The effect of rotation on the stability of nuclear burning in accreting neutron stars

Abstract

Hydrogen and/or helium accreted by a neutron star from a binary companion may undergo thermonuclear fusion. Different burning regimes are discerned at different mass accretion rates. Theoretical models predict helium fusion to proceed as a thermonuclear runaway for accretion rates below the Eddington limit and as stable burning above this limit. Observations, however, place the boundary close to 10% of the Eddington limit. We study the effect of rotationally induced transport processes on the stability of helium burning. For the first time, detailed calculations of thin-shell helium burning on neutron stars are performed using a hydrodynamic stellar evolution code including rotation and rotationally induced magnetic fields. We find that in most cases the instabilities from the magnetic field provide the dominant contribution to the chemical mixing, while Eddington-Sweet circulations become important at high rotation rates. As helium is diffused to greater depths, the stability of the burning is increased, such that the critical accretion rate for stable helium burning is found to be lower. Combined with a higher heat flux from the crust, as suggested by recent studies, turbulent mixing could explain the observed critical accretion rate. Furthermore, close to this boundary we find oscillatory burning, which previous studies have linked to mHz QPOs. In models where we continuously lower the heat flux from the crust, the period of the oscillations increases by up to several tens of percents, similar to the observed frequency drift, suggesting that this drift could be caused by the cooling of deeper layers.

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

“…For example, mHz quasi-periodic oscillations (QPOs) resulting from marginally-stable thermonuclear burning are observed at an inferred mass-accretion rate that is a factor ≃10 lower than the theoretical predictions (e.g., Revnivtsev et al 2001;Altamirano et al 2008;Keek et al 2009;Linares et al 2012). This can be reconciled if there is an additional heat flux coming from the crust.…”

On the Dependence of the X-Ray Burst Rate on Accretion and Spin Rate

“…The authors found that the mixing was more effective for slowly rotating stars. Keek et al (2009) performed more sophisticated, yet still 1D, numerical simulations showing that mixing could also be important for fast spins. They also found that mixing due to other, purely hydrodynamical effects could be important for high-enough spins.…”

“…Even at low accretion rates, the burning rate and the energy release may be above the instability threshold and the bursts begin; then, the frequency of the bursts increases with accretion rate. At the same time, accretion releases heat that eventually stabilizes the burning, preventing any bursting (Fujimoto et al 1981;Bildsten & Brown 1997;Bildsten 1998;Cumming & Bildsten 2000;Keek et al 2009;Zamfir et al 2014). The amount of heat generation from accretion comes from the gravitational energy released at the moment of accretion, the compressional heat due to the extra weight of the accumulated material, and the heat of further reactions that take place deeper than the burning layer (Cumming & Bildsten 2000).…”

“…The amount of heat generation from accretion comes from the gravitational energy released at the moment of accretion, the compressional heat due to the extra weight of the accumulated material, and the heat of further reactions that take place deeper than the burning layer (Cumming & Bildsten 2000). However, many details of the burst physics are still uncertain, mainly the reaction rates (e.g., Schatz et al 2001;Cooper & Narayan 2006a, 2006bHeger et al 2007b;Cyburt et al 2010;Davids et al 2011;Keek et al 2014;Cyburt et al 2016), or, for example, the role of mixing (e.g., Piro & Bildsten 2007;Keek et al 2009). …”

“…Furthermore, rotation affects the burning by inducing mixing of newly accreted material and ashes from previous bursts in deeper layers. It also has indirect effects since the mixing changes the temperature profile of the layer (Piro & Bildsten 2007;Keek et al 2009). Once again, the exact dependence of the bursting frequency on the accretion rate and spin frequency is still not wellunderstood.…”