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

Staubert, R. and Ducci, L. and Ji, L. and Fürst, F. and Wilms, J. and Rothschild, R. E. and Pottschmidt, K. and Brumback, M. and Harrison, F. (2020) Cyclotron line energy in Hercules X-1: stable after the decay. Astronomy and Astrophysics, 642 . Art. No. A196. ISSN 0004-6361.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Staubert, R.0000-0003-1498-1543
Fürst, F.0000-0003-0388-0560
Wilms, J.0000-0003-2065-5410
Pottschmidt, K.0000-0002-4656-6881
Brumback, M.0000-0002-4024-6967
Harrison, F.0000-0003-2992-8024
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.
Group:Astronomy Department, Space Radiation Laboratory, NuSTAR
Subject Keywords:stars: magnetic field / radiation mechanisms: general / scattering / pulsars: individual: Her X-1 / stars: neutron / X-rays: stars
Record Number:CaltechAUTHORS:20201021-160147836
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Official Citation:Cyclotron line energy in Hercules X-1: stable after the decay R. Staubert, L. Ducci, L. Ji, F. Fürst, J. Wilms, R. E. Rothschild, K. Pottschmidt, M. Brumback and F. Harrison A&A, 642 (2020) A196 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106199
Deposited By: George Porter
Deposited On:22 Oct 2020 14:48
Last Modified:22 Oct 2020 14:48

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