Two new intermediate polars with a soft X-ray component^*

¹ Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 3, 35122 Padova, Italy e-mail: gabriele.anzolin@unipd.it
² INAF – Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy e-mail: demartino@oacn.inaf.it
³ Service d'Astrophysique, DSM/DAPNIA/SAp CE Saclay, 91191 Gif-sur-Yvette, France e-mail: bonnetbidaud@cea.fr
⁴ Laboratoire Astroparticule et Cosmologie, Université Paris 7, 10 rue Alice Domon et Léonie Duquet, 75005 Paris; LUTH, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon, France e-mail: martine.mouchet@obspm.fr
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK e-mail: boris.gaensicke@warwick.ac.uk
⁶ Dipartimento di Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy e-mail: matt@fis.uniroma3.it
⁷ CRESST and X-Ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA e-mail: koji.mukai@nasa.gov

Received: 17 June 2008
Accepted: 30 July 2008

Abstract

Aims. We analyze the first X-ray observations with XMM-Newton of 1RXS J070407.9+262501 and 1RXS 180340.0+401214, in order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as intermediate polars.

Methods. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2–10.0 keV range to characterize the emission properties of both sources.

Results. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3–5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 h that is longer than previously estimated. Both systems show complex spectra with a hard (temperature up to 40 keV) optically thin and a soft (kT ~ 85–100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources.

Conclusions. Our observations confirm the two systems as intermediate polars and also add them as new members of the growing group of “soft” systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation.