A peculiar type - I X - ray burst from GRS 1747 - 312

We report the serendipitous detection with the Rossi X-ray Timing Explorer of a long and peculiar X-ray burst whose localization is consistent with one known X-ray burster (GRS 1747-312) and which occurred when that source was otherwise quiescent. The peculiar feature concerns a strong radius expansion of the neutron star photosphere, which occurred not within a few seconds from the start of the burst, as is standard in radiusexpansion bursts, but 20 s later. This suggests that two different layers of the neutron star may have undergone thermonuclear runaways: a hydrogen-rich and a hydrogen-poor layer. The reason for the delay may be related to the source being otherwise quiescent.