arXiv subjects

K. Beuermann

Publications and source records attributed to K. Beuermann.

At least 37 records · Page 2Linked to original sources

arXiv2008

Soft X-ray spectral variability of AM Herculis

Polars (AM Herculis binaries) are a prominent class of bright soft X-ray sources, many of which were discovered with ROSAT. We present a homogenous analysis of all the pointed ROSAT PSPC observations of polars subdivided into two papers that discuss the prototype polar AM Her in detail and summarize the class properties of all other polars. We derive the high-state soft X-ray flux and short-term spectral variability of AM Her using a new detector response matrix and a confirmed flux calibration of the ROSAT PSPC below 0.28 keV. The best-fit mean single-blackbody temperature and integrated bright-phase energy flux of AM Her in its April 1991 high state are 27.2 +/- 1.0 eV and (2.6 +/- 0.6) x 10^-9 erg cm^-2s^-1, respectively. The total blackbody flux of a multi-temperature model that fits both the soft X-ray and the fluctuating far-ultraviolet components is Fbb = (4.5 +/- 1.5) x 10^-9 erg cm^-2s^-1. The total accretion luminosity at a distance of 80 pc, Lbb = (2.1 +/- 0.7) x 10^33 erg s-1, implies an accretion rate of Mdot = (2.4 +/- 0.8) x 10^-10 Msun yr^-1 for an 0.78 Msun white dwarf. The soft X-ray flux displays significant variability on time scales down to 200 ms. Correlated spectral and count-rate variations are seen in flares on time scales down to 1 s, demonstrating the heating and cooling associated with individual accretion events. Our spectral and temporal analysis provides direct evidence for the blobby accretion model and suggests a connection between the soft X-ray and the fluctuating far-ultraviolet components.

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arXiv2008

Soft X-ray in-flight calibration of the ROSAT PSPC

We present an in-flight calibration of the ROSAT PSPC using the incident spectra of the hot white dwarf HZ43 and the polar AM Her. We derive an absolute flux calibration of the PSPC using the accurately known soft X-ray spectrum of HZ43. Corrections to the PSPC response matrix are derived from a comparison of predicted and observed PSPC spectra of HZ43, supplemented by results for AM Her. The calibration of the PSPC for photon energies E < 0.28 keV is found to be accurate to better than 5% refuting earlier reports of a major miscalibration. Our corrections to the detector response matrices remove systematic residuals in the pulse height spectra of soft sources.

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arXiv2008

High-resolution spectroscopy of the intermediate polar EX Hydrae. I. Kinematic study and Roche tomography

EX Hya is one of the few double-lined eclipsing cataclysmic variables that allow an accurate measurement of the binary masses. We analyze orbital phase-resolved UVES/ VLT high resolution spectroscopic observations of EX Hya with the aims of deriving the binary masses and obtaining a tomographic image of the illuminated secondary star. We present a novel method for determining the binary parameters by directly fitting an emission model of the illuminated secondary star to the phase-resolved line profiles of NaI lambda 8183/ 8195 in absorption and emission and CaII lambda 8498 in emission. The fit to the NaI and CaII line profiles, combined with the published K1, yields a white-dwarf mass M1 = 0.790 +/- 0.026 Msun, a secondary mass M2 = 0.108 +/- 0.008 Msun, and a velocity amplitude of the secondary star K2 = 432.4 +/- 4.8 km s-1. The secondary is of spectral type dM5.5 +/- 0.5 and has an absolute K-band magnitude of MK = 8.8. Its Roche radius places it on or very close to the main sequence of low-mass stars. It differs from a main sequence star by its illuminated hemisphere that faces the white dwarf. The secondary star contributes only 5% to the observed spin-phase averaged flux at 7500 A, 7.5% at 8200 A, and 37% in the K-band. We present images of the secondary star in the light of the NaI doublet and the CaII emission line derived with a simplified version of Roche tomography. We have discovered narrow spectral lines from the secondary star in EX Hya that delineate its orbital motion and allow us to derive accurate masses of both components. The primary mass significantly exceeds recently published values. The secondary is a low-mass main sequence star that displays a rich emission line spectrum on its illuminated side, but lacks chromospheric emission on its dark side.

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arXiv2006

Zeeman tomography of magnetic white dwarfs IV. The complex field structure of the polars EF Eri, BL Hyi, and CP Tuc

The magnetic fields of the accreting white dwarfs (WDs) in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects. We determine the structure of the surface magnetic fields of the WDs primaries in magnetic CVs using Zeeman tomography. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree l(max)=3 or 5 and include all tesseral and sectoral components with 0<=m<=l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, CP Tuc and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the WD. The results suggest that the field geometries of the WDs in short-period mCVs are quite complex with strong contributions from multipoles higher than the dipole in spite of a typical age of the WDs in CVs in excess of 1 Gyr. It is feasible to derive the surface field structure of an accreting WD from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure.

