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

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: staubert@astro.uni-tuebingen.de
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

Received: 6 July 2020
Accepted: 14 August 2020

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.