14,052 research outputs found
How to make a mature accreting magnetar
Several candidates for accreting magnetars have been proposed recently by
different authors. Existence of such systems contradicts the standard magnetic
field decay scenario where a large magnetic field of a neutron star reaches
fewG at ages Myr. Among other sources,
the high mass X-ray binary 4U0114+65 seems to have a strong magnetic field
around G. We develop a new Bayesian estimate for the kinematic age
and demonstrate that 4U0114+65 has kinematic age 2.4-5 Myr ( credential
interval) since the formation of the neutron star. We discuss which conditions
are necessary to explain the potential existence of magnetars in accreting
high-mass binaries with ages about few Myrs and larger. Three necessary
ingredients are: the Hall attractor to prevent rapid decay of dipolar field,
relatively rapid cooling of the crust in order to avoid Ohmic decay due to
phonons, and finally, low values of the parameter to obtain long Ohmic time
scale due to impurities. If age and magnetic field estimates for proposed
accreting magnetars are correct, then these systems set the strongest limit on
the crust impurity for a selected sample of neutron stars and provide evidence
in favour of the Hall attractor.Comment: 8 pages, 3 figures, accepted to MNRAS on September 2
Polarization of the electron and positron produced in combined Coulomb and strong laser fields
The process of production in the superposition of a Coulomb and a
strong laser field is considered. The pair production rate integrated over the
momentum and summed over the spin projections of one of the particles is
derived exactly in the parameters of the laser field and in the Born
approximation with respect to the Coulomb field. The case of a monochromatic
circularly polarized laser field is considered in detail. A very compact
analytical expression of the pair production rate and its dependence on the
polarization of one of the created particles is obtained in the quasiclassical
approximation for the experimentally relevant case of an undercritical laser
field. As a result, the polarization of the created electron (positron) is
derived.Comment: 16 pages, no figure
Electron-Positron Pair Production in Space- or Time-Dependent Electric Fields
Treating the production of electron and positron pairs by a strong electric
field from the vacuum as a quantum tunneling process we derive, in
semiclassical approximation, a general expression for the pair production rate
in a -dependent electric field pointing in the -direction. We also
allow for a smoothly varying magnetic field parallel to . The result is
applied to a confined field for , a
semi-confined field for , and a linearly increasing
field . The boundary effects of the confined fields on
pair-production rates are exhibited. A simple variable change in all formulas
leads to results for electric fields depending on time rather than space.
In addition, we discuss tunneling processes in which empty atomic bound
states are spontaneously filled by negative-energy electrons from the vacuum
under positron emission. In particular, we calculate the rate at which the
atomic levels of a bare nucleus of finite size and large
are filled by spontaneous pair creation.Comment: 33 pages and 9 figures. to appear in Phys. Rev.
Unifying neutron stars: getting to GUNS
The variety of the observational appearance of young isolated neutron stars
must find an explanation in the framework of some unifying approach. Nowadays
it is believed that such scenario must include magnetic field decay, the
possibility of magnetic field emergence on a time scale --
yrs, significant contribution of non-dipolar fields, and appropriate initial
parameter distributions. We present our results on the initial spin period
distribution, and suggest that inconsistences between distributions derived by
different methods for samples with different average ages can uncover field
decay or/and emerging field. We describe a new method to probe the magnetic
field decay in normal pulsars. The method is a modified pulsar current
approach, where we study pulsar flow along the line of increasing
characteristic age for constant field. Our calculations, performed with this
method, can be fitted with an exponential decay for ages in the range -- yrs with a time scale yrs. We
discuss several issues related to the unifying scenario. At first, we note that
the dichotomy, among local thermally emitting neutron stars, between normal
pulsars and the Magnificent Seven remains unexplained. Then we discuss the role
of high-mass X-ray binaries in the unification of neutron star evolution. We
note, that such systems allow to check evolutionary effects on a time scale
longer than what can be probed with normal pulsars alone. We conclude with a
brief discussion of importance of discovering old neutron stars accreting from
the interstellar medium.Comment: 6 pages, submitted to AN, proceedings of the workshop "The Fast and
the Furious: Energetic Phenomena in Isolated Neutron Stars, Pulsar Wind
Nebulae and Supernova Remnants" ESAC, Madrid, Spain 22nd - 24th May 201
Phase transition for the frog model
We study a system of simple random walks on graphs, known as frog model. This
model can be described as follows: There are active and sleeping particles
living on some graph G. Each active particle performs a simple random walk with
discrete time and at each moment it may disappear with probability 1-p. When an
active particle hits a sleeping particle, the latter becomes active. Phase
transition results and asymptotic values for critical parameters are presented
for Z^d and regular trees
On Systematic Design of Protectors for Employing OTS Items
Off-the-shelf (OTS) components are increasingly used in application areas with stringent dependability requirements. Component wrapping is a well known structuring technique used in many areas. We propose a general approach to developing protective wrappers that assist in integrating OTS items with a focus on the overall system dependability. The wrappers are viewed as redundant software used to detect errors or suspicious activity and to execute appropriate recovery when possible; wrapper development is considered as a part of system integration activities. Wrappers are to be rigorously specified and executed at run time as a means of protecting OTS items against faults in the rest of the system, and the system against the OTS item's faults. Possible symptoms of erroneous behaviour to be detected by a protective wrapper and possible actions to be undertaken in response are listed and discussed. The information required for wrapper development is provided by traceability analysis. Possible approaches to implementing “protectors” in the standard current component technologies are briefly outline
Worldline Instantons II: The Fluctuation Prefactor
In a previous paper [1], it was shown that the worldline expression for the
nonperturbative imaginary part of the QED effective action can be approximated
by the contribution of a special closed classical path in Euclidean spacetime,
known as a worldline instanton. Here we extend this formalism to compute also
the prefactor arising from quantum fluctuations about this classical closed
path. We present a direct numerical approach for determining this prefactor,
and we find a simple explicit formula for the prefactor in the cases where the
inhomogeneous electric field is a function of just one spacetime coordinate. We
find excellent agreement between our semiclassical approximation, conventional
WKB, and recent numerical results using numerical worldline loops.Comment: 28 pages, 1 figure; v2 references added, version in PR
Who Needs Credit and Who Gets Credit in Eastern Europe?
Based on survey data covering 8,387 firms in 20 countries we compare credit demand and credit supply for firms in Eastern Europe to those for firms in selected Western European countries.Credit Constraints;Banking sector;Transition economies
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