430 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
A Bayesian parameter estimation approach to pulsar time-of-arrival analysis
The increasing sensitivities of pulsar timing arrays to ultra-low frequency
(nHz) gravitational waves promises to achieve direct gravitational wave
detection within the next 5-10 years. While there are many parallel efforts
being made in the improvement of telescope sensitivity, the detection of stable
millisecond pulsars and the improvement of the timing software, there are
reasons to believe that the methods used to accurately determine the
time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More
specifically, the determination of the uncertainties on these TOAs, which
strongly affect the ability to detect GWs through pulsar timing, may be
unreliable. We propose two Bayesian methods for the generation of pulsar TOAs
starting from pulsar "search-mode" data and pre-folded data. These methods are
applied to simulated toy-model examples and in this initial work we focus on
the issue of uncertainties in the folding period. The final results of our
analysis are expressed in the form of posterior probability distributions on
the signal parameters (including the TOA) from a single observation.Comment: 16 pages, 4 figure
An image-based search for pulsars among Fermi unassociated LAT sources
We describe an image-based method that uses two radio criteria, compactness, and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated sources. These criteria are applied to those radio sources from the Giant Metrewave Radio Telescope all-sky survey at 150 MHz (TGSS ADR1) found within the error ellipses of unassociated sources from the 3FGL catalogue and a preliminary source list based on 7 yr of LAT data. After follow-up interferometric observations to identify extended or variable sources, a list of 16 compact, steep-spectrum candidates is generated. An ongoing search for pulsations in these candidates, in gamma rays and radio, has found 6 ms pulsars and one normal pulsar. A comparison of this method with existing selection criteria based on gamma-ray spectral and variability properties suggests that the pulsar discovery space using Fermi may be larger than previously thought. Radio imaging is a hitherto underutilized source selection method that can be used, as with other multiwavelength techniques, in the search for Fermi pulsars
A new parametric equation of state and quark stars
It is still a matter of debate to understand the equation of state of cold
supra-nuclear matter in compact stars because of unknown on-perturbative strong
interaction between quarks. Nevertheless, it is speculated from an
astrophysical view point that quark clusters could form in cold quark matter
due to strong coupling at realistic baryon densities. Although it is hard to
calculate this conjectured matter from first principles, one can expect the
inter-cluster interaction to share some general features to nucleon-nucleon
interaction. We adopt a two-Gaussian component soft-core potential with these
general features and show that quark clusters can form stable simple cubic
crystal structure if we assume Gaussian form wave function. With this
parameterizing, Tolman-Oppenheimer-Volkoff equation is solved with reasonable
constrained parameter space to give mass-radius relation of crystalline solid
quark star. With baryon densities truncated at 2 times nuclear density at
surface and range of interaction fixed at 2fm we can reproduce similar
mass-radius relation to that obtained with bag model equations of state. The
maximum mass ranges from about 0.5 to 3 solar mass. Observed maximum pulsar
mass (about 2 solar mass) is then used to constrain parameters of this simple
interaction potential.Comment: 5 pages, 2 figure
Green Bank Telescope Measurement of the Systemic Velocity of the Double Pulsar Binary J0737-3039 and Implications for its Formation
We report on the measurement at 820- and 1400-MHz of orbital modulation of
the diffractive scintillation timescale from pulsar A in the double-pulsar
system J0737-3039 using the Green Bank Telescope. Fits to this modulation
determine the systemic velocity in the plane of the sky to be V_iss ~ 140.9 +/-
6.2 km/s. The parallel and perpendicular components of this velocity with
respect to the line of nodes of the pulsar's orbit are V_plane ~ 96.0 +/- 3.7
km/s and V_perp ~ 103.1 +/- 7.7 km/s respectively. The large V_perp implies
that pulsar B was born with a kick speed of >~ 100 km/s. Future VLBA
determination of the angular proper motion in conjunction with improved V_iss
measurements should provide a precise distance to the system. Using
high-precision timing data and the V_iss model, we estimate a best-fit orbital
inclination of i = 88.7 +/- 0.9 deg.Comment: 5 pages, 3 figures. To be submitted to ApJ Letters. A version with
