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Five observations of the hard X-ray spectrum of Her X-1 from \xte show thatthe $\sim 41$ keV energy of the cyclotron scattering line is constant withinstatistics of a few percent per observtion. Comparison with other observationsover many years indicates strongly that the centroid energy of this absorptionline has increased some time between 1991 and 1993 by 23%, from 34 keV to 41keV. Moreover, the cutoff energy of the spectral continuum increased at thesame time from 16 keV to 20 keV, which is, within the statistical error of 5%,in direct proportion to the centroid. This may be a sign thatboth thesecharacteristics of the spectrum are controlled in the same way by the magneticfield strength in the region of scattering.Comment: 30 page, 5 figure

Stability of the Cyclotron Resonance Scattering Feature in Hercules X‐1 withRXTE