Cyclotron line energy in Hercules X-1: stable after the decay

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https://doi.org/10.1051/0004-6361/202038855
 http://hdl.handle.net/11603/19767

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UMBC Physics Department
UMBC Center for Space Sciences and Technology (CSST) / Center for Research and Exploration in Space Sciences & Technology II (CRSST II)
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UMBC Joint Center for Earth Systems Technology (JCET)

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https://orcid.org/0000-0002-4656-6881

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2020-10-20

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journal articles
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Citation of Original Publication

Staubert, R., “Cyclotron line energy in Hercules X-1: stable after the decay”, Astronomy and Astrophysics, vol. 642, 2020. doi:10.1051/0004-6361/202038855.

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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 Ecyc = (37.44 ± 0.07) keV (normalized to a flux level of 6.8 RXTE/ASM-cts/s). The flux dependence on 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.