274 research outputs found
Non-Steady State Accretion Disks in X-Ray Novae: Outburst Models for Nova Monocerotis 1975 and Nova Muscae 1991
We fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova
Muscae GS 1991=1124-683) using a time-dependent accretion disk model. The model
is based on a new solution for a diffusion-type equation for the
non-steady-state accretion and describes the evolution of a viscous alpha-disk
in a binary system after the peak of an outburst, when matter in the disk is
totally ionized. The accretion rate in the disk decreases according to a power
law. We derive formulas for the accretion rate and effective temperature of the
disk. The model has three free input parameters: the mass of the central object
M, the turbulence parameter alpha, and the normalization parameter delta t.
Results of the modeling are compared with the observed X-ray and optical B and
V light curves. The resulting estimates for the turbulence parameter
are similar: 0.2-0.4 for A 0620-00 and 0.45-0.65 for GS 1124-683, suggesting a
similar nature for the viscosity in the accretion disks around the compact
objects in these sources. We also derive the distances to these systems as
functions of the masses of their compact objects.Comment: 10 pages, 7 figures; style improve
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
Ultra-High-Energy Cosmic Ray Acceleration by Magnetic Reconnection in Newborn Accretion Induced Collapse Pulsars
We here investigate the possibility that the ultra-high energy cosmic ray
(UHECR) events observed above the GZK limit are mostly protons accelerated in
reconnection sites just above the magnetosphere of newborn millisecond pulsars
which are originated by accretion induced collapse (AIC). We show that
AIC-pulsars with surface magnetic fields G and spin periods , are able to
accelerate particles to energies eV. Because the expected rate
of AIC sources in our Galaxy is very small (\sim 10^{-5} yr^{-1}), the
corresponding contribution to the flux of UHECRs is neglegible, and the total
flux is given by the integrated contribution from AIC sources produced by the
distribution of galaxies located within the distance which is unaffected by the
GZK cutoff ( Mpc). We find that the reconnection efficiency factor
needs to be in order to reproduce the observed flux of
UHECRs.Comment: Latex file, 16 pages, 2 figures, replaced with revised version
accepted for publication in the ApJ letter
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