Evolution of spectral parameters during a pre-eclipse dip of HER X-1

We report on a pre-eclipse dip of the X-ray binary pulsar Her X-1 observed by the Rossi X-ray Timing Explorer (RXTE) in 1996 July. 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 photoelectric absorption and by Thomson scattering in cold material, plus an iron emission line at 6.7 keV. We present the evolution of the spectral parameters with a temporal resolution of 16 s and show that the varying flux and spectrum can be interpreted solely by a time varying column density. The data do not appear to require non-solar abundances in the absorbing material, although a slight over-abundance of the metals cannot be ruled out. We also find that the lightcurve is characterized by symmetric substructures which can be successfully modeled by Gaussian profiles. The recurrence time of these substructures is on a timescale of a few minutes. Table~2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.