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arXiv2006

Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables

Barnes-Evans type relations provide an empirical relationship between the surface brightness of stars and their color. They are widely used for measuring the distances to stars of known radii, as the Roche-lobe filling secondaries in cataclysmic variables (CVs). The calibration of the surface brightness of field dwarfs of near-solar metalicity with spectral types A0 to L8 covers all secondary spectral types detectable in CVs and related objects and will aid in the measurement of their distances. The calibrations are based on the radii of field dwarfs measured by the Infrared Flux Method and by interferometry. Published photometry is used and homogenized to the Cousins Rc and Ic and the CIT JHK photometric systems. The narrow band surface brightness at 7500A is based on our own and published spectrophotometry. Care is taken to select the dwarfs for near-solar metalicity, appropriate to CVs, and to avoid errors caused by unrecognized binarity. Relations are provided for the surface brightness in V, Rc, Ic, J, H, K and in a narrow band at 7500A as functions of V-K and of spectral type. The method is tested with selected CVs for which independent information on their distances is available. The observed spread in the radii of early M-dwarfs of given mass or luminosity and its influence on the distance measurements of CVs is discussed. As long as accurate trigonometric parallaxes are not routinely available for a large number of CVs, the surface brightness method remains a reliable means of determining distances to CVs in which a spectral signature of the secondary star can be discerned.

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arXiv2006

Establishing HZ43 A, Sirius B, and RX J185635-3754 as soft X-ray standards: a cross-calibration between the Chandra LETG+HRC-S, the EUVE spectrometer, and the ROSAT PSPC

The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. We analyze the Chandra LETG+HRC-S observations of the white dwarfs HZ43 A and Sirius B and of the neutron star RX J185635-3754 in order to resolve current uncertainties in the soft X-ray spectral fluxes and photospheric parameters of the three stars. We have obtained improved parameters for which fit the observations from the optical to the soft X-ray regime. Our approach allows us to quote their absolute spectral fluxes at selected wavelengths which may aid the calibration of other space-borne instruments.

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arXiv2006

The secondary star and distance of the polar V1309 Ori

The first phase-resolved JHK light curves of the eclipsing polar (AM Herculis binary) V1309 Ori are presented and interpreted. We separate the contributions from the secondary star and from other sources with the aim of determining a photometric distance. Simple model calculations show that the accretion stream and the cyclotron source on the accreting white dwarf are minor contributors to the infrared light, allowing an accurate determination of spectral type and absolute flux of the secondary star. The unilluminated backside of the secondary star as seen in eclipse has spectral type dM0 to dM0+. Its dereddened magnitude is K = 13.58 at orbital phase phi = 0 (eclipse). Using the calibrated surface brightness of M-stars and the published mass of the secondary, M2 = 0.46 Msun, we obtain a distance d = 600 +/- 25 pc which scales as M2^(1/2). The radius of the Roche-lobe filling secondary exceeds the main-sequence radius of an M0 star by 21 +11/-6 %. The debated origin of the infrared light of V1309 Ori has been settled in favor of the secondary star as the main contributor and an accurate distance has been derived that will place estimates of the luminosity and synchronization time scale on a more secure basis.

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arXiv2006

Zeeman tomography of magnetic white dwarfs III. The 70-80 Megagauss magnetic field of PG 1015+014

Aims: We analyse the magnetic field geometry of the magnetic DA white dwarf PG 1015+014 with our Zeeman tomography method. Methods: This study is based on rotation-phase resolved optical flux and circular polarization spectra of PG 1015+014 obtained with FORS1 at the ESO VLT. Our tomographic code makes use of an extensive database of pre-computed Zeeman spectra. The general approach has been described in Papers I and II of this series. Results: The surface field strength distributions for all rotational phases of PG 1015+014 are characterised by a strong peak at 70 MG. A separate peak at 80 MG is seen for about one third of the rotation cycle. Significant contributions to the Zeeman features arise from regions with field strengths between 50 and 90 MG. We obtain equally good simultaneous fits to the observations, collected in five phase bins, for two different field parametrizations: (i) a superposition of individually tilted and off-centred zonal multipole components; and (ii) a truncated multipole expansion up to degree l = 4 including all zonal and tesseral components. The magnetic fields generated by both parametrizations exhibit a similar global structure of the absolute surface field values, but differ considerably in the topology of the field lines. An effective photospheric temperature of T eff = 10000 ± 1000 K was found. Conclusions: Remaining discrepancies between the observations and our best-fit models suggest that additional small-scale structure of the magnetic field exists which our field models are unable to cover due to the restricted number of free parameters.

