1620006

We propose to continue the study of the long-term evolution of the cyclotron line energy Ecyc in Her X-1. The long-term decay of Ecyc discovered in data taken between 1993 and 2012 (Staubert et al. 2014) amounts to 5 keV over 20 yrs (Staubert et al. 2016). However the last observing campaign in 2016 coordinated between NuSTAR and INTEGRAL had provided evidence that the decay has ended (Staubert et al. 2017). This was confirmed by further observations until early 2018. It is now the question whether we will (at some time) see a new increase similar tothe one that was observed between 1990 and 1993. Based on the idea that both this increase and the subsequent long-term decay are 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 neutron star Staubert et al. (2016 2017) had speculated that a new increase might happen once Ecyc had reached a low level similar to the one found when the line was first discovered. We therefore propose to continue the monitoring of Her X-1 during AO-16 with two observations of 200 ks each (about half a year apart). [truncated! Please see actual data for full text]