A model for compact x-ray sources: accretion by rotating magnetic stars
The physics of accretion onto compact stars is considered, taking into account both the effects of stellar rotation and a stellar magnetic field. We show that far from the star the stellar magnetic field is screened by currents flowing in the accreting plasma, while close to the star the stellar field forces matter to corotate with the star. The location of the Alfven surface, where the transition between the two regimes occurs, depends on the flow pattern of the accreting matter beyond the Alfvea surface and the rotation period of the compact star, as well as the mass accretion rate and the strength of the stellar magnetic field. Three types of flow pattern are considered: radial inflow toward the compact object, orbital motion about it, and streaming motion past it. For the accretion rates of interest the radius of the Alfven surface r/sub A/, is found to be ~10/sup 7/-10/sup 8/ cm for a typical neutron star and ~ 10/sup 8/ -10/sup 10/cm for a typical magnetic degenerate dwarf: For neutron stars it is shown that inside the Alfvea surface there is comparatively little flow of matter across field lines, so that accreting matter is channeled toward the magnetic poles of the star where it forms hot spots. The resulting radiation is shown to emerge from the neighborhood of the stellar surface in a strongly anisotropic angular pattern with a spectrum which depends on the details of the accretion process; for an oblique rotator, one has a natural mechanism for the production of pulsed radiation, which is not generally expected to he blackbody in character, will be in excess of 6 keV. Comparison of the above model with the observations suggests strongly that the X-ray stars in the pulsating binary X-ray sources Cen X-3 and Her X-1 are accreting neutron stars with their magnetic axes inclined at substantial angles to their axes of rotation. The implications of this interpretation for the observed X-ray spectra and pulse wave-forms are discussed. The change in the X-ray pulsation period due to accretion of matter is calculated; the calculated time scales for the spin-up which occurs when there is orbital inflow toward a star rotating ia the same sense as the orbital motion, is in excellent agreement with these observed in Her X-1 and Gn X-3. The possibility of significant optical pulsations and the likely existence of a minimum pulsation period for accreting X-ray stars are discussed. (auth)
- Research Organization:
- Department of Physics, University of Illinois, Urbana
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-29-021917
- OSTI ID:
- 4324747
- Journal Information:
- Astrophys. J., v. 184, no. 1, pp. 271-289, Other Information: Orig. Receipt Date: 30-JUN-74
- Country of Publication:
- United States
- Language:
- English
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