Thermonuclear flash model for long X-ray tails from aquila X-1

The prolonged phase of X-ray emission lasting ∼2500 s after an X-ray burst event in the recurrent transient Aql X-1 is interpreted as an extended phase of hydrogen burning in the accreted envelope of a neutron star. The nuclear burning is accelerated by electron capture processes at the high densities (∼10⁷ g cm^-3) characteristic of the accreted layer. In this model, the extended phase of nuclear burning is due to the fact that the envelope is out of thermal equilibrium. Therefore only the first X-ray burst emitted by Aql X-1 during its transient outburst exhibits a long X-ray tail. In contrast, the properties of the subsequent bursts are distinguished by a lack of an X-ray tail reflecting the much smaller accumulated masses which result from the effects of thermal inertia in the neutron star envelope. The characteristics of the latter bursts are similar to those of typical X-ray bursters. For a neutron star characterized by a mass and radius of 1.4 M_⊙ and 9.1 km, respectively, the occurrence of the long X-ray tail requires that the mass of the accumulated layer ∼10²³ g and the envelope temperatures of the neutron star be less than 1.5 × 10⁷ K. This interpretation is found to be consistent with the thermal relaxation of the neutron star envelope during the quiescent state of Aql X-1 and with the mass accretion rates inferred for the transient outburst itself.