  Aims. 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 Ecyc
of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012.
Methods. 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 about 40 keV. Results. The historical evolution of the pulse phase
averaged CRSF centroid energy Ecyc since its discovery in 1976 is characterized
by an initial value around 35 keV, an abrupt jump upwards to beyond about 40
keV 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 Ecyc and the X-ray luminosity Lx 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 Ecyc on X-ray
luminosity is still valid with an increase of about 5% in energy for a factor
of two increase in luminosity. A decrease in Ecyc by 4.2 keV 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. Conclusions. 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.
