192 research outputs found
Absolute emission altitude of pulsars: PSRs B1839+09, B1916+14 and B2111+46
We study the mean profiles of the multi--component pulsars PSRs B1839+09,
B1916+14 and B2111+46. We estimate the emission height of the core components,
and hence find the absolute emission altitudes corresponding to the conal
components. By fitting Gaussians to the emission components, we determine the
phase location of the component peaks. Our findings indicate that the emission
beams of these pulsars have the nested core--cone structures. Based on the
phase location of the component peaks, we estimate the aberration--retardation
(A/R) phase shifts in the profiles. Due to the A/R phase shift, the peak of the
core component in the intensity profile and the inflection point of the
polarization angle swing are found to be symmetrically shifted in the opposite
directions with respect to the meridional plane in such a way that the core
shifts towards the leading side and the polarization angle inflection point
towards the trailing side. We have been able to locate the phase location of
the meridional plane and to estimate the absolute emission altitude of both the
core and the conal components relative to the neutron star centre, using the
exact expression for the A/R phase shift given by Gangadhara (2005).Comment: 10 pages, 6 figures, Accepted for Publication in A&
Understanding the effects of geometry and rotation on pulsar intensity profiles
We have developed a method to compute the possible distribution of radio
emission regions in a typical pulsar magnetosphere, taking into account the
viewing geometry and rotational effects of the neutron star. Our method can
estimate the emission altitude and the radius of curvature of particle
trajectory as a function of rotation phase for a given inclination angle,
impact angle, spin-period, Lorentz factor, field line constant and the
observation frequency. Further, using curvature radiation as the basic emission
mechanism, we simulate the radio intensity profiles that would be observed from
a given distribution of emission regions, for different values of radio
frequency and Lorentz factor. We show clearly that rotation effects can
introduce significant asymmetries into the observed radio profiles. We
investigate the dependency of profile features on various pulsar parameters. We
find that the radiation from a given ring of field lines can be seen over a
large range of pulse longitudes, originating at different altitudes, with
varying spectral intensity. Preferred heights of emission along discrete sets
of field lines are required to reproduce realistic pulsar profiles, and we
illustrate this for a known pulsar. Finally, we show how our model provides
feasible explanations for the origin of core emission, and also for one-sided
cones which have been observed in some pulsars.Comment: 21 pages, 11 figures, accepted for publication in MNRA
The Transition between Nonorthogonal Polarization Modes in PSR B2016+28 at 1404 MHz
Polarization observations of the radio emission from PSR B2016+28 at 1404 MHz
reveal properties that are consistent with two, very different, interpretations
of the pulsar's viewing geometry. The pulsar's average polarization properties
show a rapid change in position angle (PA) near the pulse center, suggesting
that the observer's sightline nearly intersects the star's magnetic pole. But
single pulse, polarization observations of the pulsar show nearly orthogonal
modes of polarization following relatively flat and parallel PA trajectories
across the pulse, suggesting that the sightline is far from the pole.
