Phase-resolved X-ray spectroscopy and spectral energy distribution of the X-ray soft polar RS Caeli

Abstract

Context. RS Cae is the third target in our series of XMM-Newton observations of soft X-ray-dominated polars. Aims. Our observational campaign aims to better understand and describe the multiwavelength data, the physical properties of the system components, and the short-and long-term behavior of the component fluxes in RS Cae. Methods. We employ stellar atmosphere, stratified accretion-column, and widely used X-ray spectral models. We fit the XMM-Newton spectra, model the multiband light curves, and opt for a mostly consistent description of the spectral energy distribution. Results. Our XMM-Newton data of RS Cae are clearly dominated by soft X-ray emission. The X-ray light curves are shaped by emission from the main accretion region, which is visible over the whole orbital cycle, interrupted only by a stream eclipse. The optical light curves are formed by cyclotron and stream emission. The XMM-Newton X-ray spectra comprise a black-body-like and a plasma component at mean temperatures of 36 eV and 7 keV. The spectral fits give evidence of a partially absorbing and a reflection component. Multitemperature models, covering a broader temperature range in the X-ray emitting accretion regions, reproduce the spectra appropriately well. Including archival data, we describe the spectral energy distribution with a combination of models based on a consistent set of parameters and derive a lower limit estimate of the distance d 750 pc. Conclusions. The high bolometric soft-to-hard flux ratios and short-term variability of the (X-ray) light curves are characteristic of inhomogeneous accretion. RS Cae clearly belongs in the group of polars that show a very strong soft X-ray flux compared to their hard X-ray flux. The different black-body fluxes and similar hard X-ray and optical fluxes during the XMM-Newton and ROSAT observations show that soft and hard X-ray emission are not directly correlated.

An investigation of a magnetic cataclysmic variable with a period of 14.1 ks

“…The ballistic stream or the stream to the secondary faint pole may cross our line of sight to the primary magnetic pole, thereby absorbing the soft/medium X-rays and causing the additional dip (Ramsay & Cropper 2002). This is not surprising, because these more extended, cooler parts of the accretion region, detected in the soft/medium band, are more likely to be absorbed, while the harder emission region may only be absorbed by the dense core of the accretion stream (Traulsen et al 2014).…”

Probing the Accretion Processes in Soft X-Ray Selected Polars

“…: Observed SEDs of three X-ray soft polars from IR to X-rays: our XMM-Newton and optical plus archival data at different epochs and accretion states (cf. Traulsen et al 2010Traulsen et al , 2011Traulsen et al , 2014. The Xray spectra are unfolded using the best-fit models.…”

“…Using a consistent set of parameters for all components, we thus establish a physically realistic and consistent multi-wavelength model of the whole binary system, missing only the unknown spectral contribution of the accretion stream. We describe its successful application to the multi-wavelength data of RS Cae in Traulsen et al (2014). The most relevant limitation relates to the different observational epochs of the high-state data.…”

“…Starting in 2005, we obtained XMM-Newton X-ray and ultraviolet data of AI Tri, QS Tel, and RS Cae (obs. IDs 0306840901, 0306841001, 0404710401, 0554740801; Traulsen et al, 2010Traulsen et al, , 2011Traulsen et al, , 2014, covering one to five orbital cycles per object. On the basis of optical monitoring, the TOO observations were triggered during high and intermediate high states of accretion.…”