The size of the accretion region in intermediate polars: eclipses of XY Arietis observed with RXTE

Abstract

RXTE observed 20 eclipse egresses of the intermediate polar XY Ari in order to study the size and structure of the X-ray emitting accretion regions. The spin-phase averaged egress lasts 26 s, implying a white dwarf radius of 4.3-7.0 10^8 cm. The individual egresses occur later in orbital phase with later spin phase, as expected if the white dwarf spins in the same sense as the orbital motion. The eclipse times trace out the motion of the upper pole across the white dwarf face; then, when the upper pole disappears over the white dwarf limb and the lower pole appears, they trace the motion of the lower pole across the face. Aligning all the egresses shows that the majority of the X-ray flux emerges in <2 s, implying accretion regions with area, f, < 0.002 as a fraction of the white dwarf surface. Using only the spin-phase to align the egresses, however, gives a longer (~5 s) time for the emergence of the majority of the flux, implying that the accretion regions wander over an area of f < 0.01. There is also evidence that a minority of the flux emerges from a much larger area, or that we see accretion regions at both poles simultaneously at some spin phases.