Relativistic outflow from two thermonuclear shell flashes on neutron stars

摘要

We study the exceptionally short (32-41 ms) precursors of twointermediate-duration thermonuclear X-ray bursts observed with RXTE from theneutron stars in 4U 0614+09 and 2S 0918-549. They exhibit photon fluxes thatsurpass those at the Eddington limit later in the burst by factors of 2.6 to3.1. We are able to explain both the short duration and the super-Eddingtonflux by mildly relativistic outflow velocities of 0.1$c$ to 0.3$c$ subsequentto the thermonuclear shell flashes on the neutron stars. These are the highestvelocities ever measured from any thermonuclear flash. The precursor rise timesare also exceptionally short: about 1 ms. This is inconsistent with predictionsfor nuclear flames spreading laterally as deflagrations and suggestsdetonations instead. This is the first time that a detonation is suggested forsuch a shallow ignition column depth ($y_{\rm ign}$ = 10$^{10}$ g cm$^{-2}$).The detonation would possibly require a faster nuclear reaction chain, such asbypassing the alpha-capture on $^{12}$C with the much faster$^{12}$C(p,$\gamma$)$^{13}$N($\alpha$,p)$^{16}$O process previously proposed.We confirm the possibility of a detonation, albeit only in the radialdirection, through the simulation of the nuclear burning with a large nuclearnetwork and at the appropriate ignition depth, although it remains to be seenwhether the Zel'dovich criterion is met. A detonation would also provide thefast flame spreading over the surface of the neutron star to allow for theshort rise times. (...) As an alternative to the detonation scenario, wespeculate on the possibility that the whole neutron star surface burns almostinstantly in the auto-ignition regime. This is motivated by the presence of 150ms precursors with 30 ms rise times in some superexpansion bursts from 4U1820-30 at low ignition column depths of ~10$^8$ g cm$^{-2}$.