Volume 642, October 2020
|Number of page(s)||12|
|Section||Stellar structure and evolution|
|Published online||20 October 2020|
Cyclotron line energy in Hercules X-1: stable after the decay
Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
2 European Space Agency – European Space Astronomy Center (ESA-ESAC), Operations Dpt., Camino Bajo del Castillo, s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Canada, Madrid, Spain
3 Dr. Remeis Sternwarte & Erlangen Center for Astroparticle Physics, Univ. Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
4 Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA
5 NASA-Goddard Spaceflight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
6 Department of Physics and Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
7 Dartmouth College, Department of Physics & Astronomy, 6127 Wilder Laboratory, Hannover, NH 03755, USA
8 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
Accepted: 14 August 2020
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−1). 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−1), 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.
Key words: stars: magnetic field / radiation mechanisms: general / scattering / pulsars: individual: Her X-1 / stars: neutron / X-rays: stars
© ESO 2020
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