X-ray spectra and light curves of accreting magnetic degenerate dwarfs

Abstract

We report detailed calculations of the X-ray spectra and X-ray light curves of accreting magnetic 1 M/sub sun/ degenerate dwarfs. We use accretion funnel models in which plasma flows toward the dwarf along dipole magnetic field lines and accretes onto a small cap at the dwarf's magnetic pole of area 4..pi..R/sup 2//sub asterisk/ f, where R/sub asterisk/ is the stellar radius and f<<1. Above the dwarf's surface, the plasma forms a radiating shock which produces the X-rays. We find that the X-ray spectra of weakly magnetic degenerate dwarfs are qualitatively similar to the spectra of nonmagnetic dwarfs. However, because the optical depth perpendicular to the field lines in the funnel is less than the optical depth parallel to the field lines, the majority of the X-rays escape in directions perpendicular to the field lines. This has two effects: (1) For magnetic and nonmagnetic models with equivalent accretion rates per square centimeter, i.e., equal ''effective luminosities'' L/sub f/(equivalent L/f), the emergent photons in the magnetic case suffer fewer scatterings than those in the nonmagnetic case and thus show less energy degradation. (2) The beaming of the photons into directions perpendicular to the field lines leads to X-ray light curves which exhibitmore » a wide range of behaviors. The shapes of the light curves are sensitive to f and L/sub f/, to the inclination of the magnetic field axis to the dwarf's rotation axis, and to the viewing angle of the dwarf. Thus, observed X-ray light curves can be used to constrain the physical characteristics and geometry of the accreting degenerate dwarf.« less