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 , E _ { cyc } , has recently been found to decay in Her X-1 \xspace , 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 \xspace .

Methods : We analyzed the archival Swift /BAT \xspace observations of Her X-1 \xspace 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 : { d } E _ { cyc } / { d } t \sim - 0.3 keV per year .

Conclusions : The decay of E _ { cyc } 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 , - E _ { cyc } / \dot { E } _ { cyc } \sim 100 yr , suggests that trend reversals and/or jumps of the line energy might be observed in the future .