6 research outputs found
On the role of the current loss in radio pulsar evolution
The aim of this article is to draw attention to the importance of the
electric current loss in the energy output of radio pulsars. We remind that
even the losses attributed to the magneto-dipole radiation of a pulsar in
vacuum can be written as a result of an Ampere force action of the electric
currens flowing over the neutron star surface (Michel, 1991, Beskin et al.,
1993). It is this force that is responsible for the transfer of angular
momentum of a neutron star to an outgoing magneto-dipole wave. If a pulsar is
surrounded by plasma, and there is no longitudinal current in its
magnetosphere, there is no energy loss (Beskin et al., 1993, Mestel et al.,
1999). It is the longitudinal current closing within the pulsar polar cap that
exerts the retardation torque acting on the neutron star. This torque can be
determined if the structure of longitudinal current is known. Here we remind of
the solution by Beskin, Gurevitch & Istomin (1993) and discuss the validity of
such an assumption. The behavior of the recently observed "part-time job"
pulsar B1931+24 can be naturally explained within the model of current loss
while the magneto-dipole model faces difficulties.Comment: 4 pages, to appear in Astrophysics and Space Science, Special Issue:
Isolated Neutron Stars. In the replaced paper we amended several misprints
(coefficients in equations 12,14,15) and removed the excessive explanation
for the boundary condition (4
Low Complexity Regularization of Linear Inverse Problems
Inverse problems and regularization theory is a central theme in contemporary
signal processing, where the goal is to reconstruct an unknown signal from
partial indirect, and possibly noisy, measurements of it. A now standard method
for recovering the unknown signal is to solve a convex optimization problem
that enforces some prior knowledge about its structure. This has proved
efficient in many problems routinely encountered in imaging sciences,
statistics and machine learning. This chapter delivers a review of recent
advances in the field where the regularization prior promotes solutions
conforming to some notion of simplicity/low-complexity. These priors encompass
as popular examples sparsity and group sparsity (to capture the compressibility
of natural signals and images), total variation and analysis sparsity (to
promote piecewise regularity), and low-rank (as natural extension of sparsity
to matrix-valued data). Our aim is to provide a unified treatment of all these
regularizations under a single umbrella, namely the theory of partial
smoothness. This framework is very general and accommodates all low-complexity
regularizers just mentioned, as well as many others. Partial smoothness turns
out to be the canonical way to encode low-dimensional models that can be linear
spaces or more general smooth manifolds. This review is intended to serve as a
one stop shop toward the understanding of the theoretical properties of the
so-regularized solutions. It covers a large spectrum including: (i) recovery
guarantees and stability to noise, both in terms of -stability and
model (manifold) identification; (ii) sensitivity analysis to perturbations of
the parameters involved (in particular the observations), with applications to
unbiased risk estimation ; (iii) convergence properties of the forward-backward
proximal splitting scheme, that is particularly well suited to solve the
corresponding large-scale regularized optimization problem
Pulsar Striped Winds
According to magnetohydrodynamic (MHD) models, the rotational energy of a
rapidly spinning neutron star is carried away by a relativistic wind and
deposited at a large distance, in the nebula, downstream of the wind
termination shock. The energy transport in the outflow is mediated by Poynting
flux, but it is not clear how the energy stored in the fields is transferred
into the energized population of emitting particles. The most plausible
dissipation mechanisms are thought to be related to the "striped" structure of
the wind, in particular, to the existence of a current sheet, prone to
reconnection events. In this model the current sheet is a natural place for
internal dissipation and acceleration of particles responsible for pulsed,
high-energy emission. Moreover, reconnection is a promising scenario for
explaining annihilation of fields at the shock and conversion of their energy
into the kinetic energy of particles. The shock structure, however, is likely
to differ in the low-density plasmas, in which non-MHD effects intervene. In
this regime, the striped wind can dissipate its energy via an electromagnetic
precursor of the shock.Comment: invited review at the Workshop on Modelling Nebulae, June 14-17,
2016, Sant Cugat, Spain; submitted book chapte