ON THE DISTANCE AND MASS-RADIUS RELATION OF NEUTRON-STARS IN X-RAY BURST SOURCES

We present constraints on the mass-radius relation for the neutron sta r in the X-ray burst source 4U 1820-30 by using model spectra for X-ra y bursts based on the theory developed by Titarchuk 1994. Under the as sumption of isotropic burst emission the derived results exclude the s tiffer equations of state of neutron star matter. The theory can be us ed to derive distances to X-ray burst sources from which only burst lu minosities below the Eddington limit are observed. We apply it to 4U 1 705-44 and find a distance of 7.4(-1.1)(+0.8) kpc, assuming similar ne utron star mass and radius for 4U 1705-44 and 4U 1820-30 and a neutron star atmosphere with cosmic element abundance for 4U 1705-44.