24 research outputs found
Properties of the linearly polarized radiation from PSR B0950+08
Measurements of average pulse profiles made with a single linear polarization
over the range 41-112 MHz are presented for PSR B0950+08. We show that the
observed variable structure of the pulse profiles is a result of Faraday
sinusoidal modulation of the pulse intensity with frequency. The rotation
measure corresponding to this effect, RM = 4 rad/m^2, is about 3 times greater
than the tabulated value of RM = 1.35 rad/m^2.Comment: 18 pages, including 10 figures. Accepted for publication in A&
On the Pulse Intensity Modulation of PSR B0823+26
We investigate the radio emission behaviour of PSR B0823+26, a pulsar which
is known to undergo pulse nulling, using an 153-d intensive sequence of
observations. The pulsar is found to exhibit both short (~min) and unusually
long-term (~hours or more) nulls, which not only suggest that the source
possesses a distribution of nulling timescales, but that it may also provide a
link between conventional nulling pulsars and longer-term intermittent pulsars.
Despite seeing evidence for periodicities in the pulsar radio emission, we are
uncertain whether they are intrinsic to the source, due to the influence of
observation sampling on the periodicity analysis performed. Remarkably, we find
evidence to suggest that the pulsar may undergo pre-ignition periods of
'emission flickering', that is rapid changes between radio-on (active) and -off
(null) emission states, before transitioning to a steady radio-emitting phase.
We find no direct evidence to indicate that the object exhibits any change in
spin-down rate between its radio-on and -off emission modes. We do, however,
place an upper limit on this variation to be <= 6 % from simulations. This
indicates that emission cessation in pulsars does not necessarily lead to large
changes in spin-down rate. Moreover, we show that such changes in spin-down
rate will not be discernible in the majority of objects which exhibit
short-term (<= 1 d) emission cessation. In light of this, we predict that many
pulsars could exhibit similar magnetospheric and emission properties to PSR
B0823+26, but which have not yet been observed.Comment: 13 pages, 11 figures, accepted for publication in MNRAS; 1 reference
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High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing
We demonstrate that the sensitivity of high-precision pulsar timing
experiments will be ultimately limited by the broadband intensity modulation
that is intrinsic to the pulsar's stochastic radio signal. That is, as the peak
flux of the pulsar approaches that of the system equivalent flux density,
neither greater antenna gain nor increased instrumental bandwidth will improve
timing precision. These conclusions proceed from an analysis of the covariance
matrix used to characterise residual pulse profile fluctuations following the
template matching procedure for arrival time estimation. We perform such an
analysis on 25 hours of high-precision timing observations of the closest and
brightest millisecond pulsar, PSR J0437-4715. In these data, the standard
deviation of the post-fit arrival time residuals is approximately four times
greater than that predicted by considering the system equivalent flux density,
mean pulsar flux and the effective width of the pulsed emission. We develop a
technique based on principal component analysis to mitigate the effects of
shape variations on arrival time estimation and demonstrate its validity using
a number of illustrative simulations. When applied to our observations, the
method reduces arrival time residual noise by approximately 20%. We conclude
that, owing primarily to the intrinsic variability of the radio emission from
PSR J0437-4715 at 20 cm, timing precision in this observing band better than 30
- 40 ns in one hour is highly unlikely, regardless of future improvements in
antenna gain or instrumental bandwidth. We describe the intrinsic variability
of the pulsar signal as stochastic wideband impulse modulated self-noise
(SWIMS) and argue that SWIMS will likely limit the timing precision of every
millisecond pulsar currently observed by Pulsar Timing Array projects as larger
and more sensitive antennae are built in the coming decades.Comment: 16 pages, 9 figures, accepted for publication in MNRAS. Updated
version: added DOI and changed manuscript to reflect changes in the final
published versio
RRATs: New Discoveries, Timing Solutions & Musings
We describe observations of Rotating RAdio Transients (RRATs) that were
discovered in a re-analysis of the Parkes Multi-beam Pulsar Survey (PMPS). The
sources have now been monitored for sufficiently long to obtain seven new
coherent timing solutions, to make a total of 14 now known. Furthermore we
announce the discovery of 7 new transient sources, one of which may be
extragalactic in origin (with ) and would then be a second example of
the so-called `Lorimer burst'. The timing solutions allow us to infer neutron
star characteristics such as energy-loss rate, magnetic field strength and
evolutionary timescales, as well as facilitating multi-wavelength followup by
providing accurate astrometry. All of this enables us to consider the question
of whether or not RRATs are in any way special, i.e. a distinct and separate
population of neutron stars, as has been previously suggested. We see no reason
to consider 'RRAT' as anything other than a detection label, the subject of a
selection effect in the parameter space searched. However, single-pulse
searches can be utilised to great effect to identify pulsars difficult, or
impossible, to find by other means, in particular those with long-periods (half
of the PMPS RRATs have periods greater than 4 seconds), high-magnetic field
strengths ( G) and pulsars approaching the 'death valley'.
