Abstract

view Abstract Citations (582) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Model for Compact X-Ray Sources: Accretion by Rotating Magnetic Stars Lamb, F. K. ; Pethick, C. J. ; Pines, D. Abstract 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 Alfve'n surface, where the transition between the two regimes occurs, depends on the flow pattern of the accreting matter beyond the Alfve'n 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 Alfve'n surface, , is found to be cm for a typical neutron star and cm for a typical magnetic degenerate dwarfl For neutron stars it is shown that inside the Alfve'n 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. For X-ray luminosities of the order of 10 ergs 1 it is shown that the temperature of the radiation, which is not generally expected to be 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 in the same sense as the orbital motion, is in excellent agreement with those observed in Her X-1 and Cen X-3. The possibility of significant optical pulsations and the likely existence of a minimum pulsation period for accreting X-ray stars are discussed. Subject headings: hydromagnetics - neutron stars - rotation, stellar - X-ray sources Publication: The Astrophysical Journal Pub Date: August 1973 DOI: 10.1086/152325 Bibcode: 1973ApJ...184..271L full text sources ADS |