The Bursting Behavior of the Transient X-Ray Burst Source EXO 0748-676: A Dependence between the X-Ray Burst Properties and the Strength of the Persistent Emission

Abstract

Between 1985 February 15 and July 19 EXOSAT made eight observations of the transient X-ray source EXO 0748-676. Within this time interval the persistent flux varied by a factor of 5 from a high to a low state and back to a high state. We observed 26 type I bursts, of which 22 were analyzed in detail. As the persistent flux decreased, the burst frequency increased and the burst shape changed from a "fast" to a "slow" profile. The ratio,α, of the total persistent energy before a burst to the total energy emitted in the burst varied between and and was correlated with the strength of the persistent flux. In addition, the apparent blackbody radius during the cooling tail was also correlated with the level of the persistent flux and varied from ∼ 4 to 9 km (for a fixed apparent temperature). At low persistent flux four sets of double bursts were recorded with a burst interval of minutes. The apparent radii of these bursts were identical to their predecessors (∼ 4 km). These variations in burst behavior are discussed in terms of accretion rate dependent changes in the thermonuclear flash process on the surface of a neutron star. We suggest that as the accretion rate decreases, the thermonuclear flash changes character from helium dominated to combined hydrogen/ helium burning.