Structural studies of emission regions in polars

We report on structure in the emissions region encountered in polars. This will become a travel along the accretion stream, starting at the surface of the secondary star, moving along the horizontal and vertical parts of the stream and finishing in the accretion spots on the white dwarf. For the first time we establish a detailed picture of the origin of atomic emission lines in a polar by means of Doppler tomography. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Accretion and magnetism - AM Herculies stars. Polars in the ROSAT era

The accretion processes in Am Herculis binaries are reigned by the strong magnetic field of a mass-accreting white dwarf. In the first part of this article I describe the methods applicable to infer the strength and structure of the magnetic fields in these systems. These are based on direct measurements of Zeeman split Balmer lines and/or harmonic cyclotron emission. All well-studied systems show signs of complex field structures or at least deviations from the most simple model of a centered dipole. The observed range of B-values is 10 -70 MG. The evolution of the systems is responsible for this range being much narrower than that measured in single white dwarfs, 100 kG - 1000 MG (see Schmidt 1995, this volume). The following parts of this article are devoted to other processes intimately interwoven with the presence of magnetic fields. These are the accretion streams (instead of accretion disks), the different ways of energy release in the accretion spots and the synchronous rotation of both stars. Considerable progress was reached in various fields after the launch of ROSAT which detected a lot of new and some very remarkable systems and which made detailed observational studies of individual systems possible. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The new long-period AM Herculis system RX J0203.8+2959

We present the first detailed multi-wavelength study of the ROSAT-discovered AM Herculis binary RX J0203.8+2959 comprising extended CCD and photoelectric photometry, phase-resolved spectroscopy with high and low spectral resolution and pointed ROSAT observations with the PSPC and the HRI. Between 1992 and 1997 the system displayed states of high and low accretion with mean brightness levels of V=15.5"m and 18"m, respectively. A timing analysis revealed that the binary is rotating synchronously with a period of 4.6 hr and is thus one of the longest-period polars known. The emission lines are structured showing narrow and broad components. We could distinguish between line emission arising from the heated side of the secondary star and the accretion stream. Velocity information derived for a narrow emission line with suspected origin on the heated side of the companion star allowed us to constrain its orbital velocity and to phase its inferior conjunction. In addition the photospheric spectrum of the secondary star could be detected in the near-infrared. The strength of the TiO-bands was used to determine its spectral type as dM 2.5, which is consistent with a Roche lobe filling main sequence star, and to estimate the distance of the system to be #propor to#600 pc. Definite confirmation of its magnetic nature is given by the detection of cyclotron harmonics implying a field strength of B=38#+-#2 MG. The accretion geometry could not be fixed unequivocally. The morphology of the optical light curves is suggestive of one active accretion region which undergoes no selfeclipse. A corresponding simple light curve pattern is not seen at X-ray wavelengths, it's time signature is dominated by instationary accretion of discrete blobs. We report on the occasional occurence of phase- and colour-dependent quasi-periodic oscillations at periods of #propor to#7 and #propor to#14 min. (orig.)Available from TIB Hannover: RR 7310(98-19) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman