Why does the intermediate polar V405 Aurigae show a double-peaked spin pulse?

Abstract

V405 Aurigae is an intermediate polar showing a double-peaked pulsation in soft X-rays and a single-peaked pulsation in harder X-rays. From XMM-Newton observations, we find that the soft band is dominated by blackbody emission from the heated white dwarf surface. Such emission is at a maximum when either magnetic pole points towards us, explaining the double-peaked pulsation. The symmetry of the pulses requires that the angle between the magnetic and spin axes be high. The single-peaked pulsation in harder X-rays is explained in the usual way, as a result of opacity in the accretion curtains. However, the high dipole inclination means that the accretion curtains are nearly in the plane. Thus the outer regions of the curtains do not cross the line of sight to the accretion footprints, explaining the absence of the deep absorption dip characteristic of many intermediate polars. The sawtooth profile of this pulsation requires that the magnetic axis be offset from the white dwarf centre. We remark also on the double-peaked optical emission in this star. We suggest that the difference between the spin pulse of V405 Aur and those of other intermediate polars is the result of its high dipole inclination.