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arXiv2005

Irradiated atmospheres of accreting magnetic white dwarfs with an application to the polar AM Herculis

We present a pilot study of atmospheres of accreting magnetic white dwarfs irradiated by intense fluxes at ultraviolet to infrared wavelengths. The model uses a standard LTE stellar atmosphere code which is expanded by introducing an angle-dependent external radiation source. The present results are obtained for an external source with the spectral shape of a 10000K blackbody and a freely adjustable spectral flux. The model provides an explanation for the observed largely filled-up Lyman lines in the prototype polar AM Herculis during its high states. It also confirms the hypotheses (i) that irradiation by cyclotron radiation and other radiation sources is the principle cause for the large heated polar caps surrounding the accretion spots on white dwarfs in polars and (ii) that much of the reprocessed light appears in the far ultraviolet and not in the soft X-ray regime as suggested in the original simple theories. We also briefly discuss the role played by hard X-rays in heating the polar cap.

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arXiv2005

XMM-Newton observation of the long-period polar V1309 Ori: The case for pure blobby accretion

Using XMM-Newton we have obtained the first X-ray observation covering a complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is dominated by a short, bright phase interval with EPIC pn count rates reaching up to 15 cts/sec per 30 sec resolution bin. The bright phase emission is well described by a single blackbody component with kT_bb = (45 +- 3) eV. The absence of a bremsstrahlung component at photon energies above 1 keV yields a flux ratio F_bb/F_br > 6700. This represents the most extreme case of a soft X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray emission is subdivided into a series of individual flare events supporting the hypothesis that the soft X-ray excess in V1309 is caused by accretion of dense blobs. In addition to the bright phase emission, a faint, hard X-ray component is visible throughout the binary orbit with an almost constant count rate of 0.01 cts/sec. Spectral modelling indicates that this emission originates from a complex multi-temperature plasma. At least three components of an optically thin plasma with temperatures kT= 0.065, 0.7, and 2.9 keV are required to fit the observed flux distribution. The faint phase emission is occulted during the optical eclipse. Eclipse ingress lasts about 15--20 min and is substantially prolonged beyond nominal ingress of the white dwarf. This and the comparatively low plasma temperature provide strong evidence that the faint-phase emission is not thermal bremsstrahlung from a post-shock accretion column above the white dwarf. A large fraction of the softer faint-phase emission could be explained by scattering of photons from the blackbody component in the infalling material above the accretion region. The remaining hard X-ray flux could be produced in the coupling region, so far unseen in other polars.

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arXiv2005

Zeeman tomography of magnetic white dwarfs II. The quadrupole-dominated magnetic field of HE 1045-0908

We report time-resolved optical flux and circular polarization spectroscopy of the magnetic DA white dwarf HE 1045-0908 obtained with FORS1 at the ESO VLT. Considering published results, we estimate a likely rotational period of Prot ~ 2.7 h, but cannot exclude values as high as about 9 h. Our detailed Zeeman tomographic analysis reveals a field structure which is dominated by a quadrupole and contains additional dipole and octupole contributions, and which does not depend strongly on the assumed value of the period. A good fit to the Zeeman flux and polarization spectra is obtained if all field components are centred and inclinations of their magnetic axes with respect to each other are allowed for. The fit can be slightly improved if an offset from the centre of the star is included. The prevailing surface field strength is 16 MG, but values between 10 and ~75MG do occur. We derive an effective photospheric temperature of HE 1045-0908 of Teff = 10000 +/- 1000 K. The tomographic code makes use of an extensive database of pre-computed Zeeman spectra (Paper I).

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arXiv2004

Far-ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained HST/STIS data for a total of eleven polars as part of a program aimed to compile a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the eleven polars, eight were found in a state of low accretion activity (V347Pav, VVPup, V834Cen, BLHyi, MRSer, STLMi, RXJ1554.2+2721 and V895Cen) and three in a state of high activity (CDInd, ANUMa and UWPic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10800K to 14200K for log g = 8.0. Comparing the WD temperatures of polars to those of non-magnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in non-magnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and non-magnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary.

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arXiv2004

Surface Magnetic Field Distributions of the White Dwarfs PG 1015+014 and HE 1045-0908

We have applied the method of Zeeman tomography to analyze the surface magnetic field structures of the white dwarfs PG 1015+014 and HE 1045-0908 from spin-phase resolved flux and circular polarization spectra obtained with FORS1 at the ESO VLT. We find for both objects field topologies that deviate significantly from centred dipoles. For HE 1045-0908, the frequency distribution of magnetic field strengths is sharply peaked at 16 MG for all rotational phases covered by our data but extends to field strengths at least five times this value. In the case of PG 1015+014 there are significant contributions to the frequency distribution in the range from 50 to 90 MG with the maximum near 70 MG. The detailed shape of the frequency distribution is strongly variable with respect to the rotational phase.

