  Context: The magnetic field is a crucial ingredient of neutron stars. It
governs the physics of accretion and of the resulting high-energy emission in
accreting pulsars. Studies of the cyclotron resonant scattering features
(CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit
direct measuremets of the field strength. Aims: From an analysis of a number of
pointed observations with different instruments, the energy of CRSF, Ecyc, has
recently been found to decay in Her X-1, which is one of the best-studied
accreting pulsars. We present our analysis of a homogeneous and almost
uninterrupted monitoring of the line energy with Swift/BAT. Methods: We
analyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. The
data were used to measure the CRSF energy averaged over several months.
Results: The analysis confirms the long-term decay of the line energy. The
downward trend is highly significant and consistent with the trend measured
with the pointed observations: dEcyc/dt ~-0.3 keV per year. Conclusions: The
decay of Ecyc either indicates a local evolution of the magnetic field
structure in the polar regions of the neutron star or a geometrical
displacement of the line-forming region due to long-term changes in the
structure of the X-ray emitting region. The shortness of the observed timescale
of the decay, -Ecyc/(dEcyc/dt) ~ 100 yr, suggests that trend reversals and/or
jumps of the line energy might be observed in the future.
