1RXS J173021.5-055933: a cataclysmic variable with a fast-spinning magnetic white dwarf^*

¹ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy e-mail: demartino@na.astro.it
² Dipartimento di Fisica, Universita' degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy e-mail: matt@fis.uniroma3.it
³ CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771 and Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA e-mail: mukai@milkyway.gsfc.nasa.gov
⁴ Service d'Astrophysique, DSM/DAPNIA/SAp, CE Saclay, 91191 Gif-sur-Yvette Cedex, France e-mail: [bonnetbidaud;mfalanga]@cea.fr
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK e-mail: [boris.gaensicke;Tom.Marsh]@warwick.ac.uk
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, Postfach 1312, 85741 Garching, Germany e-mail: fwh@mpe.mpg.de
⁷ Laboratory APC, University Denis Diderot, 10 Rue Alice Domon et Leonie Duquet, 75013 and LUTH, Observatoire de Paris, 92190 Meudon, France e-mail: martine.mouchet@obspm.fr
⁸ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK e-mail: [s.littlefair;vik.dhillon]@shef.ac.uk

Received: 27 July 2007
Accepted: 20 January 2008

Abstract

Aims.We present the first X-ray observations with the XMM-Newton and INTEGRAL satellites of the recently discovered cataclysmic variable 1RXS J173021.5-055933, together with simultaneous UV and coordinated optical photometry aiming at characterising its broad-band temporal and spectral properties and classifying this system as a magnetic one.

Methods.We performed a timing analysis of the X-ray, UV, and optical light curves to identify and to study the energy dependence of the fast 128 s pulsation over a wide energy range. X-ray spectral analysis in the broad 0.2-100 keV X-ray range was performed to characterise the peculiar emission properties of this source.

Results.We find that the X-ray light curve is dominated by the spin period of the accreting white dwarf in contrast to the far-UV range, which turns out to be unmodulated at a 3σ level. Near-UV and optical pulses are instead detected at twice the spin frequency. We identify the contributions from two accreting poles that imply a moderately inclined dipole field allowing, one pole to dominate at energies at least up to 10 keV, and a secondary that instead is negligible above 5 keV. X-ray spectral analysis reveals the presence of multiple emission components consisting of optically thin plasma with temperatures ranging from 0.17 keV to 60 keV and a hot blackbody at ~90 eV. The spectrum is also strongly affected by peculiar absorption components consisting of two high-density (~3  and 2  ) intervening columns, plus a warm absorber. The last is detected from an OVII absorption edge at 0.74 keV, which suggests that photoionization of pre-shock material is also occurring in this system.

Conclusions.The observed properties indicate that the accretor in 1RXS J173021.5-055933 is a white dwarf with a likely weak magnetic field, thus confirming this cataclysmic variable as an intermediate polar (IP) with one of the most extreme spin-to-orbit period ratios. This system also joins the small group of IPs showing a soft X-ray reprocessed component, suggesting that this characteristics is not uncommon in these systems.