224 research outputs found
Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves
We present an analysis of high-precision pulsar timing data taken as part of
the North American Nanohertz Observatory for Gravitational waves (NANOGrav)
project. We have observed 17 pulsars for a span of roughly five years using the
Green Bank and Arecibo radio telescopes. We analyze these data using standard
pulsar timing models, with the addition of time-variable dispersion measure and
frequency-variable pulse shape terms. Sub-microsecond timing residuals are
obtained in nearly all cases, and the best root-mean-square timing residuals in
this set are ~30-50 ns. We present methods for analyzing post-fit timing
residuals for the presence of a gravitational wave signal with a specified
spectral shape. These optimally take into account the timing fluctuation power
removed by the model fit, and can be applied to either data from a single
pulsar, or to a set of pulsars to detect a correlated signal. We apply these
methods to our dataset to set an upper limit on the strength of the
nHz-frequency stochastic supermassive black hole gravitational wave background
of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of
the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.Comment: To be submitted to Ap
Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data
Direct detection of low-frequency gravitational waves (
Hz) is the main goal of pulsar timing array (PTA) projects. One of the main
targets for the PTAs is to measure the stochastic background of gravitational
waves (GWB) whose characteristic strain is expected to approximately follow a
power-law of the form , where is the
gravitational-wave frequency. In this paper we use the current data from the
European PTA to determine an upper limit on the GWB amplitude as a function
of the unknown spectral slope with a Bayesian algorithm, by modelling
the GWB as a random Gaussian process. For the case , which is
expected if the GWB is produced by supermassive black-hole binaries, we obtain
a 95% confidence upper limit on of , which is 1.8 times
lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our
approach to the data analysis incorporates the multi-telescope nature of the
European PTA and thus can serve as a useful template for future
intercontinental PTA collaborations.Comment: 14 pages, 8 figures, 3 tables, mnras accepte
The international pulsar timing array project: using pulsars as a gravitational wave detector
The International Pulsar Timing Array project combines observations of
pulsars from both Northern and Southern hemisphere observatories with the main
aim of detecting ultra-low frequency (~10^-9 to 10^-8 Hz) gravitational waves.
Here we introduce the project, review the methods used to search for
gravitational waves emitted from coalescing supermassive binary black-hole
systems in the centres of merging galaxies and discuss the status of the
project.Comment: accepted by Classical and Quantum Gravity. Review talk for the
Amaldi8 conference serie
The High Time Resolution Universe Survey VI: An Artificial Neural Network and Timing of 75 Pulsars
We present 75 pulsars discovered in the mid-latitude portion of the High Time
Resolution Universe survey, 54 of which have full timing solutions. All the
pulsars have spin periods greater than 100 ms, and none of those with timing
solutions are in binaries. Two display particularly interesting behaviour; PSR
J1054-5944 is found to be an intermittent pulsar, and PSR J1809-0119 has
glitched twice since its discovery.
In the second half of the paper we discuss the development and application of
an artificial neural network in the data-processing pipeline for the survey. We
discuss the tests that were used to generate scores and find that our neural
network was able to reject over 99% of the candidates produced in the data
processing, and able to blindly detect 85% of pulsars. We suggest that
improvements to the accuracy should be possible if further care is taken when
training an artificial neural network; for example ensuring that a
representative sample of the pulsar population is used during the training
process, or the use of different artificial neural networks for the detection
of different types of pulsars.Comment: 15 pages, 8 figure
He star evolutionary channel to intermediate-mass binary pulsar PSR J1802-2124
The intermediate-mass binary pulsars (IMBPs) are characterized by relatively
long spin periods (10 - 200 ms) and massive (\ga 0.4 M_{\odot}) white dwarf
(WD) companions. Recently, precise mass measurements have been performed for
the pulsar and the WD in the IMBP PSR J1802-2124. Some observed properties,
such as the low mass of the pulsar, the high mass of the WD, the moderately
long spin period, and the tight orbit, imply that this system has undergone a
peculiar formation mechanism. In this work, we attempt to simulate the detailed
