1520033

We propse to continue the study of the long-termevolution of the cycloron line energy Ecyc in Her X-1. The long-term decay of Ecyc discovered in data takenbetween 1993 and 2012 (Staubert etal. 2014) was confirmed by a precise measurement by NuSTAR in August 2015 firmly estabilishing a decay by ~5 keV over the last 20 yrs (Staubert et al. 2016). However the last observing campaign in 2016 coordinated between NuSTAR and INTEGRAL has provide evidence that the decay has ended in fact hinting at a possible increase similar to the increase observed between 1990 and 1993. Based on the idea that both this increase and the subsequent long-term decay were most likely related to the evolving configuration of the magnetic field at the accretion mounds formed by matter accumulated in the polar regions of the accreting neutron star Staubert 2014 and Staubert et al. 2014 2016 had speculated that a new ncrease might happen once Ecyc had reached a low level similar to the one found whenthe line was first discovered. We therefor propose to continue the monitoring of Her X-1 during AO-15 with two observations 200 kes each (about half a year apart). [truncated! Please see actual data for full text]