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

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4. Discussion

We previously found that a two-component model provides a good explanation of the energy-independent dipping in the Exosat ME band for X 1755-338, and proposed that this model may be able to explain all of the dipping sources. We now find that this model gives good fits to the Rosat PSPC data and allows the observed features of the PSPC data to be understood.

Firstly there is no significant change in the LECO of the data between non-dip and dip data. The LECO is determined by the power law component and the spectral fitting results show there is no significant change in [FORMULA], This lack of change would be quite unexpected in terms of photoelectric absorption of a single emission component which should show a large change in LECO if the absorber has normal abundances. Thus two emission regions at least have to be present. The two-component model gives a good explanation of the lack of change in the LECO, since the blackbody is responsible for dips but does not determine the spectrum below 0.5 keV.

Secondly, the light curves show a clear decrease of count rate of 10% in the band 0.5 - 2.0 keV; in the lower band 0.1 - 0.5 keV there is little evidence for dipping but a small residual level of dipping of [FORMULA] % may persist. Thus dipping is substantially reduced especially when compared with the typical 20% dipping seen in the Exosat ME band. Spectral fitting shows that dipping is caused primarily by absorption of the blackbody. It is interesting that in this source there appears to be little absorption of the extended power law component. Comparison of the Rosat PSPC results with the previous Exosat ME results shows that the brightness of the source was [FORMULA] 1.7 times less during the Rosat observation.

The blackbody plus power law model was found to be a good fit to TTM data on X 1755-338 by Pan et al. (1995). However they were not able to find a best fit model, and several models fitted equally well including the above model and a multi-temperature disc plus power law. The latter model would allow the source to be a black hole candidate. Thus, the conclusion of Pan et al. that their results strongly support the black hole nature of the source does not seem to be justified.

It is clear that the Rosat PSPC results support the two-component model of the dippers. This model predicts that as dipping is due to absorption of the blackbody it can not extend indefinitely to energies much less than [FORMULA]. This is supported by our results which show for the first time a marked reduction in the extent of dipping at low energies in a LMXB dipping source.

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© European Southern Observatory (ESO) 1997

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