evolutionary history of PSR J1802-2124. We propose that a binary system
consisting of a neutron star (NS, of mass ) and an He star (of
mass ), and with an initial orbital period of 0.5 d, may have
been the progenitor of PSR J1802-2124. Once the He star overflows its Roche
lobe, He-rich material is transferred onto the NS at a relatively high rate of
, which is significantly higher
than the Eddington accretion rate. A large amount of the transferred material
is ejected from the vicinity of the NS by radiation pressure and results in the
birth of a mildly recycled pulsar. Our simulated results are consistent with
the observed parameters of PSR J1802-2124. Therefore, we argue that the NS + He
star evolutionary channel may be responsible for the formation of most IMBPs
with orbital periods \la 3 \rm d.Comment: 4 pages, 3 figures, Astronomy and Astrophysics in pres
The NANOGrav Nine-year Data Set: Measurement and Analysis of Variations in Dispersion Measures
We analyze dispersion measure (DM) variations of 37 millisecond pulsars in the nine-year North American Nanohertz Observatory for Gravitational Waves (NANOGrav) data release and constrain the sources of these variations. DM variations can result from a changing distance between Earth and the pulsar, inhomogeneities in the interstellar medium, and solar effects. Variations are significant for nearly all pulsars, with characteristic timescales comparable to or even shorter than the average spacing between observations. Five pulsars have periodic annual variations, 14 pulsars have monotonically increasing or decreasing trends, and 14 pulsars show both effects. Of the four pulsars with linear trends that have line-of-sight velocity measurements, three are consistent with a changing distance and require an overdensity of free electrons local to the pulsar. Several pulsars show correlations between DM excesses and lines of sight that pass close to the Sun. Mapping of the DM variations as a function of the pulsar trajectory can identify localized interstellar medium features and, in one case, an upper limit to the size of the dispersing region of 4 au. Four pulsars show roughly Kolmogorov structure functions (SFs), and another four show SFs less steep than Kolmogorov. One pulsar has too large an uncertainty to allow comparisons. We discuss explanations for apparent departures from a Kolmogorov-like spectrum, and we show that the presence of other trends and localized features or gradients in the interstellar medium is the most likely cause
Testing Theories of Gravitation Using 21-Year Timing of Pulsar Binary J1713+0747
We report 21-year timing of one of the most precise pulsars: PSR J1713+0747. Its pulse times of arrival are well modeled by a comprehensive pulsar binary model including its three-dimensional orbit and a noise model that incorporates short-and long-timescale correlated noise such as jitter and red noise. Its timing residuals have weighted root mean square similar to 92 ns. The new data set allows us to update and improve previous measurements of the system properties, including the masses of the neutron star (1.31 +/- 0.11 M-circle dot) and the companion white dwarf (0.286 +/- 0.012 M-circle dot) as well as their parallax distance 1.15 +/- 0.03 kpc. We measured the intrinsic change in orbital period, (P) over dot(b)(Int), is -0.20 +/- 0.17 ps s(-1), which is not distinguishable from zero. This result, combined with the measured (P) over dot(b)(Int) of other pulsars, can place a generic limit on potential changes in the gravitational constant G. We found that (G) over dot/G is consistent with zero [(-0.6 +/- 1.1) x 10(-12) yr(-1), 95% confidence] and changes at least a factor of 31 (99.7% confidence) more slowly than the average expansion rate of the universe. This is the best (G) over dot/G limit from pulsar binary systems. The (P) over dot(b)(Int) of pulsar binaries can also place limits on the putative coupling constant for dipole gravitational radiation kappa(D) = (-0.9 +/- 3.3) 10(-4) (95% confidence). Finally, the nearly circular orbit of this pulsar binary allows us to constrain statistically the strong-field post-Newtonian parameters Delta, which describes the violation of strong equivalence principle, and (alpha) over cap (3), which describes a breaking of both Lorentz invariance in gravitation and conservation of momentum. We found, at 95% confidence, Delta <0.01 and (3) <2 x 10(-20) based on PSR J1713+0747
The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
Observations indicate that nearly all galaxies contain supermassive black
holes (SMBHs) at their centers. When galaxies merge, their component black
holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves
(GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the
recently-released North American Nanohertz Observatory for Gravitational Waves
(NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits.
