687 research outputs found
Pulsar braking and the P-Pdot diagram
The location of radio pulsars in the period-period derivative (P-Pdot) plane
has been a key diagnostic tool since the early days of pulsar astronomy. Of
particular importance is how pulsars evolve through the P-Pdot diagram with
time. Here we show that the decay of the inclination angle (alpha-dot) between
the magnetic and rotation axes plays a critical role. In particular, alpha-dot
strongly impacts on the braking torque, an effect which has been largely
ignored in previous work. We carry out simulations which include a negative
alpha-dot term, and show that it is possible to reproduce the observational
P-Pdot diagram without the need for either pulsars with long birth periods or
magnetic field decay. Our best model indicates a birth rate of 1 radio pulsar
per century and a total Galactic population of ~20000 pulsars beaming towards
Earth.Comment: 9 pages, 5 figures, accepted by MNRA
Goonhilly Sparklers
Flux monitoring of compact radio quasars has revealed dramatic radio-wave
lensing events which challenge our understanding of the interstellar medium.
However, the data on these events remain very sparse. Here we consider how the
Goonhilly radio astronomical facility can make an impact on this problem by
dedicating one or more dishes to flux monitoring for a period of one year. Such
an experiment would be able to identify \sim6 new events and study them in
detail.Comment: Appearing in the proceedings of "Astronomy with Megastructures"
meeting in Crete, 201
Phase-resolved Faraday rotation in pulsars
We have detected significant Rotation Measure variations for 9 bright
pulsars, as a function of pulse longitude. An additional sample of 10 pulsars
showed a rather constant RM with phase, yet a small degree of RM fluctuation is
visible in at least 3 of those cases. In all cases, we have found that the
rotation of the polarization position angle across our 1.4 GHz observing band
is consistent with the wavelength-squared law of interstellar Faraday Rotation.
We provide for the first time convincing evidence that RM variations across the
pulse are largely due to interstellar scattering, although we cannot exclude
that magnetospheric Faraday Rotation may still have a minor contribution;
alternative explanations of this phenomenon, like erroneous de-dispersion and
the presence of non-orthogonal polarization modes, are excluded. If the
observed, phase-resolved RM variations are common amongst pulsars, then many of
the previously measured pulsar RMs may be in error by as much as a few tens of
rad m-2.Comment: 21 pages, 9 figures, 1 table, MNRAS accepte
The complex polarization angles of radio pulsars: orthogonal jumps and interstellar scattering
Despite some success in explaining the observed polarisation angle swing of
radio pulsars within the geometric rotating vector model, many deviations from
the expected S-like swing are observed. In this paper we provide a simple and
credible explanation of these variations based on a combination of the rotating
vector model, intrinsic orthogonally polarized propagation modes within the
pulsar magnetosphere and the effects of interstellar scattering. We use
simulations to explore the range of phenomena that may arise from this
combination, and briefly discuss the possibilities of determining the
parameters of scattering in an effort to understand the intrinsic pulsar
polarization.Comment: 5 page
Geodetic Precession in PSR B1913+16
We review the observational evidence for geodetic precession in PSR B1913+16
and present the latest observations and results from modelling the system
geometry and beam.Comment: 4 pages,to appear in "Radio Pulsars" (ASP Conf. Ser.), eds. M.
Bailes, D. Nice, & S. Thorset
Polarisation profiles of southern pulsars at 3.1 GHz
We present polarisation profiles for 48 southern pulsars observed with the
new 10-cm receiver at the Parkes telescope. We have exploited the low system
temperature and high bandwidth of the receiver to obtain profiles which have
good signal to noise for most of our sample at this relatively high frequency.
Although, as expected, a number of profiles are less linearly polarised at 3.1
GHz than at lower frequencies, we identify some pulsars and particular
components of profiles in other pulsars which have increased linear
polarisation at this frequency. We discuss the dependence of linear
polarisation with frequency in the context of a model in which emission
consists of the superposition of two, orthogonally polarised modes. We show
that a simple model, in which the orthogonal modes have different spectral
indices, can explain many of the observed properties of the frequency evolution
of both the linear polarisation and the total power, such as the high degree of
linear polarisation seen at all frequencies in some high spin-down, young
pulsars. Nearly all the position angle profiles show deviations from the
rotating vector model; this appears to be a general feature of high-frequency
polarisation observations.Comment: Accepted for publication in MNRA
An empirical model for the beams of radio pulsars
Motivated by recent results on the location of the radio emission in pulsar
magnetospheres, we have developed a model which can account for the large
diversity found in the average profile shapes of pulsars. At the centre of our
model lies the idea that radio emission at a particular frequency arises from a
wide range of altitudes above the surface of the star and that it is confined
to a region close to the last open field lines. We assert that the radial
height range over which emission occurs is responsible for the complex average
pulse shapes rather than the transverse (longitudinal) range proposed in most
current models. By implementing an abrupt change in the height range to
discriminate between young, short-period, highly-energetic pulsars and their
older counterparts, we obtain the observed transition between the simple and
complex average pulse profiles observed in each group respectively. Monte Carlo
simulations are used to demonstrate the match of our model to real
observations.Comment: Accepted for publication in MNRA
On the detectability of extragalactic fast radio transients
Recent discoveries of highly dispersed millisecond radio bursts by Thornton
et al. in a survey with the Parkes radio telescope at 1.4 GHz point towards an
emerging population of sources at cosmological distances whose origin is
currently unclear. Here we demonstrate that the scattering effects at lower
radio frequencies are less than previously thought, and that the bursts could
be detectable at redshifts out to about in surveys below 1 GHz. Using a
source model in which the bursts are standard candles with bolometric
luminosities ergs/s uniformly distributed per unit
comoving volume, we derive an expression for the observed peak flux density as
a function of redshift and use this, together with the rate estimates found by
Thornton et al. to find an empirical relationship between event rate and
redshift probed by a given survey. The non-detection of any such events in
Arecibo 1.4 GHz survey data by Deneva et al., and the Allen Telescope Array
survey by Simeon et al. is consistent with our model. Ongoing surveys in the
1--2 GHz band should result in further discoveries. At lower frequencies,
assuming a typical radio spectral index , the predicted peak flux
densities are 10s of Jy. As a result, surveys of such a population with current
facilities would not necessarily be sensitivity limited and could be carried
out with small arrays to maximize the sky coverage. We predict that sources may
already be present in 350-MHz surveys with the Green Bank Telescope. Surveys at
150 MHz with 30 deg fields of view could detect one source per hour above
30 Jy.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS on 2013 July
25. Received 2013 July 24; in original form 2013 May 3
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