EXOSAT observations of a long X-ray burst from XB 1905+000.

Abstract

During a 17-hour continuous Exosat observation of the faint X-ray source XB 1905 + 000 a single burst was detected which shows a long decay following a radius expansion-contraction phase near maximum bolometric flux. The best spectral fits for the persistent emission are found when a 10-percent contribution from a cool blackbody component is added to a hard spectrum represented by a power law. During the burst decay the blackbody burst emission evolves clearly into the small cool component seen in the persistent emission. From the relation between the observed color temperature and the bolometric flux, an allowed region in the M-R diagram is delimited for a neutron star with a hydrogen-rich atmosphere, and a distance of 8 + or - 1 kpc is derived for the source. This yields an absolute visual magnitude of + 4.8 for the optical counterpart.