108 research outputs found
Understanding the radio emission geometry of PSR B0329+54
We have analyzed high-quality single pulse data of PSR B0329+54 at 325 MHz
and 606 MHz to study the structure of the emission beam. Using the
window-threshold technique, which is suitable for detecting weak emission
components, we have detected 4 additional emission components in the pulse
window. Three of these are new components and the fourth is a confirmation of a
recently proposed component. Hence PSR B0329+54 is now known to have 9 emission
components - the highest among all known pulsars. The distribution of the pulse
components around the central core component indicates that the emission beam
consists of four nested cones. The asymmetry in the location of the conal
components in the leading versus trailing parts of the profile is interpreted
as being due to aberration and retardation in the pulsar magnetosphere. These
measurements allow us to determine the precise location of the 4 conal rings of
emission. We find that the successive outer cones are emitted at higher
altitudes in the magnetosphere. Further, for any given cone, the emission
height at the lower frequency is found to be more than that at the higher
frequency. The inferred heights range from ~160 km to ~1150 km. The set of
``active'' field lines, from which most of the conal radiation appears to
originate, are found to be confined to a region located within ~0.5 to ~0.6 of
the polar cap radius. We discuss the implications of our new findings on our
understanding of the pulsar emission geometry and its impact on the emission
mechanisms.Comment: 20 pages, 5 figures. Accepted for Astrophysical Journa
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
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&
Relativistic model on pulsar radio emission and polarization
We have developed a relativistic model for pulsar radio emission and
polarization by taking into account of detailed geometry of emission region,
rotation and modulation. The sparks activity on the polar cap leads to plasma
columns in the emission region and modulated emission. By considering
relativistic plasma bunches streaming out along the rotating dipolar field
lines as source of curvature radiation, deduced the polarization state of the
radiation field in terms of the Stokes parameters. We have simulated a set of
typical pulse profiles, and analyzed the role of viewing geometry, rotation and
modulation on the pulsar polarization profiles. Our simulations explain most of
the diverse behaviors of polarization generally found in pulsar radio profiles.
We show that both the `antisymmetric' and `symmetric' types of circular
polarization are possible within the frame work of curvature radiation. We also
show that the `kinky' nature in the polarization position angle traverses might
be due to the rotation and modulation effects. The phase lag of polarization
position angle inflection point relative to the phase of core peak also depends
up on the rotationally induced asymmetry in the curvature of source trajectory
and modulation.Comment: Accepted for the publication in Ap
Influence of Rotation on Pulsar Radiation Characteristics
We present a relativistic model for pulsar radio emission by including the
effect of rotation on coherent curvature radiation by bunches. We find that
rotation broadens the width of leading component compared to the width of
trailing component. We estimate the component widths in the average pulse
profiles of about 24 pulsars, and find that 19 of them have a broader leading
component. We explain this difference in the component widths by using the
nested cone emission geometry.
We estimate the effect of pulsar spin on the Stokes parameters, and find that
the inclination between the rotation and magnetic axes can introduce an
asymmetry in the circular polarization of the conal components. We analyze the
single pulse polarization data of PSR B0329+54 at 606 MHz, and find that in its
conal components, one sense of circular polarization dominates in the leading
component while the other sense dominates in the trailing component. Our
simulation shows that changing the sign of the impact parameter changes the
sense of circular polarization as well as the swing of polarization angle.Comment: 20 pages, 4 Postscript figures, uses aastex.cls. Accepted for
Publication in ApJ 200
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