higher resolution figures is available upon request from S. Ranso
Redesign and initial validation of an instrument to assess the motivational qualities of music in exercise: The Brunel Music Rating Inventory-2
In the present study, a measure to assess the motivational qualities of music in exercise was redesigned, extending previous
research efforts (Karageorghis et al., 1999). The original measure, the Brunel Music Rating Inventory (BMRI), had shown
limitations in its factor structure and its applicability to non-experts in music selection. Redesign of the BMRI used in-depth
interviews with eight participants (mean age 31.9 years, sÂĽ8.9 years) to establish the initial item pool, which was examined
using a series of confirmatory factor analyses. A single-factor model provided a good fit across three musical selections with
different motivational qualities (comparative fit index, CFI: 0.95 – 0.98; standardized root mean residual, SRMR: 0.03 –
0.05). The single-factor model also demonstrated acceptable fit across two independent samples and both sexes using one
piece of music (CFI: 0.86 – 1.00; SRMR: 0.04 – 0.07). The BMRI was designed for experts in selecting music for exercise
(e.g. dance aerobic instructors), whereas the BMRI-2 can be used both by exercise instructors and participants. The
psychometric properties of the BMRI-2 are stronger than those of the BMRI and it is easier to use. The BMRI-2 provides a
valid and internally consistent tool by which music can be selected to accompany a bout of exercise or a training session.
Furthermore, the BMRI-2 enables researchers to standardize music in experimental protocols involving exercise-related
tasks
High-Precision Timing of 5 Millisecond Pulsars: Space Velocities, Binary Evolution and Equivalence Principles
We present high-precision timing of five millisecond pulsars (MSPs) carried
out for more than seven years; four pulsars are in binary systems and one is
isolated. We are able to measure the pulsars' proper motions and derive an
estimate for their space velocities. The measured two-dimensional velocities
are in the range 70-210 km/s, consistent with those measured for other MSPs. We
also use all the available proper motion information for isolated and binary
MSPs to update the known velocity distribution for these populations. As found
by earlier works, we find that the velocity distribution of binary and isolated
MSPs are indistinguishable with the current data. Four of the pulsars in our
observing program are highly recycled with low-mass white dwarf companions and
we are able to derive accurate binary parameters for these systems. For three
of these binary systems we are able to place initial constraints on the pulsar
masses with best-fit values in the range 1.0-1.6 M_sun. The implications of the
results presented here to our understanding of binary pulsar evolution are
discussed. The updated parameters for the binary systems studied here, together
with recently discovered similar systems, allowed us to update previous limits
on the the violation of the strong equivalence principle through the parameter
|Delta| to 4.6x10^-3 (95% confidence) and the violation of
Lorentz-invariance/momentum-conservation through the parameter |hat{alpha}_3|
to 5.5x10^-20 (95% confidence).Comment: 24 pages, 8 figures; accepted for publication in The Astrophysical
Journa
The magnetar XTE J1810-197: variations in torque, radio flux density and pulse profile morphology
We report on 9 months of observations of the radio-emitting anomalous X-ray
pulsar XTE J1810-197 starting in 2006 May using the Nancay, Parkes, GBT, and
VLA telescopes mainly at a frequency of 1.4 GHz. The torque experienced by the
neutron star during this period, as inferred from a measurement of its
rotational frequency derivative, decreased by 60%, although not in a steady
manner. We have also observed very large ongoing fluctuations in flux density
and pulse shape. Superimposed on these, a general diminution of flux density
and a broadening of the pulse profile components occurred nearly
contemporaneously with a decrease in torque of about 10% that took place in
late 2006 July over an interval of 2 weeks. After a slight increase in average
flux density, since 2006 October the flux density has continued to decline and
the pulse profiles, while still varying, appear more uniform. In addition, a
simultaneous observation of the pulsar with the Chandra X-ray Observatory and
the GBT allows us to show how the X-ray and radio profiles are aligned. We
discuss briefly the implications of these results for the magnetospheric
currents in this remarkable object.Comment: Accepted for publication in ApJ. Major changes with respect to
submitted (v1) version: includes an additional 4 months of timing, flux and
pulse shape data, and revised discussio
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