5 Thermonuclear X-ray Bursts
245
time steps and zones as well as an extensive nuclear network, a computational task
which is not yet feasible on present-day hardware. All current simulations reduce
the problem. 2D and 3D models explicitly model turbulent mixing, but typically
simulate only part of a burst with a small selection of nuclear reactions to limit the
computational expense. On the other hand, one-dimensional simulations include an
approximation of turbulence, but this allows them to use an implicit scheme and
calculate series of multiple complete bursts. Moreover, the number of zones in such
a model is small enough to use a full nuclear network. Here we discuss the progress
in recent years for the different categories of models.
5.8.1 Single-Zone Models
The largest simplification of any simulation is the one-zone model, where the
entire neutron star envelope is reduced to a single zone. Time evolution can be
implemented by considering the changes in the the fuel column due to accretion
and nuclear burning. Such simplification is helpful when illustrating the different
regimes of nuclear burning ([48, 72]; see also Fig. 5.2). Single-zone models,
however, miss several important effects from nuclear burning taking place simultaneously at different depths or from the mixing of the fuel and ashes layers. That
behaviour can only be resolved with multi-zone models.
By considering only a single zone, most computational resources can be applied
to large nuclear networks. Such models are used to evaluate the impact on X-ray
burst light curves and burning ashes of uncertainties in a large number of reaction
rates [34, 74, 146]. Whereas some one-zone models include the thermodynamic
response of the neutron atmosphere to the nuclear burning, others rely on the
ignition conditions or even full thermodynamic trajectories from multi-zone models.
The latter is not a good approach for studying X-ray bursts, because changes to
any rate potentially produce a significant change in the temperature, invalidating
the trajectory. Such changes to the trajectory are difficult to predict a priori, as the
nuclear flow may progress through different branches, and a change in one reaction
may cause the nuclear flow to be rerouted through another path. The collective effect
of the reactions along the altered path can significantly affect the ashes composition
and the observed light curve. Therefore, an important aspect of evaluating a nuclear
reaction rate is its impact on changing the thermodynamic trajectory. Given that
multi-zone models can now be run relatively fast on current hardware, the use of
trajectories should be avoided, and the only remaining role for one-zone models
may be in exploratory studies of large parameter spaces, to be followed up with
multi-zone models [34].
5.8.2 One-Dimensional Multi-Zone Models
One-dimensional models resolve the neutron star envelope into multiple zones in
the radial direction, usually on a Lagrangian grid which employs mass coordinates.
245
time steps and zones as well as an extensive nuclear network, a computational task
which is not yet feasible on present-day hardware. All current simulations reduce
the problem. 2D and 3D models explicitly model turbulent mixing, but typically
simulate only part of a burst with a small selection of nuclear reactions to limit the
computational expense. On the other hand, one-dimensional simulations include an
approximation of turbulence, but this allows them to use an implicit scheme and
calculate series of multiple complete bursts. Moreover, the number of zones in such
a model is small enough to use a full nuclear network. Here we discuss the progress
in recent years for the different categories of models.
5.8.1 Single-Zone Models
The largest simplification of any simulation is the one-zone model, where the
entire neutron star envelope is reduced to a single zone. Time evolution can be
implemented by considering the changes in the the fuel column due to accretion
and nuclear burning. Such simplification is helpful when illustrating the different
regimes of nuclear burning ([48, 72]; see also Fig. 5.2). Single-zone models,
however, miss several important effects from nuclear burning taking place simultaneously at different depths or from the mixing of the fuel and ashes layers. That
behaviour can only be resolved with multi-zone models.
By considering only a single zone, most computational resources can be applied
to large nuclear networks. Such models are used to evaluate the impact on X-ray
burst light curves and burning ashes of uncertainties in a large number of reaction
rates [34, 74, 146]. Whereas some one-zone models include the thermodynamic
response of the neutron atmosphere to the nuclear burning, others rely on the
ignition conditions or even full thermodynamic trajectories from multi-zone models.
The latter is not a good approach for studying X-ray bursts, because changes to
any rate potentially produce a significant change in the temperature, invalidating
the trajectory. Such changes to the trajectory are difficult to predict a priori, as the
nuclear flow may progress through different branches, and a change in one reaction
may cause the nuclear flow to be rerouted through another path. The collective effect
of the reactions along the altered path can significantly affect the ashes composition
and the observed light curve. Therefore, an important aspect of evaluating a nuclear
reaction rate is its impact on changing the thermodynamic trajectory. Given that
multi-zone models can now be run relatively fast on current hardware, the use of
trajectories should be avoided, and the only remaining role for one-zone models
may be in exploratory studies of large parameter spaces, to be followed up with
multi-zone models [34].
5.8.2 One-Dimensional Multi-Zone Models
One-dimensional models resolve the neutron star envelope into multiple zones in
the radial direction, usually on a Lagrangian grid which employs mass coordinates.