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

TL;DR

Recent X-ray observations of Her X-1 indicate the end of a 20-year decay in cyclotron line energy and the start of a new increasing trend, prompting further study of accretion physics in binary pulsars.

Contribution

This paper reports the first evidence of the reversal of the long-term decay of cyclotron line energy in Her X-1 based on recent observations.

Findings

Decay of cyclotron line energy has halted and reversed.

Possible cyclic behavior in the accretion process.

Implications for theoretical models of X-ray pulsar accretion.

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.