I. Traulsen

X-ray spectroscopy and photometry of the long-period polar AI Trianguli with XMM-Newton

Context. The energy balance of cataclysmic variables with strong magnetic fields is a central subject in understanding accretion processes on magnetic white dwarfs. With XMM-Newton, we perform a spectroscopic and photometric study of soft X-ray selected polars during their high states of accretion. Aims. On the basis of X-ray and optical observations of the magnetic cataclysmic variable AI Tri, we derive the properties of the spectral components, their flux contributions, and the physical structure of the accretion region in soft polars. Methods. We use multi-temperature approaches in our XSPEC modeling of the X-ray spectra to describe the physical conditions and the structures of the post-shock accretion flow and the accretion spot on the white-dwarf surface. In addition, we investigate the accretion geometry of the system by completing a timing analysis of the photometric data. Results. Flaring soft X-ray emission from the heated surface of the white dwarf dominates the X-ray flux during roughly 70% of the binary cycle. This component deviates from a single black body and can be described by a superimposition of mildly absorbed black bodies with a Gaussian temperature distribution between kT bb,low := 2 eV and k tbb , high = 43.9 +3.3 -3.2 eV, and N H , ISM = 1.5 +0.8 -0.7 × 10 20 cm -2 . In addition, weaker hard X-ray emission is visible nearly all the time. The spectrum from the cooling post-shock accretion flow is most closely fitted by a combination of thermal plasma MEKAL models with temperature profiles adapted from prior stationary two-fluid hydrodynamic calculations. The resulting plasma temperatures lie between kT MEKAL,low = 0.8 +0.4 -0.2 keV and kT MEKAL,high = 20.0 +9.9 -6.1 keV; additional intrinsic, partial-covering absorption is on the order of N H,int = 3.3 +2.5 -1.2 × 10 23 cm -2 . The soft X-ray light curves show a dip during the bright phase, which can be interpreted as self-absorption in the accretion stream. Phase-resolved spectral modeling supports the picture of one-pole accretion and self-eclipse. One of the optical light curves corresponds to an irregular mode of accretion. During a short XMM-Newton observation at the same epoch, the X-ray emission of the system is clearly dominated by the soft component.

X-ray and optical observations of four polars

We aim to study the temporal and spectral behaviour of four polar CVs from the infrared to X-ray regimes, refine our knowledge of the physical parameters of these systems at different accretion rates, and to search for a possible excess of soft X-ray photons. We analysed four XMM X-ray observations of three of the sources, two of them discovered in SDSS, one in RASS. The X-ray data were complemented by optical photometry and spectroscopy and, for two sources, archival Swift observations. SDSSJ0328 was X-ray bright in two XMM and two Swift observations, and shows transitions from high and low accretion states over a few months. It has no strong soft excess. We measured the magnetic field strength at the main pole to be 39 MG, the inclination to be 45<i<77 deg, and we have refined the long-term ephemeris. SDSSJ1333 was X-ray faint. We measured a faint phase X-ray flux and plasma temperature for this source, which spends almost all of its time accreting at a low level. Its inclination is less than about 76 degrees. 1RXSJ1730 was X-ray bright in the XMM data. Its spectrum contained a modest soft blackbody component, not luminous enough to be considered a strong soft excess. We inferred a magnetic field strength at the main pole of 20 to 25 MG, and that the inclination is less than 77 and probably less than 63 deg. V808 Aur was faint in the Swift observation but there is still strong evidence for bright and faint phases in X-rays and perhaps in UV. Residual X-ray flux from the faint phase is hard to explain by thermal emission from the WD surface, or by accretion onto the other pole. We give a revised distance estimate of 250pc. The three systems we could study in detail appear to be normal polars, with luminosity and magnetic field strength typical for this class of CV. None of these systems shows the strong soft excess thought commonplace in polars prior to the XMM era.

A new period determination for the close PG1159 binary SDSS J212531.92–010745.9

Methods to measure masses of PG1159 stars in order to test evolutionary scenarios are currently based on spectroscopic masses or asteroseismological mass determinations. One recently discovered PG1159 star in a close binary system may finally allow the first dynamical mass determination, which has so far been analysed on the basis of one SDSS spectrum and photometric monitoring. In order to be able to phase future radial velocity measurements of the system SDSSJ212531.92-010745.9, we follow up on the photometric monitoring of this system to provide a solid observational basis for an improved orbital ephemeris determination. New white-light time series of the brightness variation of SDSSJ212531.92-010745.9 with the Tuebingen 80cm and Goettingen 50cm telescopes extend the monitoring into a second season (2006), tripling the length of overall observational data available, and significantly increasing the time base covered. We give the ephemeris for the orbital motion of the system, based on a sine fit which now results in a period of 6.95573(5)h, and discuss the associated new amplitude determination in the context of the phased light curve variation profile. The accuracy of the ephemeris has been improved by more than one order of magnitude compared to that previously published for 2005 alone.

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

RS Cae is the third target in our series of XMM-Newton observations of soft X-ray-dominated polars. 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. 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 36eV and 7keV. 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 > 750pc. 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.

