EXOSAT Observations of Double-peaked Bursts with Radius Expansion from 4U/MXB 1820-30

A detailed analysis is presented of seven X-ray bursts from the X-ray source 4U/MXB 1820-30 in the globular cluster NGC 6624. The bursts were double-peaked, and recurred at almost equal time intervals of 3.2 hr. The persistent flux varied by only 10 percent on time scales of 60 minutes. The ratio of the total persistent energy emitted before a burst to the total energy released by a burst varied between 125 and 155. The inferred blackbody radius for the burst spectra showed large variations, increasing from 10 to 200 km within the first 0.25-1.5 s, accompanied by a decrease in spectral temperature from 1.2 to 0.4 keV. The inferred source radius during burst decay was constant at about 10 km. This radius is interpreted as the size of the neutron star. During the radius expansion phase, the luminosity remained constant at a mean value of 2.5 x 10 to the 38th ergs/s, which is consistent with the helium Eddington limit for a 10 km, 1.4 solar mass neutron star.