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Abteilung Astronomie

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Preprint Kg/98

Her X-1: Spectral parameters during a pre-eclipse dip

B. Stelzer (1) , J. Wilms (2) , R. Staubert (2) , D. E. Gruber (3) , and R. E. Rothschild (3)

(1) Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) Center for Astrophysics and Space Sciences, UCSD, La Jolla, CA, USA

1999, Highlights in X-ray Astronomy, ed. B. Aschenbach, MPE Report, in press

Abstract. We report on the observation of a pre-eclipse dip of Her X-1 observed by the Rossi X-ray Timing Explorer (RXTE) in July 1996. Our analysis reveals that the energy spectra in the 3-18 keV range can be described by the sum of two power-laws, one of which is modified by photo-electric absorption and by Thomson scattering in cold material. An iron emission line at 6.7 keV is superposed on this continuum emission. We model the dip spectrum with a temporal resolution of 16 s to trace the evolution of spectral parameters and find that the variations of flux and spectrum are solely due to varying column density. The lightcurve during the dip is characterised by symmetric substructures with recurrence time of a few minutes that can be successfully modeled by Gaussian line profiles. The periodicity of these features, generally believed to reflect a clumpy absorber, allows to estimate the location of the absorbing material.

Key words: -

Paper (167k gzip'ed Postscript including figures)

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