Cataclysmic variables from a ROSAT/2MASS selection — I. Four new intermediate polars

Abstract

We report the first results from a new search for cataclysmic variables (CVs) using a combined X‐ray (ROSAT)/infrared (2MASS) target selection that discriminates against background active galactic nuclei. Identification spectra were obtained at the Isaac Newton Telescope for a total of 174 targets, leading to the discovery of 12 new CVs. Initially devised to find short‐period low‐mass‐transfer CVs, this selection scheme has been very successful in identifying new intermediate polars. Photometric and spectroscopic follow‐up observations identify four of the new CVs as intermediate polars: 1RXS J063631.9+353537 (Porb≃ 201 min, Pspin= 1008.3408 s or 930.5829 s), 1RXS J070407.9+262501 (Porb≃ 250 min, Pspin= 480.708 s), 1RXS J173021.5–055933 (Porb= 925.27 min, Pspin= 128.0 s), and 1RXS J180340.0+401214 (Porb= 160.21 min, Pspin= 1520.51 s). RX J1730, also a moderately bright hard X‐ray source in the INTEGRAL/IBIS Galactic plane survey, resembles the enigmatic AE Aqr. It is likely that its white dwarf is not rotating at the spin equilibrium period, and the system may represent a short‐lived phase in CV evolution.

Two new intermediate polars with a soft X-ray component

“…The mean optical spectrum of RX J1803 was found similar to that already reported by Gänsicke et al (2005) and is typical of magnetic CVs with strong emission lines of the Balmer series, He ii (4686 Å) and He i (4471, 5875, 6678 and 7065 Å), superimposed on a relatively blue continuum. The radial velocities for the main lines were measured using a single Gaussian least square fit procedure and periodicities were searched using a Fourier analysis.…”

“…Because of the higher accuracy of the EPIC-pn data, we adopt P ω = 480.3 ± 0.2 s. This value is consistent within 3σ with the optical photometric determination of Gänsicke et al (2005) and hence can be safely ascribed to the WD rotational period. Figure 2 shows the full band EPIC-pn light curves of both epochs folded at the spin period.…”

“…The residual light curve when fitted with a second sinusoid gives a period of 1697 ± 22 s. If this frequency represents the sideband ω − Ω (commonly named as the beat), using either the X-ray period or the optical period we derive an orbital frequency of 5.64 ± 0.69 day −1 or 5.91 ± 0.66 day −1 , respectively. This gives an orbital period of 4.25 ± 0.52 h or 4.06 ± 0.45 h which does not match either the orbital period of 2.67 h determined by Gänsicke et al (2005), or 2P Ω . As it will be shown below, the orbital period is likely 4.4 h, hence longer than previously determined.…”

“…RX J0704 was then identified as a CV by Gänsicke et al (2005) and proposed to be an IP from the detection of optical photometric and spectroscopic periodicities at 480.71 s and ∼250 min, respectively. The short period variability was ascribed to the spin period of the WD, while the other was identified as the orbital period of the binary system.…”

Properties of the Galactic population of cataclysmic variables in hard X-rays

“…Detailed spectroscopic information allows one in some cases to determine the contribution of the light of the companion star to the optical brightness of the binary (e.g. Watson et al 1995;Bonnet-Bidaud et al 2001;Gänsicke et al 2005), but this information is not available for all CVs in our sample.…”