457 research outputs found
On the duration of the subsonic propeller state of neutron stars in wind-fed mass-exchange close binary systems
The condition for the subsonic propeller - accretor state transition of
neutron stars in wind-fed mass-exchange binary systems is discussed. I show
that the value of the break period, at which the neutron star change its state
to accretor, presented by Davies & Pringle (1981) is underestimated by a factor
of 7.5. The correct value is P_{\rm br} = 450 \mu_{30}^{16/21}
\dot{M}_{15}^{-5/7} (M/M_{\sun})^{-4/21} s. This result forced us to reconsider
some basic conclusions on the efficiency of the propeller spindown mechanism.Comment: 3 pages, published in A&A 368, L
Nonadiabatic charged spherical evolution in the postquasistatic approximation
We apply the postquasistatic approximation, an iterative method for the
evolution of self-gravitating spheres of matter, to study the evolution of
dissipative and electrically charged distributions in General Relativity. We
evolve nonadiabatic distributions assuming an equation of state that accounts
for the anisotropy induced by the electric charge. Dissipation is described by
streaming out or diffusion approximations. We match the interior solution, in
noncomoving coordinates, with the Vaidya-Reissner-Nordstr\"om exterior
solution. Two models are considered: i) a Schwarzschild-like shell in the
diffusion limit; ii) a Schwarzschild-like interior in the free streaming limit.
These toy models tell us something about the nature of the dissipative and
electrically charged collapse. Diffusion stabilizes the gravitational collapse
producing a spherical shell whose contraction is halted in a short
characteristic hydrodynamic time. The streaming out radiation provides a more
efficient mechanism for emission of energy, redistributing the electric charge
on the whole sphere, while the distribution collapses indefinitely with a
longer hydrodynamic time scale.Comment: 11 pages, 16 Figures. Accepted for publication in Phys Rev
Disks Surviving the Radiation Pressure of Radio Pulsars
The radiation pressure of a radio pulsar does not necessarily disrupt a
surrounding disk. The position of the inner radius of a thin disk around a
neutron star can be estimated by comparing the electromagnetic energy density
generated by the neutron star with the kinetic energy density of the disk.
Inside the light cylinder, the near zone electromagnetic field is essentially
the dipole magnetic field, and the inner radius is the conventional Alfven
radius. Far outside the light cylinder, in the radiation zone, and the
electromagnetic energy density is where is the
Poynting vector. Shvartsman (1970) argued that a stable equilibrium can not be
found in the radiative zone because the electromagnetic energy density
dominates over the kinetic energy density, with the relative strength of the
electromagnetic stresses increasing with radius. In order to check whether this
is true also near the light cylinder, we employ global electromagnetic field
solutions for rotating oblique magnetic dipoles (Deutsch 1955). Near the light
cylinder the electromagnetic energy density increases steeply enough with
decreasing to balance the kinetic energy density at a stable equilibrium.
The transition from the near zone to the radiation zone is broad. The radiation
pressure of the pulsar can not disrupt the disk for values of the inner radius
up to about twice the light cylinder radius if the rotation axis and the
magnetic axis are orthogonal. This allowed range beyond the light cylinder
extends much further for small inclination angles. We discuss implications of
this result for accretion driven millisecond pulsars and young neutron stars
with fallback disks.Comment: Accepted by Astrophysical Journal, final version with a minor
correctio
On the state of low luminous accreting neutron stars
Observational appearance of a neutron star in the subsonic propeller state
which is a companion of a wind-fed mass-exchange close binary system is
discussed. During the subsonic propeller state the neutron star magnetosphere
is surrounded by a spherical quasi-static plasma envelope, which is extended
from the magnetospheric boundary up to the star accretion radius. The energy
input to the envelope due to the propeller action by the neutron star exceeds
the radiative losses and the plasma temperature in the envelope is of the order
of the free-fall temperature. Under this condition the magnetospheric boundary
is interchange stable. Nevertheless, I find that the rate of plasma penetration
from the envelope into the magnetic field of the neutron star due to diffusion
and magnetic field line reconnection processes is large enough for the
accretion power to dominate the spindown power. I show that the accretion
luminosity of the neutron star in the subsonic propeller state is 5*10**{30} -
10**{33} (dM/dt)_{15} erg/s, where dM/dt is the strength of the normal
companion stellar wind which is parametrized in terms of the maximum possible
mass accretion rate onto the neutron star magnetosphere. On this basis I
suggest that neutron stars in the subsonic propeller state are expected to be
observed as low luminous accretion-powered pulsars. The magnetospheric radius
of the neutron star in this state is determined by the strength of the stellar
wind, (dM/dt)_c, while the accretion luminosity is determined by the rate of
plasma penetration into the star magnetosphere, (dM/dt)_a, which is (dM/dt)_a
