2016MNRAS.463..185L

We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high-mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High-statistics, intensity-selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between ∼53 and ∼58 keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying the intensity-selected spectra built along four 33-month time intervals along the survey. For the first time, we find in this source a secular variation in the CRSF energy: independent of the source luminosity, the CRSF second harmonic energy decreases by ∼0.36 keV yr^–1 between the first and the third time intervals, corresponding to an apparent decay of the magnetic field of ∼3 x 10¹⁰ G yr^–1. The intensity-cyclotron energy pattern is consistent between the third and the last time intervals. A possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. This hypothesis seems supported by the correspondence between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in Vela X-1 and Her X-1, respectively.