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Astron. Astrophys. 317, L47-L50 (1997)

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1. Introduction

X 1755-338 is one of the most interesting members of the class of [FORMULA] 10 Low Mass X-ray Binary (LMXB) sources which show periodic dips in X-ray intensity at the orbital period. It is generally accepted that the dips are due to absorption in the bulge in the outer accretion disc caused by the impact of the accretion flow from the companion (White & Swank 1982). The evolution of the X-ray spectra during dips varies considerably between dipping sources and it is not the case that the sources in general show an increase in low energy absorption leading to a hardening of the spectrum as expected for photoelectric absorption in the relatively cool material of the outer disc. In particular cases there may be a hardening or a softening or no change in hardness. X 1755-338 is very unusual in this respect since the high quality Exosat ME spectra showed that within error, the dipping was energy independent in the band 1 - 10 keV (White et al. 1984), because of which it has been called the energy-independent dipper. Various explanations of this effect have been proposed, notably that the metallicity of the absorber is substantially reduced from solar values (White et al. 1984). Other possibilities include absorption in a region closer to the central compact object where there will be stronger photoionization (Frank, King & Lasota 1987), or partial covering of an extended source (Frank & Sztajno 1984). More recently we have suggested an explanation based on a two-component model of the source (Church & Baluciska-Church 1993) in which the total emission consists of a blackbody point source originating in the neighbourhood of the compact object plus an extended power law component probably due to Comptonisation in an Accretion Disc Corona. Dipping is seen to be primarily due to absorption of the blackbody.

[FIGURE]Fig. 1. X-ray light curves for the complete 18 h Rosat observation in two energy bands: 0.5 - 2.0 keV and 0.1 - 0.5 keV with 160 s timebins.

We have found that the same two-component model can also explain the dipping in X 1624-490 in which the spectral evolution is complex, consisting of a hardening in shallow dipping, followed by a softening in deeper dipping (Church & Bauciska-Church 1995a). It can also explain dipping in the very different source XB 1916-053 in which dipping can reach 100% (Church et al. 1997), and we have proposed that the two-component model may explain all of the dipping sources (Church & Bauciska-Church 1995b). It is important to determine which explanation of the energy-independence is correct, and one way to do this is to examine the spectrum at energies both lower and higher than the Exosat ME band. If the two-component model is correct, at photon energies much lower and higher than [FORMULA] for the blackbody (0.9 keV) there should be very little dipping since the blackbody contribution to the spectrum will be negligible.

In the present paper we examine the spectrum of the source in the PSPC band 0.1 - 2.0 keV. We find that the two-component model is a good representation of the spectra. Dipping is less obvious in the PSPC band than in the ME, but it can be seen that it is due to increases of [FORMULA] for the blackbody, and that at the lowest energies below 0.5 keV, the extent of dipping is substantially reduced.

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© European Southern Observatory (ESO) 1997