PROGRESSIVE COVERING IN DIPPING AND COMPTONIZATION IN THE SPECTRUM OF XB 1916-053 FROM THE BEPPOSAX OBSERVATION

We report results of a BeppoSAX observation of the low-mass X-ray binary (LMXB) dipping source XB 1916 053. The source joins the small group of LMXB detected at energies >100 keV. The non-dip spectrum is well fitted by an absorbed blackbody with a temperature of 1.620:05 keV and an ab- sorbed cut-off power law with a photon index of 1.610:01 and a cut-off energy of 8010 keV. Below 10 keV, where photo- electric absorption is dominant, the dramatic spectral changes observed during dips can be simply modelled by progressive covering of the blackbody and cut-off power law components. The blackbody component is very rapidly absorbed during dips, consistent with it being point-like, while the cutoff power law is more gradually absorbed, consistent with it being extended. The most likely locations for the blackbody component are the surface of the neutron star or the boundary layer between the neutron star and the accretion disk. The extended emission most probably originates in an accretion disk corona. Above 10 keV, dipping is detected up to 40 keV, and there is some evidence for an energy-independent reduction in intensity of up to 15%. This reduction could be caused by electron scattering or obscu- ration. In the first case, the change is consistent with an electron column density of 2:9 10 23 cm 2 , several times smaller than the average hydrogen column measured simultaneously.