Cosmic-abundance absorption dips in X1755 – 33

Abstract
We report results of spectral analysis for the source X1755 – 33 using the 1984 EXOSAT/ME data, in which spectral fitting was made to the complete 18-h observation by dividing it into 314 consecutive 400-s timebins (adjacent bins overlapping), and fitting the same models to the spectra of all of these. The model consisting of a blackbody plus power law, each component having its own cosmic-abundance absorption term, gave a very good fit to the whole observation. In this model, dipping is seen to be due to increases in NH for the blackbody, and possible smaller increasesin NH for the power law. Thus we are able to explain the dips by increases in NH of cosmic-abundance material. Previously, it was thought that a severe underabundance of metals was required to explain the energy-independence of the dips. In our model, the two spectral components combine in such a way that the net spectrum appears energy-independent, although the dips are caused by absorption. The considerable variability in dips can be explained simply in terms of the two-component model by structure in the outer accretion disc causing fast variations in the photoelectric absorption. Finally, we find that the blackbody emission is consistent with emission from a stellar surface, in which case the source could not be seen as a black hole candidate.