232 research outputs found
Supernova Kicks and Misaligned Be Star Binaries
Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas
in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a
neutron star. They are found to have non-zero eccentricities and there is
evidence that some systems have a misalignment between the spin axis of the
star and the spin axis of the binary orbit. The eccentricities in these systems
are thought to be caused by a kick to the neutron star during the supernova
that formed it. Such kicks would also give rise to misalignments. In this paper
we investigate the extent to which the same kick distribution can give rise to
both the observed eccentricity distribution and the observed misalignments. We
find that a Maxwellian distribution of velocity kicks with a low velocity
dispersion, , is consistent with the observed
eccentricity distribution but is hard to reconcile with the observed
misalignments, typically . Alternatively a higher velocity kick
distribution, , is consistent with the observed
misalignments but not with the observed eccentricities, unless post-supernova
circularisation of the binary orbits has taken place. We discuss briefly how
this might be achieved.Comment: Accepted for publication in MNRA
Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing
binary EXO 0748-676 started a transition from outburst to quiescence, after it
had been actively accreting for more than 24 years. In a previous work, we
discussed Chandra and Swift observations obtained during the first five months
after this transition. Here, we report on further X-ray observations of EXO
0748-676, extending the quiescent monitoring to 1.6 years. Chandra and
XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal
component that is well-fitted by a neutron star atmosphere model. An additional
hard powerlaw tail is detected that changes non-monotonically over time,
contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux.
The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal
bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the
inferred neutron star effective temperature decreases from ~124 to 109 eV. We
interpret the observed decay as cooling of the neutron star crust and show that
the fractional quiescent temperature change of EXO 0748-676 is markedly smaller
than observed for three other neutron star X-ray binaries that underwent
prolonged accretion outbursts.Comment: Moderate textual revisions according to referee report, accepted for
publication in MNRA
On the orbital parameters of Be/X-ray binaries in the Small Magellanic Cloud
The orbital motion of a neutron star about its optical companion presents a
window through which to study the orbital parameters of that binary system.
This has been used extensively in the Milky Way to calculate these parameters
for several high-mass X-ray binaries. Using several years of RXTE PCA data, we
derive the orbital parameters of four Be/X-ray binary systems in the SMC,
increasing the number of systems with orbital solutions by a factor of three.
We find one new orbital period, confirm a second and discuss the parameters
with comparison to the Galactic systems. Despite the low metallicity in the
SMC, these binary systems sit amongst the Galactic distribution of orbital
periods and eccentricities, suggesting that metallicity may not play an
important role in the evolution of high-mass X-ray binary systems. A plot of
orbital period against eccentricity shows that the supergiant, Be and low
eccentricity OB transient systems occupy separate regions of the parameter
space; akin to the separated regions on the Corbet diagram. Using a Spearman's
rank correlation test, we also find a possible correlation between the two
parameters. The mass functions, inclinations and orbital semimajor axes are
derived for the SMC systems based on the binary parameters and the spectral
classification of the optical counterpart. As a by-product of our work, we
present a catalogue of the orbital parameters for every high-mass X-ray binary
in the Galaxy and Magellanic Clouds for which they are known.Comment: 10 pages, 6 figures, Accepted for publication in MNRA
Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries
Measurement of at least three independent parameters, for example, mass,
radius and spin frequency, of a neutron star is probably the only way to
understand the nature of its supranuclear core matter. Such a measurement is
extremely difficult because of various systematic uncertainties. The lack of
knowledge of several system parameter values gives rise to such systematics.
Low-mass X-ray binaries, which contain neutron stars, provide a number of
methods to constrain the stellar parameters. Joint application of these methods
has a great potential to significantly reduce the systematic uncertainties, and
hence to measure three independent neutron star parameters accurately. Here we
review the methods based on (1) thermonuclear X-ray bursts; (2)
accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic
oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and
(6) binary orbital motions.Comment: 30 pages, 20 figures, 1 table, An Invited and Refereed Review, will
be published in "Advances in Space Research
The predictive role of symptoms in COVID-19 diagnostic models : A longitudinal insight
Acknowledgements 2019nCoV-302 Study Group Members: The NVX-CoV2373-2019nCoV-302 clinical trial was a collective group effort across multiple institutions and locations. Below is a list of sites and staff that significantly contributed to the implementation and conduct of the NVX-CoV2373-2019nCoV-302 clinical trial.Peer reviewe
The geochemical cycling of reactive chlorine through the marine troposphere
Heterogeneous reactions involving seaâsalt aerosol in the marine troposphere are the major global source for volatile inorganic chlorine. We measured reactant and product species hypothesized to be associated with these chemical transformations as a function of phase, particle size, and altitude over the North Atlantic Ocean during the summer of 1988. Concentrations of HCl were typically less than 1.0 ppbv near the sea surface and decreased with altitude and with distance from the U.S. east coast. Concentrations of Cl volatilized from aerosols were generally equivalent to the corresponding concentrations of HCl and ranged from less than detection limits to 125 nmol mâ3 STP. Highest absolute and percentage losses of particulate Cl were typically associated with elevated concentrations of anthropogenic combustion products. Concentrations of product nss SO42â and N03â in coarse aerosol fractions indicate that on average only 38% of measured Clâ deficits could be accounted for by the combined effects of acidâbase desorption and reactions involving nonacidic N gases. We hypothesize a mechanism for the Cl loss initiated by reaction of O3 at seaâsalt aerosol surfaces, generating Cl2 followed by rapid photochemical conversion of Cl2 to HCl via Cl atoms (ClË) and eventual recapture of HCl by the aerosol. Simulations with a zeroâdimension (0âD) photochemical model suggest that oxidation by ClË may be an important tropospheric sink for dimethyl sulfide and hydrocarbons. Under lowâNOx conditions, the rapid cycling of reactive Cl would provide a catalytic loss mechanism for O3, which would possibly explain the low O3 concentrations often observed above the world\u27s oceans
Safety and efficacy of the NVX-CoV2373 coronavirus disease 2019 vaccine at completion of the placebo-controlled phase of a randomized controlled trial
Background:Â The recombinant protein-based vaccine, NVX-CoV2373, demonstrated 89.7% efficacy against coronavirus disease 2019 (COVID-19) in a phase 3, randomized, observer-blinded, placebo-controlled trial in the United Kingdom. The protocol was amended to include a blinded crossover. Data to the end of the placebo-controlled phase are reported.
