EXOSAT Observations of the 35 Day Cycle of Hercules X-1: Evidence for Neutron Star Precession

Abstract

Her X-1 was observed with Exosat throughout a 35 day cycle at about 4 day sampling intervals. These measurements include, for the first time, an accurate determination of the 1.24 s pulse profiles during the short-on state at the middle of the 35 day cycle. The data show a large change in the pulse profiles between the main and short-on states, suggesting a precession of the magnetic dipole axis of the rotating neutron star. It is proposed that the clock mechanism resides in the free precession of the neutron star, and consequences of this precession with respect to the structure of the inner accretion disk are examined.