Bipolar Jets and Orbital Dynamics of the Supersoft X-Ray Source RX J0019.8+2156

Abstract

From 1994 to 1997 we monitored the optical spectrum of RXJ0019.8+2156. This supersoft X-ray source is one of only two accreting white dwarfs in the Galaxy thought to be burning hydrogen on their surface as a consequence of a high rate of mass transfer from a binary companion. Accurate orbital ephemerides are derived from radial velocity measure- ments of HeII emission lines, which exhibit a stable velocity semi- amplitude of 71.2 +- 3.6 km/s. We report the discovery of transient, low-velocity, bipolar jets. These jets are respresented by redshifted and blueshifted pairs of emission lines with an outflow velocity of v cos(i) = 815 km/s, where i is the binary inclination angle. When present, the jet lines also exhibit an orbital modulation of 71 km/s, which indicates that the jets are oriented nearly perpendicular to the orbital plane. On most occasions, the Hline profiles are further altered by P Cygni absorption. We show that the jets and the P Cygni features have very different temporal characteristics and binary phase dependence. The jet material and the absorbing wind must therefore occupy very different geometries. Finally, the measured mass function is combined with binary evolution models to suggest a limit, i < 40 deg. A particular model invoked to explain a high rate of mass transfer requires 16 < i < 25. However, at such small inclination it is difficult to explain the large amplitude of the orbital light curve. By contrast, a simple model fit to the jet outflow lines indicates an orbital inclination angle of 35 < i < 60.