Discovery of intrinsic linear polarization in SS433

Abstract

The remarkable object¹ SS433 is the optical counterpart of a discrete variable radio and X-ray source^2–7 located in an extended radio-emitting region (W50) which may be a supernova remnant^8–11. The best known attribute of SS433 is the ‘moving’ emission lines in the visible and near IR spectrum¹². The displacements of the emission features have been shown to be due to Doppler shifts^12,13 arising from velocity variations with a period of P∼164 days; velocities up to +50,000 (0.17c) and –35,000 km s^–1are attained by the redshifted and blueshifted components respectively. A favourable configuration seems to involve two highly collimated, oppositely directed near-relativistic streams emerging from the central object. We report here new observations which reveal that the linear polarization of the optical radiation from SS433 exhibits a long-term variation as well as large night-to-night changes. This establishes that a substantial fraction of the observed polarization is intrinsic to SS433 whereas the remainder may be produced in the interstellar medium. As polarization is an indicator of physical and/or geometrical asymmetries in nature, this phenomenon can have a useful role in discriminating between contending configurations for SS433.