The X-ray emission of the Intermediate Polar V709 Cas

Abstract

We present RXTE and BeppoSAX observations of the Intermediate Polar V709 Cas acquired in 1997 and 1998 respectively. The X-ray emission from 0.1 to 30keV is dominated by the strong pulsation at the rotational period of the white dwarf (312.8s) with no sign of orbital or sideband periodicity, thus confirming previous ROSAT results. However, we detect changes in the power spectra between the two epochs. While the second harmonic of the spin period is present during both observations, the first harmonic is absent in 1997. An increase in the amplitude of the spin pulsation is found between 1997 and 1998 together with a decrease in the X-ray flux. The average X-ray spectrum from 0.1 to 100keV is well described by an isothermal plasma at ~27keV plus complex absorption and an iron K(alpha) fluorescent line, due to reflection from the white dwarf surface. The rotational pulsation is compatible with complex absorption dominating the low energy range, while the high energy spin modulation can be attributed to tall shocks above the accreting poles. The RXTE spectrum in 1997 also shows the presence of an absorption edge from ionized iron likely located in the pre-shock accretion flow. The variations along the spin period of the partial covering absorber and of reflection are compatible with the classical accretion curtain scenario. The variations in the spin pulse characteristics and X-ray flux indicate that V709 Cas experiences changes in the mass accretion rate on timescales from months to years.