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Astron. Astrophys. 343, 455-465 (1999)

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4. Discussion and summary

We performed high-speed photometric and spectrophotometric observations as well as high-resolution spectroscopy of the bright galactic SSS RXJ0019 between 1992 and 1997. Our detailed optical studies result in a new determination of the orbital period. We observed a color variation in [FORMULA] symmetrical to the main minimum with an amplitude of [FORMULA] = 0.15m. The observed lightcurves change between having a flat appearance (quiet optical state) and showing humps and steps with a 1.8 h period (excited optical state). RXJ0019 can change between the two states from night to night. These variations might be due to short-term changes of the mass accretion rate from the companion star.

Our spectroscopic investigation shows the well known emission line spectrum of mainly Balmer and Helium II lines. Symmetric emission lines adjacent to the Balmer lines show the presence of high velocity outflows (jets) probably originating near the white dwarf. The jet lines show an orbital Doppler motion comparable to that of the He II lines. The velocity of the jet is quite low, indicating a medium to high inclination of the system.

From the radial velocities we calculated the mass function. For the assumed range of medium to high inclinations we derive a low mass for the secondary star ([FORMULA]). Low mass companion stars might be common among the SSS. This might also be an explanation for the absence of any spectral features of the companion star in the high resolution spectra.

We also observe P-Cygni profiles in the Balmer lines showing an orbital modulation. The P-Cygni absorption almost disappears between [FORMULA] [FORMULA] 0.7 to 0.9. Higher transitions of the Balmer series up to H12 are only detected by their corresponding P-Cygni absorption. The velocity of the directed wind responsible for the P-Cygni profiles is [FORMULA] 590 km s-1 with the absorption wings extending to 900 km s-1.

Our trailed spectra show substructures of different components. As these components are very weak or were observed in lines with bad S/N ratio the locations of these components cannot clearly be located by the means of Doppler tomography. The shifted intensity maximum in the Helium maps at [FORMULA] [FORMULA] -100 km s-1 and [FORMULA] [FORMULA] -100 km s-1 might be due to a radially extended and elevated accretion disk rim, consistent with theoretical models of SSS. But we clearly see that most of the line-emitting material is not located within the accretion disk. Due to the high accretion rate and viscous processes involved we assume that, almost the whole accretion disk is optically thick. Therefore, no emission from Balmer and Helium lines originating at the inner parts of the disk can be observed.

We only see line-emitting material with very low velocities. The emission distribution is mainly symmetrical around the center of mass. As material cannot be stationary at the center of mass the emission must originate at regions with very low velocities. Low velocities can be found far outside of the binary system. Therefore, we propose that this SSS has a circumbinary cocoon or disk of hydrogen and helium responsible for the observed emission lines.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999