The broadband spectrum of MXB 1728-34 observed by BeppoSAX

We report on the results of a broadband (0.1-100 keV) spectral analysis of the bursting atoll source MXB 1728-34 (4U 1728-34) observed by the BeppoSAX satellite. Three bursts were present during this observation. The spectrum during the bursts can be fitted by a blackbody with a temperature of simil ar to2 keV. The radius of the blackbody emitting region is compatible with the radius of the neutron star if we correct for the difference between the observed color temperature and the effective temperature. From the bursts we also estimate a distance to the source of similar to5.1 kpc. MXB 1728--3 4 was in a rather soft state during the BeppoSAX observation. The persisten t spectrum is well fitted by a continuum consisting of a soft blackbody emi ssion and a Comptonized spectrum. We interpreted the soft component as the emission from the accretion disk. Taking into account a spectral hardening factor of similar to1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk r adius is R(in)root cosi similar to 20 km, where i is the inclination angle. The Comptonized component could originate in a spherical corona, with temp erature similar to 10 keV and optical depth similar to5, surrounding the ne utron star. A broad Gaussian emission line at similar to6.7 keV is observed in the spectrum, probably emitted in the ionized corona or in the inner pa rt of the disk. Another emission line is present at similar to1.66 keV. No reflection component is detected with high statistical significance, probab ly because of the low temperature of the corona in this state of the source . If the iron emission line is caused by reflection of the Comptonized spec trum by the accretion disk, it requires a ionized disk (xi similar to 280) and a solid angle of similar to 0.2 (in units of 2 pi) subtended by the ref lector as seen from the corona.