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.
@article{arxiv.2008.13434,
title = {The cyclotron line energy in Her X-1: stable after the decay},
author = {R. Staubert and L. Ducci and L. Ji and F. Fuerst and J. Wilms and R. E. Rothschild and K. Pottschmidt and M. Brumback and F. Harrison},
journal= {arXiv preprint arXiv:2008.13434},
year = {2020}
}
Comments
13 pages, 17 figures, Astronomy and Astrophysics (in press)