UV polarimetry of the x‐ray binary systems 4U1700‐37, Vela XR‐1, and Cygnus XR‐1

Variable linear polarization has previously been observed in the visible Tapia, 1984), Vela XR‐1 (Dolan & Tapia, 1988), and Cyg XR‐1 (Kemp et al., 1976, 1978). We present the first linear polarization observations of these systems in the ultraviolet (UV), using the High Speed Photometer (HSP) aboard the Hubble Space Telescope. The UV polarimetric light curves obtained from Vela XR‐1 and 4U1700‐37 display an added component of polarization at first quadrature inconsistent with the standard model of Brown et al. (1978) for scattering in binary systems. However, the UV polarimetric light curves toward Cyg XR‐1 are well fit by the standard model. Variations attributable to changes in scattering structure from one orbit to the next are also detected in two of the systems. Possible origins of the additional polarization detected at first quadrature in Vela XR‐1 and 4U1700‐37 are evaluated.