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Preprint 03/07

Discovery of a flux-related change of the cyclotron line energy in Her X-1

R. Staubert (1), N.I. Shakura (2), K. Postnov (2), J. Wilms (3), R.E. Rothschild (4), W. Coburn (5), L. Rodina (1), D. Klochkov (1)

(1) Institut für Astronomie und Astrophysik, Universität Tübingen,Sand 1, 72076 Tübingen, Germany
(2) Sternberg Astronomical Institute, 119992, Moscow, Russia
(3) Dr. Remeis Sternwarte, Astronomisches Institut der Universität 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) Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA

To be published in: A&A Letters

Abstract. We present the results of ten years of repeated measurements of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and report the discovery of a positive correlation of the centroid energy of this absorption feature in pulse phase averaged spectra with source luminosity. Our results are based on a uniform analysis of observations by the RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source.The mean centroid energy E_c of the CRSF in pulse phase averaged spectra of Her~X-1 during this time is around 40 keV, with significant variations from one Main-On state to the next. We find that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a factor of 2 in luminosity. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars. Our finding is the first significant measurement of a positive correlation between E_c and luminosity in any X-ray pulsar. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate, which is very consistent with the effect observed in Her X-1. We urge that Her X-1 is regularly monitored further and that other X-ray pulsars are investigated for a similar behaviour.

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): astro-ph/0702490

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