Super-Eddington Winds from Type I X-Ray Bursts

Abstract

We present hydrodynamic simulations of spherically symmetric super-Eddington winds from radiusexpansion type I X-ray bursts. Previous studies assumed a steady-state wind and treated the massloss rate as a free parameter. Using MESA, we follow the multi-zone time-dependent burning, the convective and radiative heating of the atmosphere during the burst rise, and the launch and evolution of the optically thick radiation-driven wind as the photosphere expands outward to radii r ph 100 km. We focus on neutron stars (NSs) accreting pure helium and study bursts over a range of ignition depths. We find that the wind ejects ≈ 0.2% of the accreted layer, nearly independent of ignition depth. This implies that ≈ 30% of the nuclear energy release is used to unbind matter from the NS surface. We show that ashes of nuclear burning are ejected in the wind and dominate the wind composition for bursts that ignite at column depths 10 9 g cm −2 . The ejecta are composed primarily of elements with mass numbers A > 40, which we find should imprint photoionization edges on the burst spectra. Evidence of heavy-element edges has been reported in the spectra of strong, radius-expansion bursts. We find that after ≈ 1 s the wind composition transitions from mostly light elements ( 4 He and 12 C), which sit at the top of the atmosphere, to mostly heavy elements (A > 40), which sit deeper down. This may explain why the photospheric radii of all superexpansion bursts show a transition after ≈ 1 s from a superexpansion (r ph > 10 3 km) to a moderate expansion (r ph ∼ 50 km).

NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds

“…Alternatively, the apparent consistent energy shift between burst pairs 1 and 2 of all three spectral features seems to argue for a common origin, and so we briefly comment on possible origins for the 1 keV emission line in the burst-driven wind as well. As noted by Yu & Weinberg (2018) in their hydrodynamical simulations, the base of the wind is initially at relatively low column depth and so light (unburned) elements are launched first into the wind. As the burst progresses the base of the wind moves to greater column depths, eventually ejecting the burned ashes.…”

“…Another likely culprit to induce blue-shifts is the wind velocity itself. Wind velocity estimates from Yu & Weinberg (2018) suggest that the velocity is always < 0.01c, which would limit a wind-induced line ratio, s w < 1.01. Such estimates typically ignore radiative effects such as line-driving of the wind; if these are not significant, it would also appear that not all of the observed shift can be attributed to the wind.…”

“…A plausible physical interpretation for the absorption lines is that they are formed in the super-Eddington wind generated by the PRE bursts. Yu & Weinberg (2018) present hydrodynamical simulations of such winds that include modern nuclear reaction networks to follow the time-dependent burning, and composition. They find that the convective burning mixes the heavy element ashes of helium burning to sufficiently low column depths such that the ashes are eventually ejected in the wind.…”

“…in't Zand & Weinberg (2010) reported evidence from RXTE/PCA data for edges during so-called "superexpansion" bursts from 4U 0614+091, 4U 1722−30, and 4U 1820−30. These are bursts with powerful photospheric radius expansion (PRE) phases, that very likely drive outflows (in't Zand et al 2012;Yu & Weinberg 2018). The edge energies were in the 5 -12 keV range, and were detected after the strong, initial expansion phase.…”

Thermal convection in rotating spherical shells: Temperature-dependent internal heat generation using the example of triple- α burning in neutron stars

“…This means that the fluid can be Schwarzschild unstable but Ledoux stable to velocity perturbations, referred to as semiconvection in previous studies [29,38,74]. These studies (and also [44,76]) have built up the current picture of the role played by convection in the thermal evolution of a burst. Burning is mainly localised at the base of the accreted layer where temperature and density are larger.…”