540 research outputs found
Chandra Smells a RRAT: X-ray Detection of a Rotating Radio Transient
"Rotating RAdio Transients" (RRATs) are a newly discovered astronomical
phenomenon, characterised by occasional brief radio bursts, with average
intervals between bursts ranging from minutes to hours. The burst spacings
allow identification of periodicities, which fall in the range 0.4 to 7
seconds. The RRATs thus seem to be rotating neutron stars, albeit with
properties very different from the rest of the population. We here present the
serendipitous detection with the Chandra X-ray Observatory of a bright
point-like X-ray source coincident with one of the RRATs. We discuss the
temporal and spectral properties of this X-ray emission, consider counterparts
in other wavebands, and interpret these results in the context of possible
explanations for the RRAT population.Comment: 5 pages, 2 b/w figures, 1 color figure. To appear in the proceedings
of "Isolated Neutron Stars", Astrophysics & Space Science, in pres
Improving the assessment of gestational age in a Zimbabwean population
Objectives: To evaluate the performance and the utility of using birthweight‐adjusted scores of Dubowitz and Ballard methods of estimating gestational age in a Zimbabwean population. Method: The Dubowitz and the Ballard methods of estimating gestational age were administered to 364 African newborn infants with a known last menstrual period (LMP) at Harare Maternity Hospital. Results: Both methods were good predictors of gestational age useful in differentiating term from pre‐term infants. Our regression line was Y(LMP gestational age)=23.814+0.301*score for the Dubowitz and Y(LMP gestational age)=24.493+0.420*score for the Ballard method. Addition of birthweight to the regression models improved prediction of gestational age; Y(LMP gestational age)=23.512+0.219*score+0.0015*grams for Dubowitz and Y(LMP gestational age)=24.002+0.292*score+0.0016*grams for Ballard method. Conclusions: We recommend the use of our birthweight‐adjusted maturity scales; the Dubowitz for studies of prematurity, and the Ballard for routine clinical practice.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135611/1/ijgo7.pd
Dijet photoproduction of massless charm jets at next-to-leading order of QCD
We compute the charm dijet photoproduction cross section at next-to-leading
order of QCD in the zero-mass variable flavour number scheme, i.e. with active
charm quarks in the proton and photon. The results are compared to recent
measurements from the ZEUS experiment at HERA. The predictions for various
distributions agree well with the data, in particular for large momentum
fractions of the the partons in the photon, where direct photon processes
dominate. At low momentum fractions, the predictions are quite sensitive to the
charm content in the photon. The experimental data are shown to favour
parameterizations with a substantial charm quark density such as the one
proposed by Cornet et al.Comment: 18 pages, 11 figure
Twist-2 Heavy Flavor Contributions to the Structure Function
The twist--2 heavy flavor contributions to the polarized structure function
are calculated. We show that this part of is related
to the heavy flavor contribution to by the Wandzura--Wilczek
relation to all orders in the strong coupling constant. Numerical results are
presented.Comment: 17 pages LATEX, 1 style files, 4 figure
On the mechanisms of heavy-quarkonium hadroproduction
We discuss the various mechanisms potentially at work in hadroproduction of
heavy quarkonia in the light of computations of higher-order QCD corrections
both in the Colour-Singlet (CS) and Colour-Octet (CO) channels and the
inclusion of the contribution arising from the s-channel cut in the CS channel.
We also discuss new observables meant to better discriminate between these
different mechanisms.Comment: Invited review talk at 3rd International Conference On Hard And
Electromagnetic Probes Of High-Energy Nuclear Collisions (HP2008), 8-14 June
2008, Illa da Toxa, Galicia, Spain. 11 pages, 21 figures, LaTeX, uses
svjour.cls and svepj.clo (included
Inclusive Dijet Production at HERA: Direct Photon Cross Sections in Next-To-Leading Order QCD
We have calculated inclusive two-jet cross sections in next-to-leading order
QCD for direct photoproduction in low collisions at HERA. Infrared
and collinear singularities in real and virtual contributions are cancelled
with the phase space slicing method. Analytical formulas for the different
contributions giving the dependence on the slicing parameter are presented.
Various one- and two-jet distributions have been computed demonstrating the
flexibility of the method.Comment: 30 pages, latex, 11 figures appended as uuencoded fil
Gravitational collapse without a remnant
We investigate the gravitational collapse of a spherically symmetric,
inhomogeneous star, which is described by a perfect fluid with heat flow and
satisfies the equation of state or p=C\rho^\ga at its center.
Different from the ordinary process of gravitational collapsing, the energy of
the whole star is emitted into space. And the remaining spacetime is a
Minkowski one at the end of the process.Comment: 9 pages, 9 figures, to appear in Int. J. Theor. Phy
The fully differential single-top-quark cross section in next-to-leading order QCD
We present a new next-to-leading order calculation for fully differential
single-top-quark final states. The calculation is performed using phase space
slicing and dipole subtraction methods. The results of the methods are found to
be in agreement. The dipole subtraction method calculation retains the full
spin dependence of the final state particles. We show a few numerical results
to illustrate the utility and consistency of the resulting computer
implementations.Comment: 37 pages, latex, 2 ps figure
Hadronic final states in deep-inelastic scattering with Sherpa
We extend the multi-purpose Monte-Carlo event generator Sherpa to include
processes in deeply inelastic lepton-nucleon scattering. Hadronic final states
in this kinematical setting are characterised by the presence of multiple
kinematical scales, which were up to now accounted for only by specific
resummations in individual kinematical regions. Using an extension of the
recently introduced method for merging truncated parton showers with
higher-order tree-level matrix elements, it is possible to obtain predictions
which are reliable in all kinematical limits. Different hadronic final states,
defined by jets or individual hadrons, in deep-inelastic scattering are
analysed and the corresponding results are compared to HERA data. The various
sources of theoretical uncertainties of the approach are discussed and
quantified. The extension to deeply inelastic processes provides the
opportunity to validate the merging of matrix elements and parton showers in
multi-scale kinematics inaccessible in other collider environments. It also
allows to use HERA data on hadronic final states in the tuning of hadronisation
models.Comment: 32 pages, 22 figure
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