<< (dM/dt)_c. That is why the classification of the neutron star state in these
objects using the steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c)
can lead to a mistaken conclusion.Comment: 6 pages, accepted for publication in A&
Mutual Repression enhances the Steepness and Precision of Gene Expression Boundaries
Embryonic development is driven by spatial patterns of gene expression that
determine the fate of each cell in the embryo. While gene expression is often
highly erratic, embryonic development is usually exceedingly precise. In
particular, gene expression boundaries are robust not only against intrinsic
noise from gene expression and protein diffusion, but also against
embryo-to-embryo variations in the morphogen gradients, which provide
positional information to the differentiating cells. How development is robust
against intra- and inter-embryonic variations is not understood. A common motif
in the gene regulation networks that control embryonic development is mutual
repression between pairs of genes. To assess the role of mutual repression in
the robust formation of gene expression patterns, we have performed large-scale
stochastic simulations of a minimal model of two mutually repressing gap genes
in Drosophila, hunchback (hb) and knirps (kni). Our model includes not only
mutual repression between hb and kni, but also the stochastic and cooperative
activation of hb by the anterior morphogen Bicoid (Bcd) and of kni by the
posterior morphogen Caudal (Cad), as well as the diffusion of Hb and Kni. Our
analysis reveals that mutual repression can markedly increase the steepness and
precision of the gap gene expression boundaries. In contrast to spatial
averaging and cooperative gene activation, mutual repression thus allows for
gene-expression boundaries that are both steep and precise. Moreover, mutual
repression dramatically enhances their robustness against embryo-to-embryo
variations in the morphogen levels. Finally, our simulations reveal that gap
protein diffusion plays a critical role not only in reducing the width of gap
gene expression boundaries via spatial averaging, but also in repairing
patterning errors that could arise due to the bistability induced by mutual
repression.Comment: 29 pages, 9 figures, supporting text with 9 supporting figures;
accepted for publication in PLoS Comp. Bio
Superparticle actions from superfields
Gauge invariant complex covariant actions for superparticles are derived from
the field equations for the chiral superfields in a precise manner. The massive
and massless cases in four dimensions are treated both free and in interaction
with an external super Maxwell field. By means of a generalized BRST
quantization these complex actions are related to real actions with second
class constraints which are new in some cases.Comment: 23 pages, ITP-G\"{o}teborg 94-14, LATE
Where Are All The Fallback Disks? Constraints on Propeller Systems
Fallback disks are expected to form around new-born neutron stars following a
supernova explosion. In almost all cases, the disk will pass through a
propeller stage. If the neutron star is spinning rapidly (initial period ms) and has an ordinary magnetic moment ( G cm), the
rotational power transferred to the disk by the magnetic field of the neutron
star will exceed the Eddington limit by many orders of magnitude, and the disk
will be rapidly disrupted. Fallback disks can thus survive only around
slow-born neutron stars and around black holes, assuming the latter do not
torque their surrounding disks as strongly as do neutron stars. This might
explain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter
Sterile neutrinos with eV masses in cosmology -- how disfavoured exactly?
We study cosmological models that contain sterile neutrinos with eV-range
masses as suggested by reactor and short-baseline oscillation data. We confront
these models with both precision cosmological data (probing the CMB decoupling
epoch) and light-element abundances (probing the BBN epoch). In the minimal
LambdaCDM model, such sterile neutrinos are strongly disfavoured by current
data because they contribute too much hot dark matter. However, if the
cosmological framework is extended to include also additional relativistic
degrees of freedom -- beyond the three standard neutrinos and the putative
sterile neutrinos, then the hot dark matter constraint on the sterile states is
considerably relaxed. A further improvement is achieved by allowing a dark
energy equation of state parameter w<-1. While BBN strongly disfavours extra
radiation beyond the assumed eV-mass sterile neutrino, this constraint can be
circumvented by a small nu_e degeneracy. Any model containing eV-mass sterile
neutrinos implies also strong modifications of other cosmological parameters.
Notably, the inferred cold dark matter density can shift up by 20 to 75%
relative to the standard LambdaCDM value.Comment: 14 pages, 6 figures, v2: minor changes, matches version accepted for
publication in JCA
Maximum Mass-Radius Ratios for Charged Compact General Relativistic Objects
Upper limits for the mass-radius ratio and total charge are derived for
stable charged general relativistic matter distributions. For charged compact
objects the mass-radius ratio exceeds the value 4/9 corresponding to neutral
stars. General restrictions for the redshift and total energy (including the
gravitational contribution) are also obtained.Comment: 6 pages, 2 figures, RevTex. To appear in Europhys. Let
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