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Astron. Astrophys. 353, 575-582 (2000)

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4. Discussion

We report on a long (5.7 day duration) observation of most of the short-on state of Her X-1 with the BeppoSAX LECS and MECS together with a short observation during the declining phase of the preceding main-on state. We find that the main-on and early short-on state spectra can be well fit with the standard blackbody and power-law continuum model, together with Fe-L and Fe-K emission features. During the later phases of the short-on there is evidence for intervals of strong low-energy absorption. These intervals repeat each 1.7 day orbital cycle, becoming increasingly long and occurring earlier in each orbital cycle as the short-on state progresses. This behavior appears to be consistent with the predictions of the warped disk model of e.g., Schandl & Meyer (1994) and Schandl (1996) where the end of the short-on state is caused by the accretion disk moving into the line of sight to the neutron star. These results therefore support the idea that the Her X-1 system contains a warped, precessing, accretion disk.

The shape and intensity of the S4 spectrum is very similar to that measured by Mihara et al. (1991) during the low-state of Her X-1 by Ginga . They model the spectrum with "scattered" and absorbed ([FORMULA] = [FORMULA] atom cm-2) components and a power-law index equivalent to that found in the main-on state. In addition, their best-fit Fe-K line energy, FWHM and EW of [FORMULA] keV, [FORMULA] keV, and [FORMULA] keV, respectively are all strikingly similar to those reported here for interval S4 (Table 1). These BeppoSAX observations therefore support the idea, first proposed by Mihara et al. (1991), that the low-state Her X-1 emission contains at least two components, one of which is significantly absorbed.

The pulse profiles (broad and almost sinusoidal) in both the main- and short-on states are consistent with those reported by Deeter at al. (1998) obtained with Ginga at similar [FORMULA]. Deeter et al. (1998) show that the pulse shape during the main-on state changes rapidly from a clearly peaked shape to a much broader, more sinusoidal, shape during the final stages of the main-on state. The best-fit pulse periods demonstrate that Her X-1 is continuing its recent spin-up trend as shown in Bildsten et al. (1997).

The pulse phase difference of the maxima of the power-law and blackbody during the short- and main-on state observations is consistent with being the same, and consistent with the results of O97, obtained earlier in the main-on state. Such a symmetry is expected for a reprocessing region located at, or near, the inner edge of the accretion disk since at [FORMULA] and 0.5 the same behavior is predicted. This is because of the symmetrical disk shape expected unless the region of the disk where the reprocessing occurs is strongly warped (see e.g. Heemskerk & van Paradijs 1989for this "symmetry-rule"). This somewhat strengthens the association of the blackbody component with reprocessed emission originating from the inner edge of the accretion disk. Ideally, it would be preferable to track the phase difference between the components over the entire 35 day cycle. However Her X-1 is only visible as a strong X-ray source for two intervals separated by about half the 35 day cycle, giving rise to a similar phase dependence of the components. This, combined with the large pulse profile changes, make it difficult to reliably restrict the location of the reprocessing site.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999