31 research outputs found
A numerical magnetohydrodynamic scheme using the hydrostatic approximation
In gravitationally stratified fluids, length scales are normally much greater
in the horizontal direction than in the vertical one. When modelling these
fluids it can be advantageous to use the hydrostatic approximation, which
filters out vertically propagating sound waves and thus allows a greater
timestep. We briefly review this approximation, which is commonplace in
atmospheric physics, and compare it to other approximations used in
astrophysics such as Boussinesq and anelastic, finding that it should be the
best approximation to use in context such as radiative stellar zones, compact
objects, stellar or planetary atmospheres and other contexts. We describe a
finite-difference numerical scheme which uses this approximation, which
includes magnetic fields.Comment: 15 pages, 18 figures, accepted for publication by MNRA
Fast and slow magnetic deflagration fronts in Type I X-ray bursts
Type I X-ray bursts are produced by thermonuclear runaways that develop on
accreting neutron stars. Once one location ignites, the flame propagates across
the surface of the star. Flame propagation is fundamental in order to
understand burst properties like rise time and burst oscillations. Previous
work quantified the effects of rotation on the front, showing that the flame
propagates as a deflagration and that the front strongly resembles a hurricane.
However the effect of magnetic fields was not investigated, despite the fact
that magnetic fields strong enough to have an effect on the propagating flame
are expected to be present on many bursters. In this paper we show how the
coupling between fluid layers introduced by an initially vertical magnetic
field plays a decisive role in determining the character of the fronts that are
responsible for the Type I bursts. In particular, on a star spinning at 450 Hz
(typical among the bursters) we test seed magnetic fields of
G and find that for the medium fields the magnetic stresses that develop during
the burst can speed up the velocity of the burning front, bringing the
simulated burst rise time close to the observed values. By contrast, in a
magnetic slow rotator like IGR J17480--2446, spinning at 11 Hz, a seed field
G is required to allow localized ignition and the magnetic field
plays an integral role in generating the burst oscillations observed during the
bursts.Comment: Pubblished on MNRA
Flame Propagation on the Surfaces of Rapidly Rotating Neutron Stars during Type I X-ray Bursts
We present the first vertically resolved hydrodynamic simulations of a
laterally propagating, deflagrating flame in the thin helium ocean of a
rotating accreting neutron star. We use a new hydrodynamics solver tailored to
deal with the large discrepancy in horizontal and vertical length scales
typical of neutron star oceans, and which filters out sound waves that would
otherwise limit our timesteps. We find that the flame moves horizontally with
velocities of order cm s, crossing the ocean in few seconds,
broadly consistent with the rise times of Type I X-ray bursts. We address the
open question of what drives flame propagation, and find that heat is
transported from burning to unburnt fuel by a combination of top-to-bottom
conduction and mixing driven by a baroclinic instability. The speed of the
flame propagation is therefore a sensitive function of the ocean conductivity
and spin: we explore this dependence for an astrophysically relevant range of
parameters and find that in general flame propagation is faster for slower
rotation and higher conductivity.Comment: Accepted for publication by MNRA
Rotational effects in thermonuclear Type I Bursts: equatorial crossing and directionality of flame spreading
In a previous study on thermonuclear (type I) nursts on accreting neutron
stars we addressed and demonstrated the importance of the effects of rotation,
through the Coriolis force, on the propagation of the burning flame. However,
that study only analysed cases of longitudinal propagation, where the Coriolis
force coefficient was constant. In this paper, we study the
effects of rotation on propagation in the meridional (latitudinal) direction,
where the Coriolis force changes from its maximum at the poles to zero at the
equator. We find that the zero Coriolis force at the equator, while affecting
the structure of the flame, does not prevent its propagation from one
hemisphere to another. We also observe structural differences between the flame
propagating towards the equator and that propagating towards the pole, the
second being faster. In the light of the recent discovery of the low spin
frequency of burster IGR~J17480-2446 rotating at 11 Hz (for which Coriolis
effects should be negligible) we also extend our simulations to slow rotation.Comment: Accepted for publication by MNRA
On the dependence of X-ray burst rate on accretion and spin rate
Nuclear burning and its dependence on the mass accretion rate are fundamental
ingredients for describing the complicated observational phenomenology of
neutron stars in binary systems. Motivated by high quality burst rate data
emerging from large statistical studies, we report general calculations
relating bursting rate to mass accretion rate and neutron star rotation
frequency. In this first work we neglect general relativistic effects and
accretion topology, though we discuss where their inclusion should play a role.
