Relativistic outflow from two thermonuclear shell flashes on neutron stars

Abstract

We study the exceptionally short (32-43 ms) precursors of two intermediate-duration thermonuclear X-ray bursts observed with the Rossi X-ray Timing Explorer from the neutron stars in 4U 0614+09 and 2S 0918-549. They exhibit photon fluxes that surpass those at the Eddington limit later in the burst by factors of 2.6 to 3.1. We are able to explain both the short duration and the super-Eddington flux by mildly relativistic outflow velocities of 0.1c to 0.3c subsequent to the thermonuclear shell flashes on the neutron stars. These are the highest velocities ever measured from any thermonuclear flash. The precursor rise times are also exceptionally short: about 1 ms. This is inconsistent with predictions for nuclear flames spreading laterally as deflagrations and suggests detonations instead. This is the first time that a detonation is suggested for such a shallow ignition column depth (y ign ≈ 10 10 g cm −2 ). The detonation would possibly require a faster nuclear reaction chain, such as bypassing the α-capture on 12 C with the much faster 12 C(p, γ) 13 N(α, p) 16 O process previously proposed. We confirm the possibility of a detonation, albeit only in the radial direction, through the simulation of the nuclear burning with a large nuclear network and at the appropriate ignition depth, although it remains to be seen whether the Zel'dovich criterion is met. A detonation would also provide the fast flame spreading over the surface of the neutron star to allow for the short rise times. This needs to be supported by future two-dimensional calculations of flame spreading at the relevant column depth. As an alternative to the detonation scenario, we speculate on the possibility that the whole neutron star surface burns almost instantly in the auto-ignition regime. This is motivated by the presence of 150 ms precursors with 30 ms rise times in some superexpansion bursts from 4U 1820-30 at low ignition column depths of ∼10 8 g cm −2 .

Thermonuclear X-ray Bursts

“…Often the radius expansion is particularly strong, producing so-called "superexpansion" where the photospheric radius increases by a factor ∼10 2 . For two intermediate duration bursts, expansion velocities of up to 30% of the speed of light were inferred [84]. In the tail of some intermediate duration bursts strong flux variability is observed (Fig.…”

“…Furthermore, these simulations find that at large depths, the fast helium flame travels as a detonation, such that only a short simulation time is required. Fast flame propagation could be a requirement for explaining exceptionally fast rise times observed in the most powerful helium bursts [84]. Recognizing that most observed bursts ignite at a shallower depth and thus propagate more slowly as a deflagration, a new "low Mach number" code has been developed to make simulations more efficient.…”

Nuclear physics of the outer layers of accreting neutron stars

“…This expands the accessible reactions with protoncaptures, even though no hydrogen was present at ignition. The bypass of 12 C(α, γ) by the faster 12 C(p, γ), has been suggested as the explanation for observed bursts with exceptionally short rise times of ∼1 ms [86,99].…”