2015A&A...578A..88K

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 measurements of the field strength. 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, 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. 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. 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: dE_cyc/dt~-0.3keV per year. 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∼100yr, suggests that trend reversals and/or jumps of the line energy might be observed in the future.