Polarimetric Study of the Massive Interacting Binary W Serpentis: Discovery of High‐Latitude Scattering Spot/Jet

We present multicolor (UBVRI) polarimetry and additional B-band CCD polarimetry of the peculiar, strongly interacting binary W Ser, with good coverage over the 14.16 day orbital period in the three observing seasons 2001-2003. An interesting finding is that the first harmonic dominates in polarization variations over the binary cycle in each season, indicating that the main contribution to the polarized flux is coming from a localized region away from the orbital plane. We apply our new model codes for electron scattering in circumstellar matter to interpret the data and find that a "spot or jet" like region at latitude ~ 65° has clearly the best signal-to-noise ratio (>30) among the detected scattering components. Significant shell and stream components are also found, but no disk is seen in the polarized light. The upper limits for the total number of free electrons in an optically thin scattering disk are an order of magnitude lower than in the spherical shell around the primary star. A possible reason is that the accretion disk is optically thick and has a clearly defined and visible pseudophotosphere. Multiple scattering and absorption effects in an optically thick regime reduce the polarization of the observed light to small levels. Scattering from optically thin parts of the circumstellar matter, the shell, the stream, and the high-latitude spot/jet, possibly associated with optically thin polar regions of the accretion disk, dominates in the observed polarization of W Ser.