Does the neutron star in Her X-1 really show free precession?

Abstract

The accreting X-ray pulsar Her X-1 shows two types of long-term variations, both with a period of 35 days: 1) A modulation of the flux with a ten day long Main-On and a 5 d long Short-On, separated by two Off-states, and 2) A systematic variation of the shape of the 1.24 s pulse profile. While there is general consensus that the flux modulation is due to variable shading of the X-ray emitting regions on the surface of the neutron star by the precessing accretion disk, the physical reason for the variation of the pulse profiles had remained controversial. Following the suggestion by Truemper et al. (1986) that free precession of the neutron star may be responsible for the variation of the pulse profiles, we had developed physical models of strong feedback interaction between the neutron star and the accretion disk in order to explain the seemingly identical values for the periods of the two types of variations. In a deep analysis of pulse profiles observed by several different satellites over the last three decades we find now that the clock behind the pulse profile variations shows exactly the same erratic behavior as the turn-on clock, even on short time scales (100 d), suggesting that there may in fact be only one 35 d clock in the system. If this is true, it presents a serious challenge for the idea of free precession of the neutron star and calls for alternative physical models for the variation in pulse shape.