XMM-Newton observations of the X-ray soft polar QS Telescopii

Context. On the basis of XMM-Newton observations, we investigate the energy balance of selected magnetic cataclysmic variables, which have shown an extreme soft-to-hard X-ray flux ratio in the ROSAT All-Sky Survey. Aims. We intend to establish the X-ray properties of the system components, their flux contributions, and the accretion geometry of the X-ray soft polar QS Tel. In the context of high-resolution X-ray analyses of magnetic cataclysmic variables, this study will contribute to better understanding the accretion processes on magnetic white dwarfs. Methods. During an intermediate high state of accretion of QS Tel, we have obtained 20 ks of XMM-Newton data, corresponding to more than two orbital periods, accompanied by simultaneous optical photometry and phase-resolved spectroscopy. We analyze the multi-wavelength spectra and light curves and compare them to former high- and low-state observations. Results. Soft emission at energies below 2 keV dominates the X-ray light curves. The complex double-peaked maxima are disrupted by a sharp dip in the very soft energy range (0.1-0.5 keV), where the count rate abruptly drops to zero. The EPIC spectra are described by a minimally absorbed black body at 20 eV and two partially absorbed MEKAL plasma models with temperatures around 0.2 and 3 keV. The black-body-like component arises from one mainly active, soft X-ray bright accretion region nearly facing the mass donor. Parts of the plasma emission might be attributed to the second, virtually inactive pole. High soft-to-hard X-ray flux ratios and hardness ratios demonstrate that the high-energy emission of QS Tel is substantially dominated by its X-ray soft component.

V902 Monocerotis: A likely disc-accreting intermediate polar

Aims: We aim to confirm whether the eclipsing cataclysmic variable V902 Mon is an Intermediate Polar, to characterise its X-ray spectrum and flux, and to refine its orbital ephemeris and spin period. Methods: We performed spectrographic observations of V902 Mon in 2016 with the 2.2m Calar Alto telescope, and X-ray photometry and spectroscopy with XMM-Newton in October 2017. This data was supplemented by several years of AAVSO visual photometry. Results: We have confirmed V902 Mon as an IP based on detecting the spin period, with a value of 2,208s, at multiple epochs. Spectroscopy of the donor star and Gaia parallax yield a distance of 3.5+1.3-0.9, kpc, suggesting an X-ray luminosity one or two orders of magnitude lower than the 10^33 erg/s typical of previously known IPs. The X-ray to optical flux ratio is also very low. The inclination of the system is more than 79deg, with a most likely value of around 82deg. We have refined the eclipse ephemeris, stable over 14,000 cycles. The Halpha line is present throughout the orbital cycle and is clearly present during eclipse, suggesting an origin distant from the white dwarf, and shows radial velocity variations at the orbital period. The amplitude and overall recessional velocity seem inconsistent with an origin in the disc. The \emph{XMM-Newton} observation reveals a partially absorbed plasma model typical of magnetic CVs, with a fluorescent iron line at 6.4keV showing a large equivalent width of 1.4keV. Conclusions: V902 Mon is an IP, and probably a member of the hypothesized X-ray underluminous class of IPs. It is likely to be a disc accretor, though the radial velocity behaviour of the Halpha line remains puzzling. The large equivalent width of the fluorescent iron line, the small FX/Fopt ratio, and the only marginal detection of X-ray eclipses suggests that the X-ray emission arises from scattering.

Towards a dynamical mass of a PG 1159 star: radial velocities and spectral analysis of SDSS J212531−010745

The evolutionary scenarios which are commonly accepted for PG 1159 stars are mainly based on numerical simulations, which have to be tested and calibrated with real objects with known stellar parameters. One of the most crucial but also quite uncertain parameters is the stellar mass. PG 1159 stars have masses between 0.5 and 0.8 M� , as derived from asteroseismic and spectroscopic determinations. Such mass determinations are, however, themselves model-dependent. Moreover, asteroseismically and spectroscopically determined masses deviate systematically for PG 1159 stars by up to 10%. SDSS J212531.92-010745.9 is the first known PG 1159 star in a close binary with a late-main-sequence companion allowing a dynamical mass determination. We have obtained 14 Calar Alto spectra of SDSS J212531.92−010745.9 covering the full orbital phase range. A radial velocity curve was extracted for both components. With co-added phasecorrected spectra the spectral analysis of the PG 1159 component was refined. The irradiation of the companion by the PG 1159 star is modelled with PHOENIX, yielding constraints on radii, effective temperature and separation of the system’s components. The light curve of )

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.

Accretion onto selected magnetic white dwarfs as seen in X‐rays

Magnetic cataclysmic variables of AM Her type comprise an accreting white dwarf with a strong magnetic field. Under its influence, the accretion stream is channeled along the field lines towards the poles of the white dwarf, preventing the formation of an accretion disk and allowing for direct insight into the accretion regions. Due to the high temperatures developing in the accretion process, a considerable fraction of the total emission is found at X‐ray energies: hard X‐ray emission from the material which is decelerated above the white‐dwarf surface, and soft X‐ray and FUV emission from the heated photosphere, where the hard emission is reprocessed. We perform dedicated XMM‐Newton observations to study the spectral components, their flux contributions, and the physical structure of the accretion region of several AM Her systems selected by their distinct soft X‐ray fluxes. Modeling the spectral signature of these system components involves approaches to the complex and still widely unknown temperature...

Light and Heavy Metal Abundances in Hot Central Stars of Planetary Nebulae

We present new results from our spectral analyses of very hot central stars achieved since the last IAU Symposium on planetary nebulae held in Canberra 2001. The analyses are mainly based on UV and far‐UV spectroscopy performed with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer but also on ground‐based observations performed at the Very Large Telescope and other observatories. We report on temperature, gravity, and abundance determinations for the CNO elements of hydrogen‐rich central stars. In many hydrogen‐deficient central stars (spectral type PG1159) we discovered particular neon and fluorine lines, which are observed for the very first time in any astrophysical object. Their analysis strongly confirms the idea that these stars exhibit intershell matter as a consequence of a late helium‐shell flash.