Attenuation of dipping at low energies in the LMXB source X 1755-338

Abstract

We report spectral fitting results for Rosat PSPC observations of the dipping source X 1755-338. These results are consistent with the two-component model that we previously proposed consisting of a blackbody point source plus an extended power law component. Remarkably, the low energy cut-off of the spectrum does not change appreciably in dips, and it can be seen that dipping takes place in the higher part of the PSPC band where the blackbody contributes to the spectrum. Thus dipping consists primarily of absorption of this component, whereas the low energy cut-off is determined by the power law. Thus, above 0.5 keV, the dipping is approximately independent of energy as seen in Exosat and 10 +/- 0.4% deep. Below 0.5 keV, there may be a small residual dipping effect of up to 3%; however dipping is certainly substantially reduced compared with the 20% seen by Exosat, and this is the first time that the effective cessation of dipping at low energies in such sources has been seen.