774 research outputs found
Why the distance of PSR J0218+4232 does not challenge pulsar emission theories
Recent VLBI measurements of the astrometric parameters of the millisecond
pulsar J0218+4232 by Du et al. have suggested this pulsar is as distant as 6.3
kpc. At such a large distance, the large {\gamma}-ray flux observed from this
pulsar would make it the most luminous {\gamma}-ray pulsar known. This
luminosity would exceed what can be explained by the outer gap and slot-gap
pulsar emission models, potentially placing important and otherwise elusive
constraints on the pulsar emission mechanism. We show that the VLBI parallax
measurement is dominated by the Lutz-Kelker bias. When this bias is corrected
for, the most likely distance for this pulsar is 3.15(+0.85/-0.60) kpc. This
revised distance places the luminosity of PSR J0218+4232 into a range where it
does not challenge any of the standard theories of the pulsar emission
mechanism.Comment: 3 pages, 2 figures, 1 table. Accepted for publication in MNRA
Lutz-Kelker bias in pulsar parallax measurements
Lutz & Kelker showed that parallax measurements are systematically
overestimated because they do not properly account for the larger volume of
space that is sampled at smaller parallax values. We apply their analysis to
neutron stars, incorporating the bias introduced by the intrinsic radio
luminosity function and a realistic Galactic population model for neutron
stars. We estimate the bias for all published neutron star parallax
measurements and find that measurements with less than ~95% certainty, are
likely to be significantly biased. Through inspection of historic parallax
measurements, we confirm the described effects in optical and radio
measurements, as well as in distance estimates based on interstellar dispersion
measures. The potential impact on future tests of relativistic gravity through
pulsar timing and on X-ray--based estimates of neutron star radii is briefly
discussed.Comment: 9 pages, 3 tables, 1 figure. Accepted for publication in MNRA
The pulsar spectral index distribution
The flux density spectra of radio pulsars are known to be steep and, to first
order, described by a power-law relationship of the form S_{\nu} \propto
\nu^{\alpha}, where S_{\nu} is the flux density at some frequency \nu and
\alpha is the spectral index. Although measurements of \alpha have been made
over the years for several hundred pulsars, a study of the intrinsic
distribution of pulsar spectra has not been carried out. From the result of
pulsar surveys carried out at three different radio frequencies, we use
population synthesis techniques and a likelihood analysis to deduce what
underlying spectral index distribution is required to replicate the results of
these surveys. We find that in general the results of the surveys can be
modelled by a Gaussian distribution of spectral indices with a mean of -1.4 and
unit standard deviation. We also consider the impact of the so-called
"Gigahertz-peaked spectrum" pulsars. The fraction of peaked spectrum sources in
the population with significant turn-over at low frequencies appears to be at
most 10%. We demonstrate that high-frequency (>2 GHz) surveys preferentially
select flatter-spectrum pulsars and the converse is true for lower-frequency
(<1 GHz) surveys. This implies that any correlations between \alpha and other
pulsar parameters (for example age or magnetic field) need to carefully account
for selection biases in pulsar surveys. We also expect that many known pulsars
which have been detected at high frequencies will have shallow, or positive,
spectral indices. The majority of pulsars do not have recorded flux density
measurements over a wide frequency range, making it impossible to constrain
their spectral shapes. We also suggest that such measurements would allow an
improved description of any populations of pulsars with 'non-standard' spectra.Comment: 8 pages, 5 figures. Accepted by MNRA
Pulsar timing analysis in the presence of correlated noise
Pulsar timing observations are usually analysed with least-square-fitting
procedures under the assumption that the timing residuals are uncorrelated
(statistically "white"). Pulsar observers are well aware that this assumption
often breaks down and causes severe errors in estimating the parameters of the
timing model and their uncertainties. Ad hoc methods for minimizing these
errors have been developed, but we show that they are far from optimal.
