26 research outputs found
Simulating the shock dynamics of a neutron star accretion column
Accretion onto a highly-magnetised neutron star runs through a magnetospheric
flow, where the plasma follows the magnetic field lines in the force-free
regime. The flow entering the magnetosphere is accelerated by the gravity of
the star and then abruptly decelerated in a shock located above the surface of
the star. For large enough mass accretion rates, most of the radiation comes
from the radiation-pressure-dominated region below the shock, known as
accretion column. Though the one-dimensional, stationary structure of this flow
has been studied for many years, its global dynamics was hardly ever considered
before. Considering the time-dependent structure of an accretion column allows
us to test the stability of the existing stationary analytic solution, as well
as its possible variability modes, and check the validity of its boundary
conditions. Using a conservative scheme, we perform one-dimensional
time-dependent simulations of an ideal radiative MHD flow inside an aligned
dipolar magnetosphere. Whenever thermal pressure locally exceeds magnetic
pressure, the flow is assumed to lose mass. Position of the shock agrees well
with the theoretical predictions below a limit likely associated with advection
effects: if more than of the released power is advected with the flow,
the analytic solution becomes self-inconsistent, and the column starts leaking
at a finite height. Depending on the geometry, this breakdown may broaden the
column, mass-load the field lines, and produce radiation-driven, mildly
relativistic ejecta. Evolving towards the equilibrium position, the shock front
experiences damped oscillations at a frequency close to the inverse sound
propagation time.Comment: 20 pages, 11 figures; published in "MNRAS
On the appearance of non-local MRI in Keplerian accretion discs
We revisit the modal analysis of small perturbations in Keplerian ideal gas
flows with constant vertical magnetic field leading to magneto-rotational
instability (MRI) using the non-local approach. In the general case, MRI modes
are described by a Schr\"odinger-like differential equation with some effective
potential including 'repulsive' () and 'attractive' () terms and
are quantized. In shallow potentials, there are no stationary 'energy levels'.
In thin Keplerian accretion discs, the perturbation wavelengths
are smaller than the disc semi-thickness only in 'deep'
potential wells. We find that there is a critical magnetic field for the MRI to
develop. The instability arises for magnetic field below this critical value.
In thin accretion discs, at low background Alfv\'en velocity the MRI instability increment is suppressed
compared to the value obtained in the local perturbation analysis,
. We also investigate for the first
time the case of radially variable background magnetic field.Comment: 13 pages, 13 figures, to be submitted. Comments welcome
Super-Eddington accretion discs with advection and outflows around magnetized neutron stars
We present a model for a super-Eddington accretion disc around a magnetized
neutron star taking into account advection of heat and the mass loss by the
wind. The model is semi-analytical and predicts radial profiles of all basic
physical characteristics of the accretion disc. The magnetospheric radius is
found as an eigenvalue of the problem. When the inner disc is in
radiation-pressure-dominated regime but does not reach its local Eddington
limit, advection is mild, and the radius of the magnetosphere depends weakly on
the accretion rate. Once approaching the local Eddington limit, the disc
becomes advection-dominated, and the scaling for the magnetospheric radius with
the mass accretion rate is similar to the classical Alfven relation. Allowing
for the mass loss in a wind leads to an increase of the magnetospheric radius.
Our model may be applied to a large variety of magnetized neutron stars
accreting close to or above their Eddington limits: ultra-luminous X-ray
pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of
our model we discuss the observational properties of NGC 5907~X-1, the
brightest ultra-luminous pulsar known so far, and NGC 300~ULX-1 which is
apparently a Be/X-ray binary experiencing a very bright super-Eddington
outburst.Comment: Submitted to Astronomy and Astrophysics, 15 pages, 16 figure
Synthetic approaches to 2-aryl/hetaryl- and 2-(hetaryl)ylidene derivatives of fluorinated 1,3-benzothiazin-4-ones
A series of 2-hetaryl- and 2-(hetaryl)ylidene substituted 5-fluoro-8-nitro-1,3-benzothiazin-4-ones was synthesized by interaction of 2,6-difluoro-3-nitrobenzoylisothiocyanate with C-nucleophiles. Cyclocondensation of polyfluorobenzoylchlorides with aryl and hetaryl thioamides represents new approach to 1,3-benzothiazin-4-ones. Some compounds proved to be promising for further development of tuberculostatic agents