1,146 research outputs found
Gravitational wave background from rotating neutron stars
The background of gravitational waves produced by the ensemble of rotating
neutron stars (which includes pulsars, magnetars and gravitars) is
investigated. A formula for \Omega(f) (commonly used to quantify the
background) is derived, properly taking into account the time evolution of the
systems since their formation until the present day. Moreover, the formula
allows one to distinguish the different parts of the background: the
unresolvable (which forms a stochastic background) and the resolvable. Several
estimations of the background are obtained, for different assumptions on the
parameters that characterize neutron stars and their population. In particular,
different initial spin period distributions lead to very different results. For
one of the models, with slow initial spins, the detection of the background can
be rejected. However, other models do predict the detection of the background
by the future ground-based gravitational wave detector ET. A robust upper limit
for the background of rotating neutron stars is obtained; it does not exceed
the detection threshold of two cross-correlated Advanced LIGO interferometers.
If gravitars exist and constitute more than a few percent of the neutron star
population, then they produce an unresolvable background that could be detected
by ET. Under the most reasonable assumptions on the parameters characterizing a
neutron star, the background is too faint. Previous papers have suggested
neutron star models in which large magnetic fields (like the ones that
characterize magnetars) induce big deformations in the star, which produce a
stronger emission of gravitational radiation. Considering the most optimistic
(in terms of the detection of gravitational waves) of these models, an upper
limit for the background produced by magnetars is obtained; it could be
detected by ET, but not by BBO or DECIGO.Comment: 25 pages, 15 figure
A Model of the Bromine/Bromide Electrode Reaction at a Rotating Disk Electrode
A mathematical model is presented for the Br2/Br– electrode reaction at a rotating disk electrode. The model includes current density-overpotential expressions for the electrode reaction according to either the Volmer-Heyrovsky (V-H) or the Volmer-Tafel (V-T) mechanism and the transport equations including the effect of ionic migration. The model is used to predict current-overpotential curves for various cases of interest. Qualitative comparison of the model predictions to literature data shows that either the V-H or the V-T mechanism, with V controlling, may be acceptable for the Br2/Br– reaction
Prediction of the Current Density at an Electrode at Which Multiple Electrode Reactions Occur under Potentiostatic Control
It is often desirable to be able to predict the total current density at an electrode when multiple electrochemical reactions occur there under potentiostatic control. It is also sometimes desirable to include the effect of ionic migration within the diffusion layer upon the predicted total (1) and partial current densities (2). A procedure for doing this can be illustrated by considering the rotating disk electrode (RDE) system and the associated potential distribution near the RDE as shown in Fig. 1 and 2
The Effect of the Tribromide Complex Reaction on the Oxidation/Reduction Current of the Br2/Br– Electrode
The Br2/Br– electrode reaction with tribromide complex formation reaction in the solution, a chemical-electrochemical (C-E) type reaction, has been investigated in order to determine the effect of the chemical reaction on the electrode kinetics. It is shown that the chemical reaction has little effect on the electrode kinetics at very slow homogeneous reaction rates, but has a more drastic effect on the electrode kinetics at faster homogeneous reaction rates. Also, the kinetics at the electrode are affected by changes in the concentrations of the active species (Br2, Br–, and Br3–) in the bulk solution as a consequence of the coupling effect of the chemical reaction on the electrode kinetics
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
On the possible observational manifestation of supernova shock impact on the neutron star magnetosphere
Impact of supernova explosion on the neutron star magnetosphere in a massive
binary system is considered. The supernova shock striking the NS magnetosphere
filled with plasma can lead to the formation of a magnetospheric tail with
significant magnetic energy. The magnetic field reconnection in the current
sheet formed can convert the magnetic energy stored in the tail into kinetic
energy of accelerated charged particles. Plasma instabilities excited by beams
of relativistic particles can lead to the formation of a short pulse of
coherent radio emission with parameters similar to those of the observed bright
extragalactic millisecond radio burst (Lorimer et al. 2007).Comment: 8 pages, Astron. Lett. in pres
The Arecibo 430-MHz Intermediate Galactic Latitude Survey: Discovery of Nine Radio Pulsars
We have used the Arecibo Radio Telescope to search for millisecond pulsars in
two intermediate Galactic latitude regions (7 deg < | b | < 20 deg) accessible
to this telescope. For these latitudes the useful millisecond pulsar search
volume achieved by Arecibo's 430-MHz beam is predicted to be maximal. Searching
a total of 130 square degrees, we have discovered nine new pulsars and detected
four previously known objects. We compare the results of this survey with those
of other 430-MHz surveys carried out at Arecibo and of an intermediate latitude
survey made at Parkes that included part of our search area; the latter
independently found two of the nine pulsars we have discovered.
