Multiple X-ray bursts and the model of a spreading layer of accreting matter over the neutron star surface

We report the detection of series of close type I X-ray bursts consisting of two or three events with a recurrence time much shorter than the characteristic (at the observed mean accretion rate) time of matter accumulation needed for a thermonuclear explosion to be initiated on the neutron star surface during the JEM-X/INTEGRAL observations of several X-ray bursters. We show that such series of bursts are naturally explained in the model of a spreading layer of accreting matter over the neutron star surface in the case of a sufficiently high (Ṁ ≳ 1 × 10−9M⊙ yr−1) accretion rate (corresponding to a mean luminosity Ltot ≳ 1 × 1037erg s−1). The existence of triple bursts requires some refinement of the model—the importance of a central ring zone is shown. In the standard model of a spreading layer no infall of matter in this zone is believed to occur.