219 research outputs found
Electric field representation of pulsar intensity spectra
Pulsar dynamic spectra exhibit high visibility fringes arising from
interference between scattered radio waves. These fringes may be random or
highly ordered patterns, depending on the nature of the scattering or
refraction. Here we consider the possibility of decomposing pulsar dynamic
spectra -- which are intensity measurements -- into their constituent scattered
waves, i.e. electric field components. We describe an iterative method of
achieving this decomposition and show how the algorithm performs on data from
the pulsar B0834+06. The match between model and observations is good, although
not formally acceptable as a representation of the data. Scattered wave
components derived in this way are immediately useful for qualitative insights
into the scattering geometry. With some further development this approach can
be put to a variety of uses, including: imaging the scattering and refracting
structures in the interstellar medium; interstellar interferometric imaging of
pulsars at very high angular resolution; and mitigating pulse arrival time
fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA
Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries
We calculate the expected nHz--Hz gravitational wave (GW) spectrum from
coalescing Massive Black Hole (MBH) binaries resulting from mergers of their
host galaxies. We consider detection of this spectrum by precision pulsar
timing and a future Pulsar Timing Array. The spectrum depends on the merger
rate of massive galaxies, the demographics of MBHs at low and high redshift,
and the dynamics of MBH binaries. We apply recent theoretical and observational
work on all of these fronts. The spectrum has a characteristic strain
, just below the detection limit from
recent analysis of precision pulsar timing measurements. However, the amplitude
of the spectrum is still very uncertain owing to approximations in the
theoretical formulation of the model, to our lack of knowledge of the merger
rate and MBH population at high redshift, and to the dynamical problem of
removing enough angular momentum from the MBH binary to reach a GW-dominated
regime.Comment: 31 Pages, 8 Figures, small changes to match the published versio
Scattering features and variability of the Crab pulsar
We report on Westerbork Synthesis Radio Telescope observations of the Crab
pulsar at 350 MHz from 2012 November 24 until 2015 June 21. During this period
we consistently observe variations in the pulse profile of the Crab. Both
variations in the scattering width of the pulse profile as well as delayed
copies, also known as echoes, are seen regularly. These observations support
the classification of two types of echoes: those that follow the truncated
exponential shape expected for the thin-screen scattering approximation, and
echoes that show a smoother, more Gaussian shape. During a sequence of
high-cadence observations in 2015, we find that these non-exponential echoes
evolve in time by approaching the main pulse and interpulse in phase,
overlapping the main pulse and interpulse, and later receding. We find a pulse
scatter-broadening time scale, , scaling with frequency as
, with , which is consistent with expected
values for thin-screen scattering modelsComment: 10 pages, 7 figure
Effects of Intermittent Emission: Noise Inventory for Scintillating Pulsar B0834+06
We compare signal and noise for observations of the scintillating pulsar
B0834+06, using very-long baseline interferometry and a single-dish
spectrometer. Comparisons between instruments and with models suggest that
amplitude variations of the pulsar strongly affect the amount and distribution
of self-noise. We show that noise follows a quadratic polynomial with flux
density, in spectral observations. Constant coefficients, indicative of
background noise, agree well with expectation; whereas second-order
coefficients, indicative of self-noise, are about 3 times values expected for a
pulsar with constant on-pulse flux density. We show that variations in flux
density during the 10-sec integration account for the discrepancy. In the
secondary spectrum, about 97% of spectral power lies within the pulsar's
typical scintillation bandwidth and timescale; an extended scintillation arc
contains about 3%. For a pulsar with constant on-pulse flux density, noise in
the dynamic spectrum will appear as a uniformly-distributed background in the
secondary spectrum. We find that this uniform noise background contains 95% of
noise in the dynamic spectrum for interferometric observations; but only 35% of
noise in the dynamic spectrum for single-dish observations. Receiver and sky
dominate noise for our interferometric observations, whereas self-noise
dominates for single-dish. We suggest that intermittent emission by the pulsar,
on timescales < 300 microseconds, concentrates self-noise near the origin in
the secondary spectrum, by correlating noise over the dynamic spectrum. We
suggest that intermittency sets fundamental limits on pulsar astrometry or
timing. Accounting of noise may provide means for detection of intermittent
sources, when effects of propagation are unknown or impractical to invert.Comment: 38 pages, 10 figure
The Triple Pulsar System PSR B1620-26 in M4
The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a white
dwarf companion in a half-year orbit. Anomalously large variations in the
pulsar's apparent spin-down rate have suggested the presence of a second
companion in a much wider orbit. Using timing observations made on more than
seven hundred days spanning eleven years, we confirm this anomalous timing
behavior. We explicitly demonstrate, for the first time, that a timing model
consisting of the sum of two non-interacting Keplerian orbits can account for
the observed signal. Both circular and elliptical orbits are allowed, although
highly eccentric orbits require improbable orbital geometries.
The motion of the pulsar in the inner orbit is very nearly a Keplerian
ellipse, but the tidal effects of the outer companion cause variations in the
orbital elements. We have measured the change in the projected semi-major axis
of the orbit, which is dominated by precession-driven changes in the orbital
inclination. This measurement, along with limits on the rate of change of other
orbital elements, can be used to significantly restrict the properties of the
outer orbit. We find that the second companion most likely has a mass m~0.01
Msun --- it is almost certainly below the hydrogen burning limit (m<0.036 Msun,
95% confidence) --- and has a current distance from the binary of ~35 AU and
orbital period of order one hundred years. Circular (and near-circular) orbits
are allowed only if the pulsar magnetic field is ~3x10^9 G, an order of
magnitude higher than a typical millisecond pulsar field strength. In this
case, the companion has mass m~1.2x10^-3 Msun and orbital period ~62 years.Comment: 12 pages, 6 figures, 3 tables. Very minor clarifications and
rewording. Accepted for publication in the Astrophys.
The stationary phase point method for transitional scattering: diffractive radio scintillation for pulsar
The stationary phase point (SPP) method in one-dimensional case is introduced
to treat the diffractive scintillation. From weak scattering, where the SPP
number N=1, to strong scattering (N1), via transitional scattering regime
(N2,3), we find that the modulation index of intensity experiences the
monotonically increasing from 0 to 1 with the scattering strength,
characterized by the ratio of Fresnel scale \rf to diffractive scale
\rdiff.Comment: Hanas Meeting paper, appear in ChJAA, 2006, 6, Su
Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project
The first direct detection of gravitational waves may be made through
observations of pulsars. The principal aim of pulsar timing array projects
being carried out worldwide is to detect ultra-low frequency gravitational
waves (f ~ 10^-9 to 10^-8 Hz). Such waves are expected to be caused by
coalescing supermassive binary black holes in the cores of merged galaxies. It
is also possible that a detectable signal could have been produced in the
inflationary era or by cosmic strings. In this paper we review the current
status of the Parkes Pulsar Timing Array project (the only such project in the
Southern hemisphere) and compare the pulsar timing technique with other forms
of gravitational-wave detection such as ground- and space-based interferometer
systems.Comment: Accepted for publication in PAS
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