25 research outputs found
On the nature of radio pulsars with long periods
It is shown that the drift waves near the light cylinder can cause the
modulation of the emission with periods of the order several seconds. These
periods explain the intervals between successive pulses observed in "magnetars"
and radio pulsars with long periods. The model under consideration makes it
possible to calculate the real rotation periods of the host neutron stars. They
are less than 1 sec for the investigated objects. The magnetic fields at the
surface of the neutron star of the order 10^(11)-10^(13) G and equal to the
usual fields for known radio pulsarsComment: 18 pages, 4 figure
Radio Emission Signatures in the Crab Pulsar
Our high time resolution observations of individual pulses from the Crab
pulsar show that both the time and frequency signatures of the interpulse are
distinctly different from those of the main pulse. Main pulses can occasionally
be resolved into short-lived, relatively narrow-band nanoshots. We believe
these nanoshots are produced by soliton collapse in strong plasma turbulence.
Interpulses at centimeter wavelengths are very different. Their dynamic
spectrum contains regular, microsecond-long emission bands. We have detected
these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The
bands cannot easily be explained by any current theory of pulsar radio
emission; we speculate on possible new models.Comment: 26 pages, 10 figures, to appear in Ap
Frequency dependence of pulsar radiation patterns
We report on new results from simultaneous, dual frequency, single pulse
observation of PSR B0329+54 using the Giant Metrewave Radio Telescope. We find
that the longitude separation of subpulses at two different frequencies (238
and 612 MHz) is less than that for the corresponding components in the average
profile. A similar behaviour has been noticed before in a number of pulsars. We
argue that subpulses are emitted within narrow flux tubes of the dipolar field
lines and that the mean pulsar beam has a conal structure. In such a model the
longitudes of profile components are determined by the intersection of the line
of sight trajectory with subpulse-associated emission beams. Thus, we show that
the difference in the frequency dependence of subpulse and profile component
longitudes is a natural property of the conal model of pulsar emission beam. We
support our conclusions by numerical modelling of pulsar emission, using the
known parameters for this pulsar, which produce results that agree very well
with our dual frequency observations.Comment: 24 pages, 8 figures. Accepted for publication in Ap
Transverse quasilinear relaxation in inhomogeneous magnetic field
Transverse quasilinear relaxation of the cyclotron-Cherenkov instability in
the inhomogeneous magnetic field of pulsar magnetospheres is considered. We
find quasilinear states in which the kinetic cyclotron-Cherenkov instability of
a beam propagating through strongly magnetized pair plasma is saturated by the
force arising in the inhomogeneous field due to the conservation of the
adiabatic invariant. The resulting wave intensities generally have nonpower law
frequency dependence, but in a broad frequency range can be well approximated
by the power law with the spectral index -2. The emergent spectra and fluxes
are consistent with the one observed from pulsars.Comment: 14 Pages, 4 Figure
Simultaneous Dual Frequency Observations of Giant Pulses from the Crab Pulsar
Simultaneous measurements of giant pulses from the Crab pulsar were taken at
two widely spaced frequencies using the real-time detection of a giant pulse at
1.4 GHz at the Very Large Array to trigger the observation of that same pulse
at 0.6 GHz at a 25-m telescope in Green Bank, WV. Interstellar dispersion of
the signals provided the necessary time to communicate the trigger across the
country via the Internet. About 70% of the pulses are seen at both 1.4 GHz and
0.6 GHz, implying an emission mechanism bandwidth of at least 0.8 GHz at 1 GHz
for pulse structure on time scales of one to ten microseconds.
The arrival times at both frequencies display a jitter of 100 microseconds
within the window defined by the average main pulse profile and are tightly
correlated. This tight correlation places limits on both the emission mechanism
and on frequency dependent propagation within the magnetosphere.
At 1.4 GHz the giant pulses are resolved into several, closely spaced
components. Simultaneous observations at 1.4 GHz and 4.9 GHz show that the
component splitting is frequency independent. We conclude that the multiplicity
of components is intrinsic to the emission from the pulsar, and reject the
hypothesis that this is the result of multiple imaging as the signal propagates
through the perturbed thermal plasma in the surrounding nebula. At both 1.4 GHz
and 0.6 GHz the pulses are characterized by a fast rise time and an exponential
decay time which are correlated. The pulse broadening with its exponential
decay form is most likely the result of multipath propagation in intervening
ionized gas.Comment: LaTeX, 18 pages, 7 figures, accepted for publication in The
Astrophysical Journa
Formation of a Partially-Screened Inner Acceleration Region in Radio Pulsars: Drifting Subpulses and Thermal X-Ray Emission from Polar Cap Surface
The subpulse drifting phenomenon in pulsar radio emission is considered
within the partially screened inner gap model, in which the
sub-Goldreich-Julian thermionic flow of iron ions or electrons coexists with
the spark-associated electron-positron plasma flow. We derive a simple formula
that relates the thermal X-ray luminosity from the spark-heated
polar cap and the \EB subpulse periodicity (polar cap carousel
time). For PSRs B0943+10 and B1133+16, the only two pulsars for which both
and are known observationally, this formula holds well.