Additionally, PA histograms reveal a "modal connecting bridge", of unknown
origin, joining the modal PA trajectories over much of the pulse and following
the rapid PA change shown in the average data. The nonorthogonality of
polarization modes is incorporated in a statistical model of radio polarization
to account for the deviations from mode orthogonality that are observed in the
pulsar. The model is used to interpret the rapid PA change and modal connecting
bridge as a longitudinally-resolved transition between modes of nonorthogonal
polarization. Thus, the modal PA trajectories are argued to reflect the
pulsar's true viewing geometry. This interpretation is consistent with the
pulsar's morphological classification, preserves the Radhakrishnan & Cooke
model of pulsar radio emission, and avoids the complication that the modal
connecting bridge might be produced by some other emission mechanism. The
statistical model's ability to simulate the rich variety of polarization
properties observed in the emission lends additional support to the model's
applicability and its underlying assumption that the polarization modes occur
simultaneously.Comment: Accepted for publication in Ap
Radio Emission by Particles due to Pulsar Spin
We present a relativistic model for the motion of charged particles in
rotating magnetic field lines projected on to a plane perpendicular to the
rotation axis. By making an approximation that the projected field lines are
straight, an analytical expression is obtained for the particle trajectory. The
motive behind developing this model is to elucidate some of the effects of
rotation in pulsar profiles. There is a significant contribution to the
curvature of particle trajectory due to the rotation of pulsar, which is in
addition to the inherent curvature of the field lines. The asymmetry in the
observed pulse shapes can be explained by considering the
aberration-retardation effects. The single sign circular polarization that has
been observed in many pulsars, might be due to the relative orientation of
sight line with respect to the particle trajectory plane.Comment: 19 pages, 6 figues. Submitted to Astronomy and Astrophysic
Dynamics of Charged Particles in the Radio Emission Region of Pulsar Magnetosphere
We consider the classical picture of three dimensional motion of charged
particles in pulsar magnetosphere. We adopt a perturbative method to solve the
equation of motion, and find the trajectory of particles as they move along the
rotating dipolar magnetic field lines. Our aim is to study the influence of
rotation on the pulsar radio emission by considering the constrained motion of
particles along the open dipolar magnetic field lines. We find that the
rotation induces a significant curvature into the particle trajectories. Our
model predicts the intensity on leading side dominates over that of trailing
side. We expect that if there is any curvature induced radio emission from the
region close to the magnetic axis then it must be due to the rotation induced
curvature. Our model predicts the radius--to--frequency mapping (RFM) in the
core emissions.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysics (2007
Radio astronomical polarimetry and phase-coherent matrix convolution
A new phase-coherent technique for the calibration of polarimetric data is
presented. Similar to the one-dimensional form of convolution, data are
multiplied by the response function in the frequency domain. Therefore, the
system response may be corrected with arbitrarily high spectral resolution,
effectively treating the problem of bandwidth depolarization. As well, the
original temporal resolution of the data is retained. The method is therefore
particularly useful in the study of radio pulsars, where high time resolution
and polarization purity are essential requirements of high-precision timing. As
a demonstration of the technique, it is applied to full-polarization baseband
recordings of the nearby millisecond pulsar, PSR J0437-4715.Comment: 8 pages, 4 figures, accepted for publication in Ap
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Centrifugally driven electrostatic instability in extragalactic jets
The stability problem of the rotation induced electrostatic wave in
extragalactic jets is presented. Solving a set of equations describing dynamics
of a relativistic plasma flow of AGN jets, an expression of the instability
rate has been derived and analyzed for typical values of AGNs. The growth rate
was studied versus the wave length and the inclination angle and it has been
found that the instability process is much efficient with respect to the
accretion disk evolution, indicating high efficiency of the instability.Comment: 7 pages, 4 figure
Effect of Rotation on Ferromagnetic Porous Convection with a Thermal Non-Equilibrium Model
The effect of rotation on the onset of thermal convection in a horizontal layer of ferrofluid saturated Brinkman porous medium is investigated in the presence of a uniform vertical magnetic field using a local thermal non-equilibrium (LTNE) model. A two-field model for temperature representing the solid and fluid phases separately is used for energy equation. The condition for the occurrence of stationary and oscillatory convection is obtained analytically. The stability of the system has been analyzed when the magnetic and buoyancy forces are acting together as well as in isolation and the similarities as well as differences between the two are highlighted. In contrast to the non-rotating case, it is shown that decrease in the Darcy number Da and an increase in the ratio of effective viscosity to fluid viscosity Λ is to hasten the onset of stationary convection at high rotation rates and a coupling between these two parameters is identified in destabilizing the system. Asymptotic solutions for both small and large values of scaled interphase heat transfer coefficient H t are presented and compared with those computed numerically. Besides, the influence of magnetic parameters and also parameters representing LTNE on the stability of the system is discussed and the veracity of LTNE model over the LTE model is also analyzed
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