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Preprint 14a/06

Variable cyclotron line in Her X-1

R. Staubert (1), N.I. Shakura (2), K.P. Postnov (2), J. Wilms (3), W. Coburn (4), D. Klochkov (1), L. Rodina (1), M. Kuster (5), R. Rothschild (6)

(1) Institute für Astronomie und Astrophysik, Univ. Tübingen
(2) Sternberg Astronomical Institute, Lomonossov Univ. Moscow
(3) Dr. Remeis-Sternwarte Bamberg, Univ. Erlangen
(4) Space Science Laboratory, Univ. of California, Berkeley
(5) Institut für Kernphysik, Techn. Univ. Darmstadt
(6) Center for Astrophysics and Space Sciences, Univ. of California, San Diego

To be published in: Proceed. 6th INTEGRAL Conf., Moscow, July 2006

Abstract. We present new results on the cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of the binary X-ray pulsar Her X-1 from repeated observations by RXTE between 2000 and 2005, together with earlier results from RXTE and historical data. The centroid line energy Ec in pulse phase averaged spectra shows considerable variation, both in the long-term historical development as well as during the time of the RXTE observations. Using RXTE data only, we find that the cyclotron line energy correlates with the observed maximum flux of the corresponding 35 day Main-On state, as measured by the RXTE/ASM. Contrary to what is observed in some high luminosity transient pulsars, the correlation found here is positive, with a change of ~5% in Ec for a change of a factor of two in X-ray flux. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate.

Key words: X-ray binarie, X-ray pulsars, neutron stars, cyclotron lines

Preprint (81 kb PDF file including figures)

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