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

TL;DR

Between 2012 and 2019, Her X-1's cyclotron line energy stabilized after a 20-year decay, with a confirmed flux dependence and correlations with X-ray flux, providing new insights into its magnetic field behavior.

Contribution

This study provides the first high-precision measurement confirming the cessation of the long-term decay of the cyclotron line energy in Her X-1 and details its flux dependence.

Findings

Cyclotron line energy stabilized after 2012.

Confirmed flux dependence with a slope of 0.675 keV/(ASM-cts/s).

All line and continuum parameters correlate with X-ray flux.

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.