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We study the long-term evolution of the centroid energy of cyclotron lines -often referered to as Cyclotron Resonance Scattering Features (CRSF) - in HerX-1, Vela X-1 and Cen X-3, using survey observations of the Burst AlertTelescope (BAT) onboard Swift. We find a significant decrease of thefundamental CRSF energy in Her X-1 and the first harmonic line energy in VelaX-1, since the launch of Swift in 2004 and until 2010 and 2012, respectively.Inboth sources the decreases stopped at some time, with a quite stable centroidenergy thereafter. Unlike in Her X-1 and Vela X-1, the CRSF energy in Cen X-3does not show a long-term decrease. It is observed not to change for at leastthe past 14 years. The long-term variation of the line energy is a direct wayto investigate the magnetic field structure in the polar regions of pulsars.Our results may stimulate the development of theoretical models, especiallyregarding to how the accreted mass accumulates in the accretion mound or howthe magnetic field distorts around the polar cap.

Long-term evolutions of the cyclotron line energies in Her X-1, Vela X-1 and Cen X-3 as observed with Swift/BAT