The X-ray emission of the intermediate polar V 709 Cas

¹ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
² Dipartimento di Fisica, Universita' degli Studi Roma Tre, Via della Vasca Navale, Roma, Italy e-mail: matt@fis.uniroma3.it
³ Laboratory for High Energy Physics, NASA/GSFC, Code 662, Greenbelt, MD 20771, USA
⁴ Universities Space Research Association e-mail: mukai@milkyway.gsfc.nasa.gov
⁵ Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy e-mail: belloni@merate.mi.astro.it
⁶ Service d'Astrophysique Saclay, Gif-Sur-Yvette, France e-mail: bobi@discovery.saclay.cea.fr
⁷ Istituto di Fisica Cosmica del CNR, Via Bassini 15, Milano, Italy e-mail: lucio@ifctr.mi.cnr.it
⁸ Universitäts Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany e-mail: boris@uni-sw.gwdg.de
⁹ Max Planck Institut für Extraterrestrische Physik, Garching, Germany e-mail: fwh@xray.mpe.mpg.de
¹⁰ DAEC et UMR 8631 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
¹¹ Université Denis Diderot, Place Jussieu, 75005 Paris, France e-mail: martine.mouchet@obspm.fr

Corresponding author: D. de Martino, demartino@na.astro.it

Received: 25 April 2001
Accepted: 19 July 2001

Abstract

We present RXTE and BeppoSAX observations of the Intermediate Polar V 709 Cas acquired in 1997 and 1998 respectively. The X-ray emission from 0.1 to 30 keV is dominated by the strong pulsation at the rotational period of the white dwarf (312.8 s) 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 100 keV is well described by an isothermal plasma at ~27 keV plus complex absorption and an iron K_α 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 V 709 Cas experiences changes in the mass accretion rate on timescales from months to years.