111 research outputs found
Low-level accretion in neutron-star X-ray binaries
We search the literature for reports on the spectral properties of
neutron-star low-mass X-ray binaries when they have accretion luminosities
between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon
index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV
range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the
spectrum softens). Such behaviour has been reported before for individual
sources, but here we demonstrate that very likely most (if not all)
neutron-star systems behave in a similar manner and possibly even follow a
universal relation. When comparing the neutron-star systems with black-hole
systems, it is clear that most black-hole binaries have significantly harder
spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data
points, there are indications that these spectral differences also extend to
the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star
binaries and black-hole ones suggests that the spectral properties (between
0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the
nature of the accretor in unclassified X-ray binaries. We discuss our results
in the context of properties of the accretion flow at low luminosities and we
suggest that the observed spectral differences likely arise from the
neutron-star surface becoming dominantly visible in the X-ray spectra. We also
suggest that both the thermal component and the non-thermal component might be
caused by low-level accretion onto the neutron-star surface for luminosities
below a few times 1E34 erg/s.Comment: Accepted for publication in MNRA
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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 timescales 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
A study of radial self-similar non-relativistic MHD outflow models: parameter space exploration and application to the water fountain W43A
Outflows, spanning a wide range of dynamical properties and spatial extensions, have now been associated with a variety of accreting astrophysical objects, from supermassive black holes at the core of active galaxies to young stellar objects. The role of such outflows is key to the evolution of the system that generates them, for they extract a fraction of the orbiting material and angular momentum from the region close to the central object and release them in the surroundings. The details of the launching mechanism and their impact on the environment are fundamental to understand the evolution of individual sources and the similarities between different types of outflow-launching systems. We solve semi-analytically the non-relativistic, ideal, magnetohydrodynamics equations describing outflows launched from a rotating disc threaded with magnetic fields using our new numerical scheme. We present here a parameter study of a large sample of new solutions. We study the different combinations of forces that lead to a successfully launched jet and discuss their global properties. We show how these solutions can be applied to the outflow of the water fountain W43A for which we have observational constraints on magnetic field, density and velocity of the flow at the location of two symmetrical water maser emitting regions
X-ray burst ignition location on the surface of accreting X-ray pulsars: Can bursts preferentially ignite at the hotspot?
Hotspots on the surface of accreting neutron stars have been directly
observed via pulsations in the lightcurves of X-ray pulsars. They are thought
to occur due to magnetic channelling of the accreted fuel to the neutron star
magnetic poles. Some X-ray pulsars exhibit burst oscillations during Type I
thermonuclear X-ray bursts which are thought to be caused by asymmetries in the
burning. In rapidly rotating neutron stars, it has been shown that the lower
gravity at the equator can lead to preferential ignition of X-ray bursts at
this location. These models, however, do not include the effect of accretion
hotspots at the neutron star surface. There are two accreting neutron star
sources in which burst oscillations have been observed to track exactly the
neutron star spin period. We analyse whether this could be due to the X-ray
bursts igniting at the magnetic pole of the neutron star, because of heating in
the accreted layers under the hotspot causing ignition conditions to be reached
earlier. We investigate heat transport in the accreted layers using a 2D model
and study the prevalence of heating down to the ignition depth of X-ray bursts
for different hotspot temperatures and sizes. We perform calculations for
accretion at the pole and at the equator, and infer that ignition could occur
away from the equator at the magnetic pole for hotspots with temperatures
greater than K. However, current observations have not identified
such high temperatures in accreting X-ray pulsars.Comment: 13 pages, 8 figures, accepted for publication in MNRA
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