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We summarize the results of a dedicated effort between 2012 and 2019 tofollow the evolution of the cyclotron line in Her~X-1 through repeated NuSTARobservations. The previously observed nearly 20-year long decay of thecyclotron line energy has ended around 2012: from there onward the pulse phaseaveraged flux corrected cyclotron line energy has remained stable and constantat an average value of Ecyc= (37.44+/-0.07) keV (normalized to a flux level of6.8 RXTE/ASM-cts/s). The flux dependence of Ecyc discovered in 2007 is nowmeasured with high precision, giving a slope of (0.675+/-0.075)keV/(ASM-cts/s), corresponding to an increase of 6.5% of Ecyc for an increasein flux by a factor of two. We also find that all line parameters as well asthe continuum parameters show a correlation with X-ray flux. While acorrelation between Ecyc and X-ray flux (both positive and negative) is nowknown for several accreting binaries with various suggestions for theunderlying physics, the phenomenon of a long-term decay has so far only beenseen in Her~X-1 and Vela~X-1, with far less convincing explanations.

The cyclotron line energy in Her X-1: stable after the decay