440 research outputs found
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
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
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
Geodetic Precession and the Binary Pulsar B1913+16
A change of the component separation in the profiles of the binary pulsar PSR
B1913+16 has been observed for the first time (Kramer 1998) as expected by
geodetic precession. In this work we extend the previous work by accounting for
recent data from the Effelsberg 100-m telescope and Arecibo Observatory and
testing model predictions. We demonstrate how the new information will provide
additional information on the solutions of the system geometry.Comment: 2 pages, 1 figure, IAU 177 Colloquium: Pulsar Astronomy - 2000 and
Beyon
Simultaneous single-pulse observations of radio pulsars III. The behaviour of circular polarization
We investigate circular polarization in pulsar radio emission through
simultaneous observations of PSR B1133+16 at two frequencies. In particular, we
investigate the association of the handedness of circular polarization with the
orthogonal polarization mode phenomenon at two different frequencies. We find
the association to be significant across the pulse for PSR B1133+16, making a
strong case for orthogonal polarization modes determining the observed circular
polarization. The association however is not perfect and decreases with
frequency. Based on these results and assuming emission occurs in superposed
orthogonal polarization modes, we present a technique of mode decomposition
based on single pulses. Average profiles of the polarization of each mode can
then be computed by adding the individual mode-separated single pulses. We show
that decomposing single pulses produces different average profiles for the
orthogonal polarization modes from decomposing average profiles. Finally, we
show sample single pulses and discuss the implications of the frequency
dependence of the correlation of the circular polarization with the orthogonal
polarization mode phenomenon.Comment: accepted for publication in A&
Observations of transients and pulsars with LOFAR international stations
The LOw FRequency ARray - LOFAR is a new radio telescope that is moving the
science of radio pulsars and transients into a new phase. Its design places
emphasis on digital hardware and flexible software instead of mechanical
solutions. LOFAR observes at radio frequencies between 10 and 240 MHz where
radio pulsars and many transients are expected to be brightest. Radio frequency
signals emitted from these objects allow us to study the intrinsic pulsar
emission and phenomena such as propagation effects through the interstellar
medium. The design of LOFAR allows independent use of its stations to conduct
observations of known bright objects, or wide field monitoring of transient
events. One such combined software/hardware solution is called the Advanced
Radio Transient Event Monitor and Identification System (ARTEMIS). It is a
backend for both targeted observations and real-time searches for millisecond
radio transients which uses Graphical Processing Unit (GPU) technology to
remove interstellar dispersion and detect millisecond radio bursts from
astronomical sources in real-time using a single LOFAR station.Comment: To appear in the proceedings of the Electromagnetic Radiation from
Pulsars and Magnetars conference, Zielona Gora, 2012. 4 pages, 1 figur
A transient component in the pulse profile of PSR J0738-4042
One of the tenets of the radio pulsar observational picture is that the
integrated pulse profiles are constant with time. This assumption underpins
much of the fantastic science made possible via pulsar timing. Over the past
few years, however, this assumption has come under question with a number of
pulsars showing pulse shape changes on a range of timescales. Here, we show the
dramatic appearance of a bright component in the pulse profile of PSR
J0738-4042 (B0736-40). The component arises on the leading edge of the profile.
It was not present in 2004 but strongly present in 2006 and all observations
thereafter. A subsequent search through the literature shows the additional
component varies in flux density over timescales of decades. We show that the
polarization properties of the transient component are consistent with the
picture of competing orthogonal polarization modes. Faced with the general
problem of identifying and characterising average profile changes, we outline
and apply a statistical technique based on a Hidden Markov Model. The value of
this technique is established through simulations, and is shown to work
successfully in the case of low signal-to-noise profiles.Comment: Accepted for publication in MNRA
Evidence for alignment of the rotation and velocity vectors in pulsars. II. Further data and emission heights
We have conducted observations of 22 pulsars at frequencies of 0.7, 1.4 and
3.1 GHz and present their polarization profiles. The observations were carried
out for two main purposes. First we compare the orientation of the spin and
velocity vectors to verify the proposed alignment of these vectors by Johnston
et al. (2005). We find, for the 14 pulsars for which we were able to determine
both vectors, that 7 are plausibly aligned, a fraction which is lower than, but
consistent with, earlier measurements. Secondly, we use profiles obtained
simultaneously at widely spaced frequencies to compute the radio emission
heights. We find, similar to other workers in the field, that radiation from
the centre of the profile originates from lower in the magnetosphere than the
radiation from the outer parts of the profile.Comment: Accepted by MNRAS. 14 page
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