars in the Einstein database, and present the results of our finds.; As part of a search for thermal surface radiation from nearby neutron stars, we have carried out a 45,000 s observation of the nearby radio pulsar PSR 1929+10 with the ROSAT PSPC. After background subtraction, a net of {dollar}420pm25{dollar} photons in the 0.1-2.0 keV band were detected at the position of the pulsar, corresponding to a luminosity of position of the pulsar, corresponding to a luminosity of {dollar}1.2 times 10sp{lcub}30{rcub}{dollar} erg/s for a source distance of 250 pc, or {dollar}{lcub}sim{rcub}3 times 10sp{lcub}-4{rcub}{dollar} of the pulsar's spin-down luminosity. We find coherent pulsations from PSR 1929+10 at the radio period of 0.2265 s. The folded light curve is well fit by a sinusoidal oscillation with a pulsed fraction of about 30%. The total spectrum is fit by a blackbody with a temperature {dollar}Tsbinftyapprox3.2times10sp6{dollar} K; the implied emitting area has a radius of less than 50 meters. The maximum of the X-ray light curve coincides with the radio pulse, suggesting we are detecting the hot magnetic polar cap of the star. We discuss the implications of our results for the temperature distribution over the surface of the star, and use this detection to constrain various heating mechanisms for rotation-powered neutron stars. We also use a simple model of general relativistic light bending near the stellar surface in conjunction with the radio emission geometry and the X-ray pulsed fraction to derive a radius for the neutron star of {dollar}R=9pm3{dollar} km.; Finally, we describe a search conducted in the Einstein database for the nearest, oldest, lowest luminosity, below 10{dollar}sp{lcub}31{rcub}{dollar} erg/s, isolated neutron stars accreting from the interstellar medium and emitting with a thermal black body spectrum of {dollar}sim{dollar}0.1 keV or less. We present our findings for of the order of 30 candidates of which two were also firm ROSAT detections. For these, we determine blackbody temperatures in the expected range, and by their fitted neutral column densities establish the plausibility of their being within {dollar}sim{dollar}100 pc of the sun. (Abstract shortened by UMI.)...