THE MASS AND THE RADIUS OF THE NEUTRON STAR IN THE TRANSIENT LOW-MASS X-RAY BINARY SAX J1748.9-2021

Abstract

We use time-resolved spectroscopy of thermonuclear X-ray bursts observed from SAX J1748.9-2021 to infer the mass and the radius of the neutron star in the binary. Four X-ray bursts observed from the source with the Rossi X-ray Timing Explorer enable us to measure the angular size and the Eddington limit on the neutron star surface. Combined with a distance measurement to the globular cluster NGC 6440, in which SAX J1748.9-2021 resides, we obtain two solutions for the neutron star radius and mass, R = 8.18 +/- 1.62 km and M = 1.78 +/- 0.3 M-circle dot or R = 10.93 +/- 2.09 km and M = 1.33 +/- 0.33 M-circle dot.