313 research outputs found
Three-dimensional numerical simulations of the pulsar magentoshere: Preliminary results
We investigate the three-dimensional structure of the pulsar magnetosphere
through time-dependent numerical simulations of a magnetic dipole that is set
in rotation. We developed our own Eulerian finite difference time domain
numerical solver of force-free electrodynamics and implemented the technique of
non-reflecting and absorbing outer boundaries. This allows us to run our
simulations for many stellar rotations, and thus claim with confidence that we
have reached a steady state. A quasi-stationary corotating pattern is
established, in agreement with previous numerical solutions. We discuss the
prospects of our code for future high-resolution investigations of dissipation,
particle acceleration, and temporal variability.Comment: 9 pages, 4 figures. Astronomy & Astrophysics, in pres
MeV Pulsars: Modeling Spectra and Polarization
A sub-population of energetic rotation-powered pulsars show high fluxes of
pulsed non-thermal hard X-ray emission. While this MeV pulsar population
includes some radio-loud pulsars like the Crab, a significant number have no
detected radio or GeV emission, a mystery since gamma- ray emission is a common
characteristic of pulsars with high spin-down power. Their steeply rising hard
X-ray spectral energy distributions (SEDs) suggest peaks at 0.1 - 1 MeV but
they have not been detected above 200 keV. Several upcoming and planned
telescopes may shed light on the MeV pulsars. The Neutron star Interior
Composition ExploreR (NICER) will observe pulsars in the 0.2 - 12 keV band and
may discover additional MeV pulsars. Planned telescopes, such as All-Sky
Medium-Energy Gamma-Ray Observatory (AMEGO) and e-ASTROGAM, will detect
emission above 0.2 MeV and polarization in the 0.2 - 10 MeV band. We present a
model for the spectrum and polarization of MeV pulsars where the X-ray emission
comes from electron- positron pairs radiating in the outer magnetosphere and
current sheet. This model predicts that the peak of the SED increases with
surface magnetic field strength if the pairs are produced in polar cap
cascades. For small inclination angles, a range of viewing angles can miss both
the radio pulse and the GeV pulse from particles accelerating near the current
sheet. Characterizing the emission and geometry of MeV pulsars can thus provide
clues to the source of pairs and acceleration in the magnetosphere.Comment: 8 pages, 5 figures, published in Proceedings of Scienc
Axisymmetric force-free magnetosphere of a pulsar. I. The structure close to the magnetic axis
The stationary axisymmetric force-free magnetosphere of a pulsar is studied
analytically. The pulsar equation is solved in the region close to the magnetic
axis. Proceeding from linearization of the current function in the axial
region, we find the axial magnetic flux function valid at any altitude above
the neutron star. This function is used as a starting approximation to develop
series for the non-linear pulsar equation in the polar region. Taking into
account the quasi-monopolar character of the pulsar magnetic flux at infinity,
we obtain unique asymptotic series for the flux and current functions. At
infinity, both functions are close but not equivalent to those known for the
case of a force-free monopole. The flux function at the top of the polar gap is
found to differ from the dipolar one at the neutron star surface. With our
results, the transverse current sheet closing the pulsar circuit at the neutron
star surface is consistently incorporated into the global magnetospheric
structure, the backward particle flow at small polar angles can be excluded and
the stationary cascade scenario looks admissible. The present paper is the
first step toward complete analytic description of the pulsar force-free
magnetosphere allowing for the plasma-producing gaps and pulsar current circuit
closure.Comment: 8 pages, 2 figures; accepted for publication in MNRA
Effective power-law dependence of Lyapunov exponents on the central mass in galaxies
Using both numerical and analytical approaches, we demonstrate the existence
of an effective power-law relation between the mean Lyapunov
exponent of stellar orbits chaotically scattered by a supermassive black
hole in the center of a galaxy and the mass parameter , i.e. ratio of the
mass of the black hole over the mass of the galaxy. The exponent is found
numerically to obtain values in the range --. We propose a
theoretical interpretation of these exponents, based on estimates of local
`stretching numbers', i.e. local Lyapunov exponents at successive transits of
the orbits through the black hole's sphere of influence. We thus predict
with --. Our basic model refers to elliptical
galaxy models with a central core. However, we find numerically that an
effective power law scaling of with holds also in models with central
cusp, beyond a mass scale up to which chaos is dominated by the influence of
the cusp itself. We finally show numerically that an analogous law exists also
in disc galaxies with rotating bars. In the latter case, chaotic scattering by
the black hole affects mainly populations of thick tube-like orbits surrounding
some low-order branches of the family of periodic orbits, as well as its
bifurcations at low-order resonances, mainly the Inner Lindbland resonance and
the 4/1 resonance. Implications of the correlations between and to
determining the rate of secular evolution of galaxies are discussed.Comment: 27 pages, 19 figure
Gamma-Ray Emission in Dissipative Pulsar Magnetospheres: From Theory to Fermi Observations
We compute the patterns of -ray emission due to curvature radiation
in dissipative pulsar magnetospheres. Our ultimate goal is to construct
macrophysical models that are able to reproduce the observed -ray
light-curve phenomenology recently published in the Second Fermi Pulsar
Catalog. We apply specific forms of Ohm's law on the open field lines using a
broad range for the macroscopic conductivity values that result in solutions
ranging, from near-vacuum to near Force-Free. Using these solutions, we
generate model -ray light curves by calculating realistic trajectories
and Lorentz factors of radiating particles, under the influence of both the
accelerating electric fields and curvature radiation-reaction. We further
constrain our models using the observed dependence of the phase-lags between
the radio and -ray emission on the -ray peak-separation. We
perform a statistical comparison of our model radio-lag vs peak-separation
diagram and the one obtained for the Fermi standard pulsars. We find that for
models of uniform conductivity over the entire open magnetic field line region,
agreement with observations favors higher values of this parameter. We find,
however, significant improvement in fitting the data with models that employ a
hybrid form of conductivity; specifically, infinite conductivity interior to
the light-cylinder and high but finite conductivity on the outside. In these
models the -ray emission is produced in regions near the equatorial
current sheet but modulated by the local physical properties. These models have
radio-lags near the observed values and statistically best reproduce the
observed light-curve phenomenology. Additionally, these models produce GeV
photon cut-off energies.Comment: Accepted for publication in ApJ (revised version, 26 pages, 23
figures
- …