Inversion of the decay of the cyclotron line energy in Hercules X-1

Creators: Staubert, R.; Klochkov, D.; Fürst, F.; Wilms, J.; Rothschild, R. E.; Harrison, F. A.

Abstract

Recent observations of Her X-1 with NuSTAR and INTEGRAL in 2016 have provided evidence that the 20-year decay of the cyclotron line energy found between 1996 and 2015 has ended and that an inversion with a new increase, possibly similar to the one observed around 1990-1993, has started. We consider this a strong motivation for further observations and for enhanced efforts to significantly improve our theoretical understanding of the accretion process in binary X-ray pulsars. We speculate about the physics behind the long-term decay and its inversion, a possible cyclic behavior, and correlations with other variable observables.

Additional Information

© 2017 ESO. Article published by EDP Sciences. Received 11 September 2017; Accepted 7 October 2017; Published online 27 October 2017. The motivation for this paper are new observational data taken by the NASA satellite NuSTAR and the ESA satellite INTEGRAL. We would like to acknowledge the dedication of all the people who have contributed to the great success of these missions, especially the "schedulers" for their efforts with respect to the nonstandard scheduling of the observations of Her X-1. Earlier important data were provided by the equally successful NASA missions RXTE and Swift. We thank the anonymous referee for useful suggestions.

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