532 research outputs found
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.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
The New Eclipsing Cataclysmic Variable SDSS 154453+2553
The cataclysmic variable SDSS154453+2553 was recently identified in the Sloan
Digital Sky Survey. We obtained spectra and photometry at the MDM Observatory,
which revealed an eclipse with a 6.03 hour period. The H{\alpha} emission line
exhibits a strong rotational disturbance during eclipse, indicating that it
arises in an accretion disk. A contribution from an M-type companion is also
observed. Time-series photometry during eclipse gives an ephemeris of
2454878.0062(15) + 0.251282(2)E. We present spectroscopy through the orbit and
eclipse photometry. Our analysis of the secondary star indicates a distance of
800 {\pm} 180 pc.Comment: 6 pages, 3 figures, Accepted for publication in PAS
A new soft X-ray spectral model for polars with an application to AM Herculis
We present a simple heuristic model for the time-averaged soft X-ray
temperature distribution in the accretion spot on the white dwarf in polars.
The model is based on the analysis of the Chandra LETG spectrum of the
prototype polar AM Her and involves an exponential distribution of the emitting
area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter
besides the normalization, it is mathematically as simple as the single
blackbody, but is physically more plausible and fits the soft X-ray and
far-ultraviolet spectral fluxes much better. The model yields more reliable
values of the wavelength-integrated flux of the soft X-ray component and the
implied accretion rate than reported previously.Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae
Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects.
Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography.
Methods. 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.
Results. 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 lmax = 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, and 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 white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with
strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr.
Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf 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
Spectroscopy of Four Cataclysmic Variables with Periods above 7 Hours
We present spectroscopy of four cataclysmic variables. Using radial velocity
measurements, we find orbital periods for the first time. The stars and their
periods are GY Hya, 0.347230(9) d; SDSS J204448-045929, 1.68(1) d; V392 Hya,
0.324952(5) d; and RX J1951.7+3716, 0.492(1) d. We also detect the spectra of
the secondary stars, estimate their spectral types, and derive distances based
on surface brightness and Roche lobe constraints.Comment: 22 pages, 3 figures, 5 tables, to be published in December 2006 PAS
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.Comment: 10 pages, 5 figures, A&A publishe
The high-field magnetic white dwarf LP 790-29: not a fast rotator
We have investigated the nature of the magnetic white dwarf LP 790-29 = LHS
2293 by polarimetric monitoring, searching for short-term variability. No
periodicity was found and we can exclude rotation periods between 4 sec and 1.5
hour with a high confidence. Maximum amplitudes of sinusoidal variations are
Delta R < 0.009 mag and Delta V_R < 0.7% for a mean value of the R-band
circular polarization of V_R = +9.1+/-0.3%. Combined with earlier results by
other authors, our observation suggests that LP 790-29 is, in fact, an
extremely slowly rotating single white dwarf and not an unrecognized fast
rotator and/or disguised cataclysmic variable.Comment: 4 pages, 2 figures, accepted for publication by A&
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