Compensation for temporal correlation can be done optimally if the covariance
matrix of the residuals is known using a linear transformation that whitens
both the residuals and the timing model. We adopt a transformation based on the
Cholesky decomposition of the covariance matrix, but the transformation is not
unique. We show how to estimate the covariance matrix with sufficient accuracy
to optimize the pulsar timing analysis. We also show how to apply this
procedure to estimate the spectrum of any time series with a steep red
power-law spectrum, including those with irregular sampling and variable error
bars, which are otherwise very difficult to analyse.Comment: Accepted by MNRA
Low-Frequency Spectral Turn-Overs in Millisecond Pulsars Studied from Imaging Observations
Measurements of pulsar flux densities are of great importance for
understanding the pulsar emission mechanism and for predictions of pulsar
survey yields and the pulsar population at large. Typically these flux
densities are determined from phase-averaged "pulse profiles", but this method
has limited applicability at low frequencies because the observed pulses can
easily be spread out by interstellar effects like scattering or dispersion,
leading to a non-pulsed continuum component that is necessarily ignored in this
type of analysis. In particular for the class of the millisecond pulsars (MSPs)
at frequencies below 200MHz, such interstellar effects can seriously compromise
de- tectability and measured flux densities. In this paper we investigate MSP
spectra based on a complementary approach, namely through investigation of
archival con- tinuum imaging data. Even though these images lose sensitivity to
pulsars since the on-pulse emission is averaged with off-pulse noise, they are
insensitive to effects from scattering and provide a reliable way to determine
the flux density and spectral indices of MSPs based on both pulsed and unpulsed
components. Using the 74MHz VLSSr as well as the 325MHz WENSS and 1.4GHz NVSS
catalogues, we investigate the imaging flux densities of MSPs and evaluate the
likelihood of spectral turn-overs in this population. We determine three new
MSP spectral indices and identify six new MSPs with likely spectral turn-overs.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
Limits on the Mass, Velocity and Orbit of PSR J19336211
We present a high-precision timing analysis of PSR J19336211, a
millisecond pulsar (MSP) with a 3.5-ms spin period and a white dwarf (WD)
companion, using data from the Parkes radio telescope. Since we have accurately
measured the polarization properties of this pulsar we have applied the matrix
template matching approach in which the times of arrival are measured using
full polarimetric information. We achieved a weighted root-mean-square timing
residuals (rms) of the timing residuals of 1.23 , 15.5
improvement compared to the total intensity timing analysis. After studying the
scintillation properties of this pulsar we put constraints on the inclination
angle of the system. Based on these measurements and on mapping we put
a 2- upper limit on the companion mass (0.44 M). Since this
mass limit cannot reveal the nature of the companion we further investigate the
possibility of the companion to be a He WD. Applying the orbital period-mass
relation for such WDs, we conclude that the mass of a He WD companion would be
about 0.260.01 M which, combined with the measured mass function
and orbital inclination limits, would lead to a light pulsar mass
1.0 M. This result seems unlikely based on current neutron star
formation models and we therefore conclude that PSR J19336211 most likely
has a CO WD companion, which allows for a solution with a more massive pulsar
The impact of a stochastic gravitational-wave background on pulsar timing parameters
Gravitational waves are predicted by Einstein's theory of general relativity
as well as other theories of gravity. The rotational stability of the fastest
pulsars means that timing of an array of these objects can be used to detect
and investigate gravitational waves. Simultaneously, however, pulsar timing is
used to estimate spin period, period derivative, astrometric, and binary
parameters. Here we calculate the effects that a stochastic background of
gravitational waves has on pulsar timing parameters through the use of
simulations and data from the millisecond pulsars PSR J0437--4715 and PSR
J1713+0747. We show that the reported timing uncertainties become
underestimated with increasing background amplitude by up to a factor of
for a stochastic gravitational-wave background amplitude of , where is the amplitude of the characteristic strain spectrum at
one-year gravitational wave periods. We find evidence for prominent
low-frequency spectral leakage in simulated data sets including a stochastic
gravitational-wave background. We use these simulations along with independent
Very Long Baseline Interferometry (VLBI) measurements of parallax to set a
2--sigma upper limit of . We find that different
supermassive black hole assembly scenarios do not have a significant effect on
the calculated upper limits. We also test the effects that ultralow--frequency
(10--10 Hz) gravitational waves have on binary pulsar parameter
measurements and find that the corruption of these parameters is less than
those due to -- Hz gravitational waves.Comment: 16 pages, 7 figures, accepted to MNRA
A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves
In this paper we study the effect of cosmic neutrino decoupling on the
spectrum of cosmological gravitational waves (GWs). At temperatures T>>1 MeV,
neutrinos constitute a perfect fluid and do not hinder GW propagation, while
for T<<1 MeV they free-stream and have an effective viscosity that damps
cosmological GWs by a constant amount. In the intermediate regime,
corresponding to neutrino decoupling, the damping is frequency-dependent. GWs
entering the horizon during neutrino decoupling have a frequency f ~ 1 nHz,
corresponding to a frequency region that will be probed by Pulsar Timing Arrays
(PTAs). In particular, we show how neutrino decoupling induces a spectral
feature in the spectrum of cosmological GWs just below 1 nHz. We briefly
discuss the conditions for a detection of this feature and conclude that it is
unlikely to be observed by PTAs.Comment: 11 pages, 2 figures. V2: References Adde
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