As we did not find strong evidence for GWs in our data, we placed 95\% upper
limits on the strength of GWs from such sources as a function of GW frequency
and sky location. We placed a sky-averaged upper limit on the GW strain of at nHz. We also developed a
technique to determine the significance of a particular signal in each pulsar
using ``dropout' parameters as a way of identifying spurious signals in
measurements from individual pulsars. We used our upper limits on the GW strain
to place lower limits on the distances to individual SMBHBs. At the
most-sensitive sky location, we ruled out SMBHBs emitting GWs with
nHz within 120 Mpc for , and
within 5.5 Gpc for . We also determined that
there are no SMBHBs with emitting
GWs in the Virgo Cluster. Finally, we estimated the number of potentially
detectable sources given our current strain upper limits based on galaxies in
Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris
cosmological simulation project. Only 34 out of 75,000 realizations of the
local Universe contained a detectable source, from which we concluded it was
unsurprising that we did not detect any individual sources given our current
sensitivity to GWs.Comment: 10 pages, 11 figures. Accepted by Astrophysical Journal. Please send
any comments/questions to S. J. Vigeland ([email protected]
The High Time Resolution Universe Survey II: Discovery of 5 Millisecond Pulsars
We present the discovery of 5 millisecond pulsars found in the mid-Galactic
latitude portion of the High Time Resolution Universe (HTRU) Survey. The
pulsars have rotational periods from ~2.3 to ~7.5 ms, and all are in binary
systems with orbital periods ranging from ~0.3 to ~150 d. In four of these
systems, the most likely companion is a white dwarf, with minimum masses of
~0.2 Solar Masses. The other pulsar, J1731-1847, has a very low mass companion
and exhibits eclipses, and is thus a member of the "black widow" class of
pulsar binaries. These eclipses have been observed in bands centred near
frequencies of 700, 1400 and 3000 MHz, from which measurements have been made
of the electron density in the eclipse region. These measurements have been
used to examine some possible eclipse mechanisms. The eclipse and other
properties of this source are used to perform a comparison with the other known
eclipsing and "black widow" pulsars.
These new discoveries occupy a short-period and high-dispersion measure (DM)
region of parameter space, which we demonstrate is a direct consequence of the
high time and frequency resolution of the HTRU survey. The large implied
distances to our new discoveries makes observation of their companions unlikely
with both current optical telescopes and the Fermi Gamma-ray Space Telescope.
The extremely circular orbits make any advance of periastron measurements
highly unlikely. No relativistic Shapiro delays are obvious in any of the
systems, although the low flux densities would make their detection difficult
unless the orbits were fortuitously edge-on.Comment: 11 pages, 5 figures, 4 tables, for publication in MNRA
Observations and Modelling of Relativistic Spin Precession in PSR J1141-6545
Observations of the binary pulsar PSR J1141-6545 using the Parkes radio
telescope over 9.3 years show clear time-variations in pulse width, shape and
polarization. We interpret these variations in terms of relativistic precession
of the pulsar spin axis about the total angular momentum vector of the system.
Over the nine years, the pulse width at the 50% level has changed by more than
a factor of three. Large variations have also been observed in the 1400-MHz
mean flux density. The pulse polarization has been monitored since 2004 April
using digital filterbank systems and also shows large and systematic variations
in both linear and circular polarization. Position angle variations, both
across the pulse profile and over the data span, are complex, with major
differences between the central and outer parts of the pulse profile. Modelling
of the observed position angle variations by relativistic precession of the
pulsar spin axis shows that the spin-orbit misalignment angle is about 110 deg
and that the precessional phase has passed through 180 deg during the course of
our observations. At the start of our observations, the line-of-sight impact
parameter was about 4 deg in magnitude and it reached a minimum very close to 0
deg around early 2007, consistent with the observed pulse width variations. We
have therefore mapped approximately one half of the emission beam, showing that
it is very asymmetric with respect to the magnetic axis. The derived
precessional parameters imply that the pre-supernova star had a mass of about 2
Msun and that the supernova recoil kick velocity was relatively small. With the
reversal in the rate of change of the impact parameter, we predict that over
the next decade we will see a reversed "replay" of the variations observed in
the past decade.Comment: 45 pages, 19 figures, 6 tables, accepted by Astrophysical Journa
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