XMM-Newton EPIC observations of Her X-1

Abstract

We present spin-resolved X-ray data of the neutron star binary Her X-1 taken using the EPIC detectors on XMM-Newton. The data were taken at three distinct epochs through the 35-d precession period. The energy-dependent light curves of this source vary significantly from epoch to epoch. It is known that the relative phasing of the soft (≲1 keV) and hard (≳2 keV) X-rays varies. Here, we find that the phase shift between the soft and hard bands during the main-on state is considerably different from that observed in the past. Further, it continues to change significantly during the other two observations. This suggests that we are observing, for the first time, a substantial and continuous variation in the tilt of the disc, as it is expected if the accretion disc is precessing in the system. Analysis of the spin resolved data confirms that the equivalent width variation of the fluorescence Fe K line at ∼6.4 keV follows that of the soft X-ray emission in the main-on state, thus suggesting a common origin for Fe K line and thermal component. The Fe K line is considerably broader when the source is brightest.