XMM-Newton and optical observations of the eclipsing polar CSS081231:071126+440405

Aims: We aim to study the temporal and spectral behaviour of the eclipsing polar CSS081231:071126+440405 from the infrared to the X-ray regime. Methods: We obtained phase-resolved XMM-Newton X-ray observations on two occasions in 2012 and 2013 in different states of accretion. In 2013 the XMM-Newton X-ray and UV data were complemented by optical photometric and spectroscopic observations. Results: CSS081231 displays two-pole accretion in the high state. The magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar field geometry. The X-ray spectrum of the main accreting pole with the lower field comprises a hot thermal component from the cooling accretion plasma, $kT_{plas}$ of a few tens of keV, and a much less luminous blackbody-like component from the accretion area with $kT_{\rm bb} \sim$ 50-100\,eV. The high-field pole which was located opposite to the mass-donating star accretes at a low rate and has a plasma temperature of about 4\,keV. At both occasions the X-ray eclipse midpoint precedes the optical eclipse midpoint by 3.2 seconds. The center of the X-ray bright phase shows accretion-rate dependent longitudinal motion of ~20 degrees. Conclusions: CSS081231 is a bright polar that escaped detection in the RASS survey because it was in a low accretion state. Even in the high state it lacks the prominent soft component previously thought ubiquitous in polars. Such an excess may still be present in the unobserved extreme ultraviolet. All polars discovered in the XMM-Newton era lack the prominent soft component. The intrinsic spectral energy distribution of polars still awaits characterisation by future X-ray surveys such as eROSITA. The trajectory taken by material to reach the second pole is still uncertain.Comment: 12 pages, 14 figure

XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars

Archival X-ray observations of EF Eridani obtained in a low state revealed distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders of magnitude below its high state value. The plasma temperature was found to be as low as kT \loa 2 keV, a factor 10 below the high state. The X-ray/UV/IR spectral energy distribution suggests faint residual accretion rather than coronal emission as being responsible for the low-state X-ray emission. EF Eri thus showed a clear transition from being shock-dominated in the high state to be cyclotron-dominated in the low state. From the optical/UV spectral energy distribution we re-determine the photospheric temperature of the white dwarf to \~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.Comment: A&A, in pres

Probing the accretion processes in soft X-ray selected polars

High-energy data of accreting white dwarfs give access to the regime of the primary accretion-induced energy release and the different proposed accretion scenarios. We perform XMM-Newton observations of polars selected due to their ROSAT hardness ratios close to -1.0 and model the emission processes in accretion column and accretion region. Our models consider the multi-temperature structure of the emission regions and are mainly determined by mass-flow density, magnetic field strength, and white-dwarf mass. To describe the full spectral energy distribution from infrared to X-rays in a physically consistent way, we include the stellar contributions and establish composite models, which will also be of relevance for future X-ray missions. We confirm the X-ray soft nature of three polars.Comment: Accepted for publication in Acta Polytechnica, Proceedings of "The Golden Age of Cataclysmic Variables and Related Objects II

A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris

We searched for long-term period changes in the polar EK UMa using new optical data and archival X-ray/EUV data. An optical ephemeris was derived from data taken remotely with the MONET/N telescope and compared with the X-ray ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the combined data sets yields the epoch, the period, and the phase offset between the optical minima and the X-ray absorption dips. An added quadratic term is insignificant and sets a limit to the period change. The derived linear ephemeris is valid over 30 years and the common optical and X-ray period is P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that the observed period is the orbital period and that the system is tightly synchronized. The limit on Delta P and the phase constancy of the bright part of the light curve indicate that O-C variations of the type seen in the polars DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some insight into the accretion geometry.Comment: 4 pages, 2 Postscript figures, accepted for publication in Astronomy & Astrophysic

Mapping the secondary star in QQ Vulpeculae

We present high- and medium-resolution phase-resolved far-red spectra of the magnetic cataclysmic variable QQ Vul. The spectra show the Na i doublet absorption features near λ 8190 Å from the cool secondary star, and the lines of He ii, O i, Mg ii, C i, N i, Ca ii and Paschen in emission. Using a Doppler imaging technique, we find that the H i, He ii, C i and O i lines have a narrow component originating near the L1 point and a strong component from the stream, while the Mg ii and Ca ii emission arises solely from the illuminated hemisphere of the red dwarf. We carry out an exhaustive analysis of the emission- and absorption-line velocities and fluxes seen in the QQ Vul spectrum. By simultaneously fitting the radial velocity and flux information we are able to produce surface maps of each line on the secondary star using a technique analogous to the one employed by Davey. The Na i and Mg ii maps show an asymmetric distribution akin to that seen in AM Her. Although the observed velocity semi-amplitudes (K2) of the lines can potentially be corrected for the effects of irradiation, we find that time-dependent changes in the degree of heating on the secondary can lead to large discrepancies in the results, significant enough to give inconsistent values from data taken at different epochs. We discuss the limitations of the surface mapping method as a means of correcting the observed K2. Our results also suggest that the emission features from the red dwarf are likely to be formed at quite high levels of the stellar chromosphere, in some cases probably even beyond the L1 point and inside the Roche lobe of the white dwarf, with the different lines possibly forming at different depths. Using the Na i absorption doublet, we find a velocity semi-amplitude for the secondary star of K2=219±6 km s−1 and a projected rotational velocity of vrot sin i=110±15 km s−1. Thus we estimate the mass ratio to be q=0.54±0.14. Based on the results of the best-fitting surface maps on all the lines, and the nature of the phase-dependent variations of the continuum and lines, we infer a binary inclination of i=65°±7°, and obtain a complete set of binary parameters for QQ Vul. We classify the secondary star as M4V from the TiO band ratios