arXiv subjects

Axel D. Schwope

Publications and source records attributed to Axel D. Schwope.

40 records · Page 3Linked to original sources

arXiv1997

On the mass of the white dwarf in UZ Fornacis

We present phase-resolved spectroscopy of the eclipsing AM Herculis star UZ For obtained when the system was in its low state of accretion. Faint residual Halpha-emission and NaI absorption were used to trace the secondary star and infer its orbital velocity K_2. The measured radial velocity amplitude of NaI K_2 * sin(i) = 285 \pm 50 kmps suggests a low-mass white dwarf with M_wd = 0.44 \pm 0.15 Msun (1 sigma-errors). The Halpha emission line on the other hand, visible only for part of the orbital cycle and supposed to originate only on the illuminated hemisphere facing the white dwarf, displays a similar radial velocity amplitude, K'_2 sin(i) = 308 \pm 27 kmps. The standard K_2-correction applied by us then suggests a white dwarf mass of up to 1 Msun. Compared with earlier results the new ones enlarge the window in which the white dwarf mass may lie and resolves the conflict between mass estimates based on photometry and spectroscopy. They leave some ambiguity in the location of emission and absorption components in these and former observations.

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arXiv1997

Phase-resolved high-resolution spectrophotometry of the eclipsing polar HU Aquarii

We present phase-resolved spectroscopy of the bright, eclipsing polar HU Aqr obtained with high time (~30sec) and spectral (1.6 A) resolution when the system was in a high accretion state. The trailed spectrograms reveal clearly the presence of three different line components with different width and radial velocity variation. By means of Doppler tomography their origin could be located unequivocally (a) on the secondary star, (b) the ballistic part of the accretion stream (horizontal stream), and (c) the magnetically funnelled part of the stream (vertical stream). For the first time we were able to derive a (near-)complete map of the stream in a polar. We propose to use Doppler tomography of AM Herculis stars as a new tool for the mass determination of these binaries. This method, however, still needs to be calibrated by an independent method. The asymmetric light curve of the narrow emission line originating on the mass-donating companion star reveals evidence for significant shielding of 60% of the leading hemisphere by the gas between the two stars.

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arXiv1997

Phase-resolved spectroscopy and photometry of the eclipsing polar EP Draconis (=H1907+690)

We present phase-resolved optical spectroscopy and CCD photometry of the faint eclipsing polar EP Dra (H1907+690). A revised ephemeris is derived which connects all 32000 binary cycles since its discovery by Remillard et al. (1991). We found no difference between spin and and orbital periods of the white dwarf. Changes in the light curve morphology are attributed to a different beaming behaviour which might change on timescales as short as one or several orbital periods. Optical light curve modelling was used to estimate the co-latitude of the accretion spot, which must be larger than 40 degrees. We have detected Zeeman absorption lines of Halpha originating in an accretion halo in a field of 16MG. The low-resolution spectra reveal no indication of resolved cyclotron harmonics, which is also suggestive of a relatively low field strength in the accretion region. The Balmer emission lines contain significant contributions from the UV-illuminated hemisphere of the companion star, whereas the HeII4686 emission originates predominantly from the accretion stream. The emission lines have a multi-component structure and we could single out a narrow emission line in the Hbeta and Hgamma lines. Its radial velocity amplitude suggests a low mass for the white dwarf, if the lines are interpreted as being of reprocessed origin from the whole illuminated hemisphere of the companion star.

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arXiv1997

Cyclotron spectroscopy of VV Puppis

We present phase-resolved spectrophotometric observations of VV Puppis obtained during two different states of accretion. We confirm the detection of cyclotron lines from emission regions at both poles (Wickramasinghe et al. 1989) having significantly different field strength of B_1 ~ 31MG and B_2 ~ 54MG. Our phase-resolved data allowed the detection of phase-dependent wavelength shifts of the cyclotron lines from the main accretion pole which is due to the varying aspect of the observer. A corresponding motion of the cyclotron lines from the secondary pole appears likely. Compared to 1984, the cyclotron lines from the main pole appeared redshifted in 1989, during an episode of higher system brightness. This shift can be explained assuming either different locations of the accretion spot and, hence, variations of the magnetic field, or variations of the plasma temperature both triggered by variations of the mass accretion rate. The cyclotron lines from the second pole do not show such changes suggesting that the coupling region for the weakly accreting pole is more or less stationary whereas the coupling region of the main pole varies in space, most likely depending on the mass accretion rate.

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