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arXiv2004

The Structure and Origin of Magnetic Fields on Accreting White Dwarfs

We have started a systematic study of the field topologies of magnetic single and accreting white dwarfs using Zeeman tomography. Here we report on our analysis of phase-resolved flux and circular polarization spectra of the magnetic cataclysmic variables BL Hyi and MR Ser obtained with FORS1 at the ESO VLT. For both systems we find that the field topologies are more complex than a dipole or an offset dipole and require at least multipole expansions up to order l = 3 to adequately describe the observed Zeeman features and their variations with rotational phase. Overall our model fits are in excellent agreement with observations. Remaining residuals indicate that the field topologies might even be more complex. It is, however, assuring that the global characteristics of our solutions are consistent with the average effective field strengths and the halo field strengths derived from intensity spectra in the past.

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arXiv2004

A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721

We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721. Fitting the STIS data with magnetic white dwarf model spectra, we derive a field strength of B~144MG and an effective temperature of 17000K<Teff<23000K. This measurement makes RX J1554.2+2721 only the third cataclysmic variable containing a white dwarf with a field exceeding 100MG. Similar to the other high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble mass transfer, which suggests that a considerable number of high-field polars may still remain undiscovered.

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arXiv2004

An HST parallax of the distant cataclysmic variable V1223 Sgr, its system parameters, and accretion rate

Using the Hubble space Telescope Fine Guidance Sensor, we have measured the trigonometric parallax of the bright cataclysmic variable 1223 Sgr. The absolute parallax is pi(abs)= 1.96+-0.18mas, making V1223 Sgr the most distant CV with a well-determined trigonometric parallax. This distance, a Lutz-Kelker correction, and the previously measured extinction yield an absolute visual high-state magnitude Mv=4.0+-0.2. We outline a model, which is consistent with the observed spin-down of the white dwarf and provides for much of the UV/optical emission by reverberation of X-rays. From previous X-ray and UV/optical data, we derive an accretion luminosity Lacc=(2.6+-0.8)e34 erg/s, a white dwarf mass M_1=0.93+-0.12Msun, and an accretion rate dot M=(1.4+-0.3)e17 g/s.

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arXiv2003

AstroCat/CVcat: A catalogue on Cataclysmic Variables based on a new framework for online interactive astronomical databases

We report on the progress of the development of CVcat, an interactive catalogue on Cataclysmic Variables, which is the first application based on AstroCat, a general framework for the installation and maintenance of web-based interactive astronomical databases. Registered users can contribute directly to the catalogue content by adding new objects, object properties, literature references, and annotations. The scientific quality control of the catalogue is carried out by a distributed editorial team. Searches in CVcat can be performed by object name, classification, certain properties or property ranges, and coordinates. Search results can be retrieved in several output formats, including XML. Old database states can be restored in order to ensure the citability of the catalogue. Furthermore, CVcat is designed to serve as a repository for reduced data from publications. Future prospects include the integration of AstroCat-based catalogues in the international network of Virtual Observatories.

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arXiv2003

A precise HST parallax of the cataclysmic variable EX Hydrae, its system parameters, and accretion rate

Using the HST Fine Guidance Sensor, we have measured a high precision astrometric parallax of the cataclysmic variable EX Hydrae, pi=15.50+-0.29mas. From the wavelength-integrated accretion-induced energy flux, we derive a quiescent accretion luminosity for EX Hya of Lacc = (2.6+-0.6)x10e32 erg. The quiescent accretion rate then is Mdot=(6.2\+-1.5)x10e-11 (M1/0.5Msun)^(-1.61})Msun/yr. The time-averaged accretion rate, which includes a small correction for the rare outbursts, is 6% higher. We discuss the system parameters of EX Hya and deduce M1=0.4-0.7Msun, M2=0.07-0.10Msun, and i=76.0deg-77.6deg, using recent radial velocity measurements of both components and restrictions imposed by other observational and theoretical constraints. We conclude that the secondary is undermassive, overluminous, and expanded over a ZAMS star of the same mass. Near the upper limit to M1, the accretion rate of the white dwarf coincides with that due to near-equilibrium angular momentum loss by gravitational radiation and angular momentum transfer from the orbit into the spin-up of the white dwarf. Near the lower mass limit, the correspondingly higher accretion rate requires that either an additional angular momentum loss process is acting besides gravitational radiation or that accretion occurs on a near-adiabatic time scale. The latter possibility would imply that EX Hya is in a transient phase of high mass transfer and the associated spin-up of the white dwarf.

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