The relations we derive are suitable for different burning regimes and provide
a direct link between parameters predicted by theory and what is to be expected
in observations. We illustrate this for analytical relations of different
unstable burning regimes that operate on the surface of an accreting neutron
star. We also use the observed behaviour of burst rate to suggest new
constraints on burning parameters. We are able to provide an explanation for
the long standing problem of the observed decrease of burst rate with
increasing mass accretion that follows naturally from these calculations: when
accretion rate crosses a certain threshold, ignition moves away from its
initially preferential site and this can cause a net reduction of the burst
rate due to the effects of local conditions that set local differences in both
burst rate and stabilization criteria. We show under which conditions this can
happen even if locally the burst rate keeps increasing with accretion.Comment: Accepted for publication on Ap
The jet-disk symbiosis without maximal jets: 1-D hydrodynamical jets revisited
In this work we discuss the recent criticism by Zdziarski of the maximal jet
model derived in Falcke & Biermann (1995). We agree with Zdziarski that in
general a jet's internal energy is not bounded by its rest-mass energy density.
We describe the effects of the mistake on conclusions that have been made using
the maximal jet model and show when a maximal jet is an appropriate assumption.
The maximal jet model was used to derive a 1-D hydrodynamical model of jets in
agnjet, a model that does multiwavelength fitting of quiescent/hard state X-ray
binaries and low-luminosity active galactic nuclei. We correct algebraic
mistakes made in the derivation of the 1-D Euler equation and relax the maximal
jet assumption. We show that the corrections cause minor differences as long as
the jet has a small opening angle and a small terminal Lorentz factor. We find
that the major conclusion from the maximal jet model, the jet-disk symbiosis,
can be generally applied to astrophysical jets. We also show that isothermal
jets are required to match the flat radio spectra seen in low-luminosity X-ray
binaries and active galactic nuclei, in agreement with other works.Comment: 7 pages, accepted by A&
Dynamics of Laterally Propagating Flames in X-ray Bursts. I. Burning Front Structure
We investigate the structure of laterally-propagating flames through the
highly-stratified burning layer in an X-ray burst. Two-dimensional
hydrodynamics simulations of flame propagation are performed through a rotating
plane-parallel atmosphere, exploring the structure of the flame. We discuss the
approximations needed to capture the length and time scales at play in an X-ray
burst and describe the flame acceleration observed. Our studies complement
other multidimensional studies of burning in X-ray bursts.Comment: Submitted to Ap
Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts
Accreting neutron stars (NS) can exhibit high frequency modulations in their
lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The
frequencies can be offset from the spin frequency of the NS by several Hz, and
can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean,
the frequency of which would decrease (in the rotating frame) as the burst
cools, hence explaining the drifts. Most burst oscillations have been observed
during H/He triggered bursts, however there has been one observation of
oscillations during a superburst; hours' long Type I X-ray bursts caused by
unstable carbon burning deeper in the ocean. This paper calculates the
frequency evolution of an oceanic r-mode during a superburst. The rotating
frame frequency varies during the burst from 4-14 Hz, and is sensitive to the
background parameters, in particular the temperature of the ocean and ignition
depth. This calculation is compared to the superburst oscillations observed on
4U-1636-536. The predicted mode frequencies ( 10 Hz) would require a spin
frequency of 592 Hz to match observations; 6 Hz higher than the spin
inferred from an oceanic r-mode model for the H/He triggered burst
oscillations. This model also over-predicts the frequency drift during the
superburst by 90 %.Comment: Accepted for publication in MNRA
The low luminosity behaviour of the 4U 0115+63 Be/X-ray transient
The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in
October 2015. The source did not decay to its quiescent state but settled in a
meta-stable plateau state (a factor ~10 brighter than quiescence) in which its
luminosity slowly decayed. We used XMM-Newton to observe the system during this
phase and we found that its spectrum can be well described using a black-body
model with a small emitting radius. This suggests emission from hot spots on
the surface, which is confirmed by the detection of pulsations. In addition, we
obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our
XMM-Newton one. We found that the source luminosity was significantly higher
and, although the spectrum could be fitted with a black-body model the
temperature was higher and the emitting radius smaller. Several weeks later the
system started a sequence of type-I accretion outbursts. In between those
outbursts, the source was marginally detected with a luminosity consistent with
its quiescent level. We discuss our results in the context of the three
proposed scenarios (accretion down to the magnestospheric boundary, direct
accretion onto neutron star magnetic poles or cooling of the neutron star
crust) to explain the plateau phase.Comment: 8 pages, 4 figures, 2 tables, accepted for publication in MNRA