19 research outputs found
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
Corona of Magnetars
We develop a theoretical model that explains the formation of hot coronae
around strongly magnetized neutron stars -- magnetars. The starquakes of a
magnetar shear its external magnetic field, which becomes non-potential and is
threaded by an electric current. Once twisted, the magnetosphere cannot untwist
immediately because of its self-induction. The self-induction electric field
lifts particles from the stellar surface, accelerates them, and initiates
avalanches of pair creation in the magnetosphere. The created plasma corona
maintains the electric current demanded by curl(B) and regulates the
self-induction e.m.f. by screening. This corona persists in dynamic
equilibrium: it is continually lost to the stellar surface on the
light-crossing time of 10^{-4} s and replenished with new particles. In
essence, the twisted magnetosphere acts as an accelerator that converts the
toroidal field energy to particle kinetic energy. Using a direct numerical
experiment, we show that the corona self-organizes quickly (on a millisecond
timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic
lines. The heating rate of the corona is ~10^{36} erg/s, in agreement with the
observed persistent, high-energy output of magnetars. We deduce that a static
twist that is suddenly implanted into the magnetosphere will decay on a
timescale of 1-10 yrs. The particles accelerated in the corona impact the solid
crust, knock out protons, and regulate the column density of the hydrostatic
atmosphere of the star. The transition layer between the atmosphere and the
corona is the likely source of the observed 100-keV emission from magnetars.
The corona emits curvature radiation and can supply the observed IR-optical
luminosity. (Abridged)Comment: 70 pages, 14 figures, accepted to Ap
Transient Crossing of Phantom divide line under Gauss-Bonnet interaction
Smooth double crossing of the phantom barrier has been
found possible in cosmological model with Gauss-Bonnet-scalar interaction, in
the presence of background cold dark matter. Such crossing has been observed to
be a sufficiently late time phenomena and independent of the sign of
Gauss-Bonnet-scalar interaction. The luminosity distance versus redshift curve
shows a perfect fit with the model up to .Comment: 9 pages, 9 figure
Scientific Prospects for Hard X-ray Polarimetry
X-ray polarimetry promises to give qualitatively new information about
high-energy sources. Examples of interesting source classes are binary black
hole systems, rotation and accretion powered neutron stars, Microquasars,
Active Galactic Nuclei and Gamma-Ray Bursts. Furthermore, X-ray polarimetry
affords the possibility for testing fundamental physics, e.g. to observe
signatures of light bending in the strong gravitational field of a black hole,
to detect third order Quantum Electrodynamic effects in the magnetosphere of
Magnetars, and to perform sensitive tests of Lorentz Invariance. In this paper
we discuss scientific drivers of hard (>10 keV) X-ray polarimetry emphasizing
how observations in the hard band can complement observations at lower energies
(0.1 - 10 keV). Subsequently, we describe four different technical realizations
of hard X-ray polarimeters suitable for small to medium sized space borne
missions, and study their performance in the signal-dominated case based on
Monte Carlo simulations. We end with confronting the instrument requirements
for accomplishing the science goals with the capabilities of the four
polarimeters.Comment: Accepted for publication in Astroparticle Physic