The Polarization of X-Ray Binaries in the Ultraviolet. I. Cygnus XR-1, 4U 0900-40, and 4U 1700-37

Abstract

Cyg XR-1, 4U 0900 - 40, and 4U 1700 - 37 were monitored polarimetrically and photometrically in the ultraviolet (UV) with the High Speed Photometer (HSP) on the Hubble Space Telescope (HST) at 6-10 different phases during a single binary orbit. Polarimetric observations were obtained in a bandpass centered at 2770 Å; 4U 0900-40 was also monitored polarimetrically in a bandpass centered at 3270 Å. Photometric observations were obtained in a bandpass centered at 2480 Å. The UV polarization is variable in all three systems, consistent with a previously discovered intrinsic component of polarization attributed to scattering of light from the supergiant primary by a gas stream between the primary and the neutron star (4U 0900-40 and 4U 1700-37) or black hole (Cyg XR-1) secondary. The amplitude of the polarimetric variability is much larger in the UV than in the visible, confirming a prediction of the model that identifies Rayleigh scattering as the principal mechanism producing the intrinsic polarization. The polarimetric variability as a function of phase fits the standard scattering model of Brown, McLean, & Emslie only if an additional component of polarization occurs near first quadrature in the two systems with neutron star secondaries.