Discovery of X-ray burst triplets in EXO 0748-676

Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet, T1, T2, and T3. We also detect 14 doublets with similar wait times between the two components of the doublet, D1 and D2. We characterize this behavior to try and obtain a better understanding of bursts with short wait times. We measure the burst peak flux, fluence, wait time and time profile, and study correlations between these parameters and with the persistent flux representing the mass accretion rate. (i) For all bursts with a long wait time, the fluence is tightly correlated with the wait time, whereas burst with short wait times generally have higher fluences than expected from this relationship; (ii) wait times tend to be longer after doublets and triplets; (iii) the time profile of single bursts, S1, and of the first burst in a double or triple burst, D1 and T1, always contains a slow component which is generally absent in the D2, T2 and T3 bursts; (iv) the peak flux is highest for S1, D1 and T1 bursts, but this is still a factor of 7 lower than the highest peak flux ever seen for a burst in this system; (v) the persistent flux, representing the mass accretion rate onto the neutron star, is about 1% of Eddington, which is among the lowest value so far measured for this system. The amount of energy per gram of accreted mass liberated during bursts is consistent with a fuel mixture of hydrogen-rich material. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts. For example the 12 min wait time may be associated with a nuclear beta decay timescale. Appendix is only available in electronic form at http://www.aanda.org