The detailed nulling properties of such pulsars are unknown but the mounting
evidence suggests a broad range of behaviour in the pulsar population. The
group of RRATs fit in to the picture where pulsar magnetospheres switch between
stable configurations.Comment: accepted for publication in MNRAS, 17 pages, 7 figures, 3 table
S2DFS: Analysis of temporal changes of drifting subpulses
We introduce a new technique, called the Sliding Two-Dimensional Fluctuation
Spectrum, used for detecting and characterising the temporal changes of
drifting subpulses from radio pulsars. The method was tested using simulated
data as well as archived observations made with the WSRT at wavelengths of 92
and 21 cm. The drifting subpulse phenomenon is a well known property of radio
pulsars. However the properties of the temporal behaviour of drifting subpulses
are not fully explored. The drifting can also be non-coherent and the presence
of effects like nulling or drift rate changing can mask the drifting behaviour
of the pulsar. The S2DFS is a robust method for investigating this phenomenon
and by introducing it we aim to expand our knowledge of the temporal drifting
subpulse properties. Our new analysis method uses horizonally collapsed
fluctuation spectra obtained with the Two-Dimensional Fluctuation Spectrum
method. Stacking the collapsed spectra obtained in a 256 pulse window which
slides by a pulse at a time produces a map of the collapsed fluctuation
spectrum. By analysing the maps one can easily determine the presence of any
temporal drift changes. Simulated data showed that the technique can reveal the
presence of any temporal changes in drift behaviour like mode changing or
nulling. We have also analysed data of three pulsars, PSRs B0031-07, B1819-22
and B1944+17, which were selected based on the quality of the data and their
known drift properties. All three sources are known to exhibit mode changes
which could easily be seen in the S2DFS. The results from the analysis of the
data sets used in this paper have shown that the S2DFS method is robust and
complimentary to the 2DFS method in detecting and characterising the temporal
changes in drifting subpulses from radio pulsars.Comment: accepted for publication in A&A, 10 pages, 7 figures, 1 tabl
Populating the Galaxy with pulsars I: stellar & binary evolution
The computation of theoretical pulsar populations has been a major component
of pulsar studies since the 1970s. However, the majority of pulsar population
synthesis has only regarded isolated pulsar evolution. Those that have examined
pulsar evolution within binary systems tend to either treat binary evolution
poorly or evolve the pulsar population in an ad-hoc manner. Thus no complete
and direct comparison with observations of the pulsar population within the
Galactic disk has been possible to date. Described here is the first component
of what will be a complete synthetic pulsar population survey code. This
component is used to evolve both isolated and binary pulsars. Synthetic
observational surveys can then be performed on this population for a variety of
radio telescopes. The final tool used for completing this work will be a code
comprised of three components: stellar/binary evolution, Galactic kinematics
and survey selection effects. Results provided here support the need for
further (apparent) pulsar magnetic field decay during accretion, while they
conversely suggest the need for a re-evaluation of the assumed \textit{typical}
MSP formation process. Results also focus on reproducing the observed
diagram for Galactic pulsars and how this precludes short timescales
for standard pulsar exponential magnetic field decay. Finally, comparisons of
bulk pulsar population characteristics are made to observations displaying the
predictive power of this code, while we also show that under standard binary
evolutionary assumption binary pulsars may accrete much mass.Comment: 26 pages, 16 figures, 1 table, accepted for publication in MNRA
The bright spiky emission of pulsar B0656+14
We present a detailed study of the single radio pulses of PSR B0656+14, a
pulsar also known to be a strong pulsed source of high-energy emission. Despite
the extensive studies at high-energy wavelengths, there is little or no
published work on its single-pulse behaviour in the radio band. In this report
we rectify this omission. We have found that the shape of the pulse profile of
PSR B0656+14 requires an unusually long timescale to achieve stability (over
25,000 pulses at 327 MHz). This instability is caused by very bright and narrow
pulses with widths and luminosities comparable to those observed for the RRATs.
Many pulses are bright enough to qualify as "giant pulses", but are broader
than those usually meant by this term. At 327 MHz the brightest pulse was about
116 times brighter than the average pulse. Although the most powerful pulses
peak near the centre of the profile, occasional sudden strong pulses are also
found on the extreme leading edge of the profile. One of them has a peak flux
of about 2000 times the average flux at that pulse longitude. No "break" in the
pulse-energy distributions is observed, but nevertheless there is evidence of
two separate populations of pulses: bright pulses have a narrow "spiky"
appearance consisting of short quasi-periodic bursts of emission with
microstructure, in contrast to the underlying weaker broad pulses. Furthermore,
the spiky pulses tend to appear in clusters which arise and dissipate over
about 10 periods. We demonstrate that the spiky emission builds a narrow and
peaked profile, whereas the weak emission produces a broad hump, which is
largely responsible for the shoulders in the total emission profiles at both
high and low frequencies.Comment: 17 pages, 18 figures, accepted for publication in A&
Massive stars as thermonuclear reactors and their explosions following core collapse
Nuclear reactions transform atomic nuclei inside stars. This is the process
of stellar nucleosynthesis. The basic concepts of determining nuclear reaction
rates inside stars are reviewed. How stars manage to burn their fuel so slowly
most of the time are also considered. Stellar thermonuclear reactions involving
protons in hydrostatic burning are discussed first. Then I discuss triple alpha
reactions in the helium burning stage. Carbon and oxygen survive in red giant
stars because of the nuclear structure of oxygen and neon. Further nuclear
burning of carbon, neon, oxygen and silicon in quiescent conditions are
discussed next. In the subsequent core-collapse phase, neutronization due to
electron capture from the top of the Fermi sea in a degenerate core takes
place. The expected signal of neutrinos from a nearby supernova is calculated.
The supernova often explodes inside a dense circumstellar medium, which is
established due to the progenitor star losing its outermost envelope in a
stellar wind or mass transfer in a binary system. The nature of the
circumstellar medium and the ejecta of the supernova and their dynamics are
revealed by observations in the optical, IR, radio, and X-ray bands, and I
discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure