1320027

We propose to study the long-term evolution of the CRSF centroid energy in three persistentaccreting pulsars showing a well measurable cyclotron line and for which a considerable record of previousCRSF measurements exists: Her X-1 1H 1538522 and Cen X-3. Such a long-term CRSF evolution is a newlydiscovered phenomenon which has so far only been observed in Her X-1 with an indication of an evolution alsopresent in 1H 1538522. The variability is most probably related to the evolving configuration of the magneticfield and of the mounds formed by matter accumulated in the polar regions of the accreting neutron star.The proposed measurements of the CRSF energy in 1H 1538522 and Cen X-3 will allow us to verify and toconstrain its long-term trend in these sources. In case of Her X-1 the new measurements will allow us to followthe on-going decay of the CRSF energy and to search for jumps in its behavior. Such jumps are most probablyassociated with catastrophic re-adjustments of the accretion mound back to the unperturbed configuration. [truncated! Please see actual data for full text]