For a few other pulsars, for which only one quantity is measured
observationally, we predict the value of the other quantity and propose
relevant observations that can confirm or discard the model. Then we further
study the detailed physical conditions that allow such partially screened inner
gap to form. By means of the condition (where is the critical temperature above which the surface delivers a thermal flow
to adequately supply the corotation charge density, and is the
actual surface temperature), it is found that a partially-screened gap (PSG)
can be formed given that the near surface magnetic fields are very strong and
curved. We consider both curvature radiation (CR) and resonant inverse Compton
scattering (ICS) to produce seed photons for pair production, and find that the
former is the main agency to produce gamma-rays to discharge PSG
The spark-associated soliton model for pulsar radio emission
We propose a new, self-consistent theory of coherent pulsar radio emission
based on the non-stationary sparking model of Ruderman & Sutherland (1975),
modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these
authors, the polar cap is populated as densely as possible by a number of
sparks with a characteristic perpendicular dimension D approximately equal to
the polar gap height scale h, separated from each other also by about h. Each
spark reappears in approximately the same place on the polar cap for a time
scale much longer than its life-time and delivers to the open magnetosphere a
sequence of electron-positron clouds which flow orderly along a flux tube of
dipolar magnetic field lines. The overlapping of particles with different
momenta from consecutive clouds leads to effective two-stream instability,
which triggers electrostatic Langmuir waves at the altitudes of about 50
stellar radii. The electrostatic oscillations are modulationally unstable and
their nonlinear evolution results in formation of ``bunch-like'' charged
solitons. A characteristic soliton length along magnetic field lines is about
30 cm, so they are capable of emitting coherent curvature radiation at radio
wavelengths. The net soliton charge is about 10^21 fundamental charges,
contained within a volume of about 10^14 cm^3. For a typical pulsar, there are
about 10^5 solitons associated with each of about 25 sparks operating on the
polar cap at any instant. One soliton moving relativisticaly along dipolar
field lines with a Lorentz factor of the order of 100 generates a power of
about 10^21 erg/s by means of curvature radiation. Then the total power of a
typical radio pulsar can be estimated as being about 10^(27-28) erg/s.Comment: 27 pages, 5 figures, accepted by Ap
Vacuum gaps in pulsars and PSR J2144-3933
In this paper we revisit the radio pulsar death line problem within the
framework of curvature radiation and/or inverse compton scattering induced
vacuum gap model above neutron star polar caps. Our special interest is in the
recently detected pulsar PSR J2144-3933 with extremal period 8.5 seconds, which
lies far beyond conventional death lines. We argue, that formation of vacuum
gaps requires a complicated multipolar surface magnetic field, with a strenght
much higher than the surface dipolar component , and radii of
curvature much smaller than the neutron star radius cm.
Such a multipolar surface field is also consistent with death lines including
the extremal pulsar PSR J2144-3933. Since vacuum gap models produce sparks, our
paper naturally supports the spark related models of subpulse drift phenomenon
as well as to the spark associated models of coherent pulsar radio emission.Comment: 19 pages, 1 postscript figure, Latex, uses aastex.st
Modelling of surface magnetic field in neutron stars: application to radio pulsars
We propose a vacuum gap (VG) model which can be applied uniformly for normal
and high magnetic field pulsars. The model requires strong and non-dipolar
surface magnetic field near the pulsar polar cap. We assume that the actual
surface magnetic field in pulsars results from a superposition of global dipole
field and crust-anchored small scale magnetic anomaly. We provide a numerical
formalism for modelling such structures of surface magnetic field and explore
it within the framework of VG model, which requires strong surface fields more
than 10^{13} G.Comment: Submitted to A&A, 11 pages, 9 figure
On the mean profiles of radio pulsars I: Theory of the propagation effects
We study the influence of the propagation effects on the mean profiles of
radio pulsars using the Kravtsov-Orlov method of the wave propagation in the
inhomogeneous media. This approach allows us firstly to include into
consideration the transition from geometrical optics to vacuum propagation, the
cyclotron absorption, and the wave refraction simultaneously. In addition,
arbitrary non-dipole magnetic field configuration, drift motion of plasma
particles, and their realistic energy distribution are taken into account. The
one-to-one correspondence between the signs of circular polarization and
position angle (p.a.) derivative along the profile for both ordinary and
extraordinary waves is predicted. Using the numerical integration we now can
model the main profiles of radio pulsars. It is shown that standard S-shape
form of the p.a. swing can be realized for small enough pair production
multiplicity and large enough bulk plasma Lorentz factor only. It is also shown
that the value of p.a. maximum derivative, that is often used for determination
the angle between magnetic dipole and rotation axis, depends on the plasma
parameters and could differ from the rotation vector model (RVM) prediction.Comment: 20 pages, 16 figures, accepted MNRA