Methods:Â Adults aged 18â84 years received 2 doses of NVX-CoV2373 or placebo (1:1) and were monitored for virologically confirmed mild, moderate, or severe COVID-19 (onset from 7 days after second vaccination). Participants who developed immunoglobulin G (IgG) against nucleocapsid protein but did not show symptomatic COVID-19 were considered asymptomatic. Secondary outcomes included anti-spike (S) IgG responses, wild-type virus neutralization, and T-cell responses.
Results:Â Of 15 185 participants, 13 989 remained in the per-protocol efficacy population (6989 NVX-CoV2373, 7000 placebo). At a maximum of 7.5 months (median, 4.5) postvaccination, there were 24 cases of COVID-19 among NVX-CoV2373 recipients and 134 cases among placebo recipients, a vaccine efficacy of 82.7% (95% confidence interval [CI], 73.3%â88.8%). Vaccine efficacy was 100% (95% CI, 17.9%â100.0%) against severe disease and 76.3% (95% CI, 57.4%â86.8%) against asymptomatic disease. High anti-S and neutralization responses to vaccination were evident, together with S-proteinâspecific induction of interferon-Îł secretion in peripheral blood T cells. Incidence of serious adverse events and adverse events of special interest were similar between groups.
Conclusions:Â A 2-dose regimen of NVX-CoV2373 conferred a high level of ongoing protection against asymptomatic, symptomatic, and severe COVID-19 through >6 months postvaccination. A gradual decrease of protection suggests that a booster may be indicated.
Clinical Trials Registration:Â EudraCT, 2020-004123-16
Safety and Efficacy of the NVX-CoV2373 Coronavirus Disease 2019 Vaccine at Completion of the Placebo-Controlled Phase of a Randomized Controlled Trial
Acknowledgements The study and article were funded by Novavax. We would like to thank all the study participants for their commitment to this study. We also acknowledge the investigators and their study teams for their hard work and dedication. In addition, we would like to thank the National Institute for Health Research, representatives from the Department of Health and Social Care laboratories and NHS Digital and the members of the UK Vaccine Task Force. Editorial support was provided by Kelly Cameron of Ashfield MedComms, an Inizio company Funding This work was funded by Novavax, and the sponsor had primary responsibility for study design, study vaccines, protocol development, study monitoring, data management, and statistical analyses. All authors reviewed and approved the manuscript before submission. LF reports a position as a prior full-time employee, now contractor to Novavax re-imbursed hourly for work performed on this study and in analyses and drafting this report. IC reports providing medical writing support for this work as an employee of NovavaxPeer reviewedPublisher PD
A Complete Pathway Model for Lipid A Biosynthesis in Escherichia coli.
Lipid A is a highly conserved component of lipopolysaccharide (LPS), itself a major component of the outer membrane of Gram-negative bacteria. Lipid A is essential to cells and elicits a strong immune response from humans and other animals. We developed a quantitative model of the nine enzyme-catalyzed steps of Escherichia coli lipid A biosynthesis, drawing parameters from the experimental literature. This model accounts for biosynthesis regulation, which occurs through regulated degradation of the LpxC and WaaA (also called KdtA) enzymes. The LpxC degradation signal appears to arise from the lipid A disaccharide concentration, which we deduced from prior results, model results, and new LpxK overexpression results. The model agrees reasonably well with many experimental findings, including the lipid A production rate, the behaviors of mutants with defective LpxA enzymes, correlations between LpxC half-lives and cell generation times, and the effects of LpxK overexpression on LpxC concentrations. Its predictions also differ from some experimental results, which suggest modifications to the current understanding of the lipid A pathway, such as the possibility that LpxD can replace LpxA and that there may be metabolic channeling between LpxH and LpxB. The model shows that WaaA regulation may serve to regulate the lipid A production rate when the 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) concentration is low and/or to control the number of KDO residues that get attached to lipid A. Computation of flux control coefficients showed that LpxC is the rate-limiting enzyme if pathway regulation is ignored, but that LpxK is the rate-limiting enzyme if pathway regulation is present, as it is in real cells. Control also shifts to other enzymes if the pathway substrate concentrations are not in excess. Based on these results, we suggest that LpxK may be a much better drug target than LpxC, which has been pursued most often
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