INFRARED PHOTOMETRY AND POLARIMETRY OF CYGNUS-X-3

We present photometry and linear polarimetry of Cygnus X-3 at K (2.2 m um) obtained over a 5 yr period. Photometry and polarimetry at J, H, a nd K of nearby field stars is also presented. From an analysis of thes e data we find: (1) Using the x-ray ephemeris of Kitamoto et al. [ApJ, 384, 263 (1992)], including the first and second derivatives of the p eriod, the leading edge of the decline to minimum in the quiescent K l ight curve has not changed in phase since 1974. The duration of the mi nimum in the light curve has changed significantly between different e pochs, becoming much broader in 1993 than it was previously. (2) In ad dition to an interstellar polarization component, it is likely Cyg X-3 has an intrinsic polarization component that is variable. The variati ons in the polarization do not show any diagnostic pattern with orbita l phase. A crude analysis of the polarization suggests the intrinsic p olarization of Cyg X-3 has a mean position angle of approximately 12-d egrees, nearly the same as the direction of the expanding radio lobes. This is consistent with perpendicular electrons scattering in an equa torial disk that is perpendicular to the lobe axis. (3) The mean posit ion angle for the interstellar polarization in the direction of Cyg X- 3 is 150-degrees: This is nearly perpendicular to the axis of interste llar radio scattering seen in the extended VLBI images. Since the posi tion angle of interstellar polarization is the same as the projected m agnetic field direction, this suggests the interstellar (not circumste llar) scattering must be taking place perpendicular to the interstella r magnetic field lines. (4) Cyg X-3 was observed at K during a flare o n 1992 September 30 with a temporal resolution of 6 s. The flaring had rise and fall times of approximately 50 s with peak intensities up to 80 mJy. The flux between individual flare events never dropped to qui escent levels for the duration of our observations (approximately 2000 s).