Cyclotron line energy in Hercules X-1: stable after the decay
Abstract
We summarize the results of a dedicated effort made 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 in 2012: from then on, the pulse-phase-averaged flux-corrected cyclotron line energy has remained stable and constant at an average value of E_(cyc) = (37.44 ± 0.07) keV (normalized to a flux level of 6.8 RXTE/ASM-cts s⁻¹). The flux dependence of E_(cyc) discovered in 2007 is now measured with high precision, giving a slope of (0.675 ± 0.075) keV/(ASM-cts s⁻¹), corresponding to an increase of 6.5% of E_(cyc) 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 E_(cyc) 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.
Additional Information
© ESO 2020. Received: 6 July 2020. Accepted: 14 August 2020. We would like to acknowledge the dedication of all the people who have contributed to the great success of all relevant missions, in particular NuSTAR. We especially thank the "schedulers", foremost Karl Forster, for his efforts with respect to the non-standard scheduling of the observations of Her X-1. We acknowledge the historical contributions to the subject at hand by Dmitry Klochkov. We thank the anonymous referee for valuable comments.Attached Files
Published - aa38855-20.pdf
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Additional details
- Eprint ID
- 106199
- Resolver ID
- CaltechAUTHORS:20201021-160147836
- Created
-
2020-10-22Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Astronomy Department, Space Radiation Laboratory, NuSTAR