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Context: The magnetic field is a crucial ingredient of neutron stars. Itgoverns the physics of accretion and of the resulting high-energy emission inaccreting pulsars. Studies of the cyclotron resonant scattering features(CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permitdirect measuremets of the field strength. Aims: From an analysis of a number ofpointed observations with different instruments, the energy of CRSF, Ecyc, hasrecently been found to decay in Her X-1, which is one of the best-studiedaccreting pulsars. We present our analysis of a homogeneous and almostuninterrupted monitoring of the line energy with Swift/BAT. Methods: Weanalyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. Thedata were used to measure the CRSF energy averaged over several months.Results: The analysis confirms the long-term decay of the line energy. Thedownward trend is highly significant and consistent with the trend measuredwith the pointed observations: dEcyc/dt ~-0.3 keV per year. Conclusions: Thedecay of Ecyc either indicates a local evolution of the magnetic fieldstructure in the polar regions of the neutron star or a geometricaldisplacement of the line-forming region due to long-term changes in thestructure of the X-ray emitting region. The shortness of the observed timescaleof the decay, -Ecyc/(dEcyc/dt) ~ 100 yr, suggests that trend reversals and/orjumps of the line energy might be observed in the future.

Swift/BAT measurements of the cyclotron line energy decay in the accreting neutron star Her X-1: indication of an evolution of the magnetic field?