Continued decay in the cyclotron line energy in Hercules X-1

The centroid energy Ecyc of the cyclotron line in the spectrum of the binary X-ray pulsar Her X-1 had been found to decrease with time on a time scale of a few tens of years - surprisingly short astrophysically. This was found for the pulse phase averaged line centroid energy using observational data from various X-ray satellites over the time frame 1996 to 2012, establishing a reduction of about 4 keV. Here we report on the result of a new observation by NuSTAR performed in August 2015. The earlier results are confirmed and strengthened, with respect to both, the dependence of Ecyc on flux (it is still present after 2006) and the dependence on time: the long-term decay continued with the same rate, corresponding to a reduction of about 5 keV in 20 years.
Comment: 7 pages, 5 figures, accepted by Astronomy and Astrophysics (18 March 2016)