X-Ray Polarization Measurements of the Crab Nebula and Pulsar and Centaurus X-3 and Hercules X-1 Pulsars.

The Crab pulsar is known to be linearly polarized at radio and optical wavelengths, and the polarization vector rotates as a function of pulse phase. A similar behavior at X-ray energies would suggest a common emission mechanism. The OSO-8 polarimeters were therefore employed to search for pulse dependent X-ray polarization. There is marginal evidence that the behavior of the X-ray polarization position angles is the same as that at optical frequencies. Three-sigma upper limits to the polarization of 13.5 percent and 19 percent at 2.6 keV and 5.2 keV, respectively, were obtained for Cen X-3 when the data were averaged over the pulse and binary periods. The upper limit for Her X-1 at 2.6 keV is 60 percent. At the 90 percent confidence level, the leading edge of the Cen X-3 pulse profile is 9 percent polarized at a position angle of 94 deg; the wing of the pulse is 29 percent polarized at a position angle of 127 deg. Three sigma upper limits to the polarization at 2.6 and 5.2 keV are presented for several pulse phase regions of the light curve.