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

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1. Introduction

Her X-1 is an eclipsing X-ray pulsar with a pulse period of 1.24 s and an orbital period of 1.70 days (Tananbaum et al. 1972; Giacconi et al. 1973). The source exhibits a 35 day X-ray intensity cycle consisting of a [FORMULA]10 day duration main on-state between [FORMULA] (using the ephemeris of Scott & Leahy (1999) where [FORMULA] is defined as the turn-on to the main-on state) and a fainter [FORMULA]5 day duration secondary, or short, on-state between [FORMULA] = 0.57-0.79. At other [FORMULA] Her X-1 is still visible as a low-level X-ray source (Jones & Forman 1976). This modulation has been ascribed to a tilted precessing accretion disk (Gerend & Boynton 1976) and an accretion disk corona Schandl & Meyer (1994) that periodically obscure the line of sight to the neutron star. In the warped disk model of Schandl & Meyer (1994) and Schandl (1996), the onset of the main-on state is caused by the edge of the accretion disk moving out of the line of sight to the neutron star and the end of the main-on state is caused by the corona crossing the line of sight. The onset of the main-on state is associated with an increase in photoelectric absorption by cold material, while no such increase is seen during its more gradual decay (e.g., Becker et al. 1977; Parmar et al. 1980). In addition, a regular pattern of narrow X-ray intensity dips are observed which repeat every 1.65 days. These are probably caused by obscuration from cold material in the outer regions of the accretion disk (Crosa & Boynton 1980)

The broad-band on-state X-ray spectrum of Her X-1 is known to be complex and consists of at least the following components: (1) a power-law with a photon index, [FORMULA], of [FORMULA]0.9 in the energy range 2-20 keV. (2) Cyclotron absorption (Trümper et al. 1978; Mihara et al. 1990) at energies [FORMULA]20 keV. (3) A broad Fe emission feature near 6.4 keV (Pravdo et al. 1977; Choi et al. 1994). (4) A blackbody with a temperature, kT, of [FORMULA]0.1 keV (Shulman et al. 1975; Catura & Acton 1975; McCray et al. 1982; Mavromatakis 1993; Vrtilek et al. 1994; Choi et al. 1997; Oosterbroek et al. 1997, hereafter O97) and (5) a broad emission feature between 0.8-1.4 keV, which is probably unresolved Fe L shell emission (McCray et al. 1982; Mihara & Soong 1994; O97).

Pulse-phase spectroscopy using the Einstein Objective Grating (OGS) and Solid State Spectrometers (SSS) in the energy range 0.15-4.5 keV by McCray et al. (1982) revealed that the maximum intensities of the blackbody and power-law components are shifted by 240o during the main-on state. It is likely that the blackbody component results from hard X-rays that are reprocessed in the illuminated inner regions of the accretion disk. McCray et al. (1982) note that the phase of maximum intensity of the unresolved 0.8-1.4 keV feature appears coincident with that of the blackbody. The pulse-phase dependence of the blackbody component was investigated with better quality data by O97 who confirmed the above phase-difference, and established that the Fe-L line and the blackbody exhibit a similar phase-dependent behavior during the main-on state.

In this paper we report on a BeppoSAX observation designed to investigate the nature of the short-on state modulation. An additional short observation was performed during the declining phase of the preceding main-on state for comparison purposes. We concentrate on the 0.1-10 keV spectrum of Her X-1, which contains all the continuum and reprocessed components.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999