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arXiv · 2008.13434

The cyclotron line energy in Her X-1: stable after the decay

R. Staubert·L. Ducci·L. Ji·F. Fuerst·J. Wilms·R. E. Rothschild·K. Pottschmidt·M. Brumback·F. Harrison
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Abstract

We summarize the results of a dedicated effort between 2012 and 2019 to follow the evolution of the cyclotron line in Her~X-1 through repeated NuSTAR observations. The previously observed nearly 20-year long decay of the cyclotron line energy has ended around 2012: from there onward the pulse phase averaged flux corrected cyclotron line energy has remained stable and constant at an average value of Ecyc= (37.44+/-0.07) keV (normalized to a flux level of 6.8 RXTE/ASM-cts/s). The flux dependence of Ecyc discovered in 2007 is now measured with high precision, giving a slope of (0.675+/-0.075) keV/(ASM-cts/s), corresponding to an increase of 6.5% of Ecyc for an increase in flux by a factor of two. We also find that all line parameters as well as the continuum parameters show a correlation with X-ray flux. While a correlation between Ecyc and X-ray flux (both positive and negative) is now known for several accreting binaries with various suggestions for the underlying physics, the phenomenon of a long-term decay has so far only been seen in Her~X-1 and Vela~X-1, with far less convincing explanations.

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R. Staubert, L. Ducci, L. Ji, F. Fuerst, J. Wilms, R. E. Rothschild, K. Pottschmidt, M. Brumback, F. Harrison. 2020-08-31. The cyclotron line energy in Her X-1: stable after the decay. https://doi.org/10.1051/0004-6361/202038855

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