At least six of our discoveries are isolated pulsars with ages between 5 and
300 Myr; one of these, PSR J1819+1305, exhibits very marked and periodic
nulling. We have also found a recycled pulsar, PSR J2016+1948. With a
rotational period of 65 ms, this is a member of a binary system with a 635-day
orbital period. We discuss some of the the properties of this system in detail,
and indicate its potential to provide a test of the Strong Equivalence
Principle. This pulsar and PSR J0407+16, a similar system now being timed at
Arecibo, are by far the best systems known for such a test.Comment: Accepted for publication in ApJ Referee format: 22 pages, 7 figure
Are Supershells Powered by Multiple Supernovae? Modeling the Radio Pulsar Population Produced by OB Associations
Traditional searches for radio pulsars have targeted individual small regions
such as supernova remnants or globular clusters, or have covered large
contiguous regions of the sky. None of these searches has been specifically
directed towards giant supershells, some of which are likely to have been
produced by multiple supernova (SN) explosions from an OB association. Here we
perform a Montecarlo simulation of the pulsar population associated with
supershells powered by multiple SNe. We predict that several tens of radio
pulsars could be detected with current instruments associated with the largest
Galactic supershells (with kinetic energies >~ 10^{53} ergs), and a few pulsars
with the smaller ones. We test these predictions for some of the supershells
which lie in regions covered by past pulsar surveys. For the smaller
supershells, our results are consistent with the few detected pulsars per
bubble. For the giant supershell GSH 242-03+37, we find the multiple SN
hypothesis inconsistent with current data at the 95% level. We stress the
importance of undertaking deep pulsar surveys in correlation with supershells.
Failure to detect any pulsar enhancement in the largest of them would put
serious constraints on the multiple SN origin for them. Conversely, the
discovery of the pulsar population associated with a supershell would allow a
different/independent approach to the study of pulsar properties.Comment: accepted to ApJ; 17 pages, 2 figures, 1 tabl
Discovery of 10 pulsars in an Arecibo drift-scan survey
We present the results of a 430-MHz survey for pulsars conducted during the
upgrade to the 305-m Arecibo radio telescope. Our survey covered a total of
1147 square degrees of sky using a drift-scan technique. We detected 33
pulsars, 10 of which were not known prior to the survey observations. The
highlight of the new discoveries is PSR J0407+1607, which has a spin period of
25.7 ms, a characteristic age of 1.5 Gyr and is in a 1.8-yr orbit about a
low-mass (>0.2 Msun) companion. The long orbital period and small eccentricity
(e = 0.0009) make the binary system an important new addition to the ensemble
of binary pulsars suitable to test for violations of the strong equivalence
principle. We also report on our initially unsuccessful attempts to detect
optically the companion to J0407+1607 which imply that its absolute visual
magnitude is > 12.1. If, as expected on evolutionary grounds, the companion is
an He white dwarf, our non-detection imples a cooling age of least 1 Gyr.Comment: 8 pages, 3 figures, accepted for publication in MNRA
The SPORTSMART study: a pilot randomised controlled trial of sexually transmitted infection screening interventions targeting men in football club settings
Background:
Uptake of chlamydia screening by men in England has been substantially lower than by women. Non-traditional settings such as sports clubs offer opportunities to widen access. Involving people who are not medically trained to promote screening could optimise acceptability.
Methods:
We developed two interventions to explore the acceptability and feasibility of urine-based sexually transmitted infection (STI) screening interventions targeting men in football clubs. We tested these interventions in a pilot cluster randomised control trial. Six clubs were randomly allocated, two to each of three trial arms: team captain-led and poster STI screening promotion; sexual health adviser-led and poster STI screening promotion; and poster-only STI screening promotion (control/comparator). Primary outcome was test uptake.
Results:
Across the three arms, 153 men participated in the trial and 90 accepted the offer of screening (59%, 95% CI 35% to 79%). Acceptance rates were broadly comparable across the arms: captain-led: 28/56 (50%); health professional-led: 31/46 (67%); and control: 31/51 (61%). However, rates varied appreciably by club, precluding formal comparison of arms. No infections were identified. Process evaluation confirmed that interventions were delivered in a standardised way but the control arm was unintentionally ‘enhanced’ by some team captains actively publicising screening events.
Conclusions:
Compared with other UK-based community screening models, uptake was high but gaining access to clubs was not always easy. Use of sexual health advisers and team captains to promote screening did not appear to confer additional benefit over a poster-promoted approach. Although the interventions show potential, the broader implications of this strategy for UK male STI screening policy require further investigation
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