The Broadband Spectrum of MXB 1728−34 Observed byBeppoSAX

Abstract

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 ~2 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 ~5.1 kpc. MXB 1728-34 was in a rather soft state during the BeppoSAX observation. The persistent spectrum is well fitted by a continuum consisting of a soft blackbody emission and a Comptonized spectrum. We interpreted the soft component as the emission from the accretion disk. Taking into account a spectral hardening factor of ~1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin ~ 20 km, where i is the inclination angle. The Comptonized component could originate in a spherical corona, with temperature ~10 keV and optical depth ~5, surrounding the neutron star. A broad Gaussian emission line at ~6.7 keV is observed in the spectrum, probably emitted in the ionized corona or in the inner part of the disk. Another emission line is present at ~1.66 keV. No reflection component is detected with high statistical significance, probably 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 spectrum by the accretion disk, it requires a ionized disk (ξ ~ 280) and a solid angle of ~0.2 (in units of 2π) subtended by the reflector as seen from the corona.