2014A&A...572A.119S

We investigate the long-term evolution of the cyclotron resonance scattering feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true long-term decrease in the centroid energy E_cyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of ∼40keV; these are RXTE, INTEGRAL, Suzaku, and NuSTAR. We consider results based on our own successful observing proposals as well as results from the literature. The historical evolution of the pulse phase averaged CRSF centroid energy E_cyc since its discovery in 1976 is characterized by an initial value around 35keV, an abrupt jump upwards to beyond ∼40keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between E_cyc and the X-ray luminosity L_x discovered in 2007. In observations after 2006, however, we now find a statistically significant true decrease in the cyclotron line energy. At the same time, the dependence of E_cyc on X-ray luminosity is still valid with an increase of ∼5% in energy for a factor of two increase in luminosity. We also report on the first evidence of a weak dependence of E_cyc on phase of the 35d precessional period, which manifests itself not only in the modulation of the X-ray flux, but also in the systematic variation in the shape of the 1.24s pulse profile. One of our motivations for repeatedly observing Her X-1, namely the suspicion that the cyclotron line energy may be gradually decreasing after its strong upward jump in the early 1990s, is finally confirmed. A decrease in E_cyc by 4.2keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. The observed timescale for the decrease in E_cyc of a few decades is too short for a decay of the global magnetic field. We speculate that the physical reason could be connected to a geometric displacement of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion. In the second scenario, the upward jump in E_cyc observed around 1991 may have been due to a relatively fast event in which the polar magnetic field rearranged itself after releasing part of the accumulated material to larger areas of the neutron star surface.