The broad-band X-ray spectrum of the dipping low mass X-ray binary EXO 0748-676

We present results of a 0.1-100 keV BeppoSAX observation of the dipping low-mass X-ray binary (LMXRB) EXO 0748-676 performed in 2000 November. During the observation EXO 0748-676 exhibited X-ray eclipses, type I X-ray bursts and dipping activity over a wide range of orbital phases. The 0.1-100 keV ``dip-free''(i.e. dipping and eclipsing intervals excluded) spectrum is complex, especially at low energies where a soft excess is present. Two very different spectral models give satisfactory fits. The first is the progressive covering model, consisting of separately absorbed black body and cut-off power law components. This model is often used to study the dipping activity in LMXRB. The second model is an absorbed cut-off power law together with a moderately ionized absorber with a sub-solar abundance of Fe and a 2.13 keV absorption feature (tentatively identified with Si XIII). This ionized absorber may be the same feature as seen by Chandra during dips from EXO 0748-676. The fact that these two physically very different models both give acceptable fits to the data and plausible values of the best fit parameters indicates how difficult it is to reliably model such complex dipping spectra with moderate spectral resolution data. The blackbody component required by the progressive covering LMXRB model probably has a more complex underlying nature, due to the strong effects of ionized absorption present.