Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

Preprint 10/14

Secular change in the cyclotron line energy in Hercules X-1

R. Staubert (1), D. Klochkov (1), J. Wilms (2), K. Postnov (3), N.I. Shakura (3), R.E. Rothschild (4), F. Fürst (5), F.A. Harrison (5)

(1) Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
(2) Dr. Remeis Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
(3) Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, 119992, Moscow, Russia
(4) Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA
(5) Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

To be published in: A&A

Abstract. Aims. We investigate the long-term evolution of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true secular decrease in the centroid energy Ecyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Methods. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of about 40 keV.
Results. The historical evolution of the pulse phase averaged CRSF centroid energy Ecyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond about 40 keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between Ecyc and the X-ray luminosity Lx discovered in 2007. In observations after 2006, however, we now find a statistically significant true secular decrease in the cyclotron line energy. At the same time, the dependence of Ecyc on X-ray luminosity is still valid with an increase of about 5% in energy for a factor of two increase in luminosity. A decrease in Ecyc by 4.2 keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. Conclusions. We speculate that the physical reason could be connected to a geometric displace ment of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion.
 
Key words: magnetic fields, neutron stars, - radiation mechanisms, cyclotron scattering features - accretion, accretion columns - binaries: eclipsing - stars: Her X-1 - X-rays: general - X-rays: stars

Astrophysics (astro-ph): 1410.3647

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