Accretion by rotating magnetic neutron stars. III. Accretion torques and period changes in pulsating X-ray sources
We use the solutions of the two-dimensional hydromagnetic equations obtained previously to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. We find that the magnetic coupling between the star and the plasma outside the inner edge of the disk is appreciable. As a result of this coupling the spin-up torque on fast rotators is substantially less than that on slow rotators; for sufficiently high stellar angular velocities or sufficiently low accretion rates this coupling dominates that de to the plasma and the magnetic field at the inner edge of the disk, braking the star's rotation even while accretion, and hence X-ray emission, continues.We apply these results to pulsating X-ray sources, and show that the observed secular spin-up rates of all the sources in which this rate has been measured can be accounted for quantitatively if one assumes that these sources are accreting from Keplerian disks and have magnetic moments approx.10/sup 29/--10/sup 32/ gauss cm/sup 3/. The reduction of the torque on fast rotators provides a natural explanation of the spin-up rate of Her X-1, which is much below that expected for slow rotators. We show further that a simple relation between the secular spin-up rate : P and the quantity PL/sup 3/7/ adequately represents almost all the observational data, P and L being the pulse period and the luminosity of the source, respectively. This ''universal'' relation enables one to estimate any one of the parameters P, P, and L for a given source if the other two are known. We show that the short-term period fluctuations observed in Her X-1, Cen X-3, Vela X-1, and X Per can be accounted for quite naturally as consequences of torque variations caused by fluctuations in the mass transfer rate. We also indicate how the spin-down torque at low luminosities found here may account for the paradoxical existence of a large number of long-period sources with short spin-up time scales.
- Research Organization:
- Department of Physics, University of Illinois at Urbana-Champaign
- OSTI ID:
- 5731419
- Journal Information:
- Astrophys. J.; (United States), Vol. 233:4
- Country of Publication:
- United States
- Language:
- English
Similar Records
Accretion by rotating magnetic neutron stars
Period variations in pulsating X-ray sources. I. Accretion flow parameters and neutron star structure from timing observations
Related Subjects
GENERAL PHYSICS
COSMIC X-RAY SOURCES
PULSATIONS
VARIATIONS
NEUTRON STARS
ROTATION
STAR ACCRETION
FLUID FLOW
MAGNETIC FIELDS
MAGNETIC MOMENTS
MAGNETIC STARS
MAGNETOHYDRODYNAMICS
SPIN
TORQUE
ANGULAR MOMENTUM
COSMIC RAY SOURCES
FLUID MECHANICS
HYDRODYNAMICS
MECHANICS
MOTION
PARTICLE PROPERTIES
STAR EVOLUTION
STARS
640102* - Astrophysics & Cosmology- Stars & Quasi-Stellar
Radio & X-Ray Sources
640103 - Astrophysics & Cosmology- Quasi-Stellar
Radio
& X-Ray Sources- (-1987)