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Hercules X-1 - another 'first': long-term decay of the cyclotron line energy

Her~X-1 is one of the most remarkable members of the class of binary X-ray pulsars. It does not only show a large number of observable features, but has repeatedly been the first object for which fundamental discoveries were made: it was the first to show a super-orbital modulation, the first to reveal a cyclotron line in its spectrum and the first in which systematic variations of the cyclotron line energy were detected, namely variations with pulse phase (by $\sim$ 25\%) and a positive correlation with X-ray luminosity ($\sim$ 5\% increase for a factor of two increase in luminosity). Now we have found another 'first': a long-term decrease of the pulse phase averaged cyclotron line energy E$_{\rm cyc}$ by $\sim$ 5\,keV in 20 years. At the time of the discovery of the cyclotron line in 1976, its energy was $\sim$ 35\,keV, remeasured around a similar energy by various instruments until 1990. Between 1990 and 1994 a jump upwards beyond 41\,keV occurred, we are now back at $\sim$ 37\,keV With respect to the physical cause of the discovered effect, we suggest it to be connected to a geometric displacement of the cyclotron resonant scattering region in the polar magnetic field or due to to a true physical change in the field configuration at the accretion mound or column by the continued accretion. The variation with time might be due to a non-perfect equilibrium between the rate at which material is accreted and the rate at which material is lost at the base of the accretion mound, allowing for a variation of the configuration of the accretion mound (height, total mass, field distribution). We do believe that we see the signature of a local change in the field configuration, rather than a decay of the global magnetic field, since the observed timescale of a few decades is very short.