15,036 research outputs found
The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005
On 2004 December 27, a highly-energetic giant flare was recorded from the
magnetar candidate SGR 1806-20. In the months preceding this flare, the
persistent X-ray emission from this object began to undergo significant
changes. Here, we report on the evolution of key spectral and temporal
parameters prior to and following this giant flare. Using the Rossi X-ray
Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the
spin-down rate of this SGR varied erratically in the months before and after
the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for
a discrete jump in spin frequency at the time of the December 27th flare
(|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong
correlation between pulsed flux and torque consistent with the model for
magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov &
Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR
1806-20 changes drastically following the flare. Using the Chandra X-ray
Observatory and other publicly available imaging X-ray detector observations,
we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual
magnetar persistent emission spectral model of a power-law plus a blackbody
provides an excellent fit to the data. We confirm the earlier finding by
Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between
1993 and 2004. Contrary to the direct correlation between torque and spectral
hardness proposed by Mereghetti et al., we find evidence for a sudden torque
change that triggered a gradual hardening of the energy spectrum on a timescale
of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and
phase-averaged of SGR 1806-20 all peak months before the flare epoch.Comment: 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To
appear in the Oct 20 2006 editio
Physical Mechanisms for the Variable Spin-down of SGR 1900+14
We consider the physical implications of the rapid spindown of Soft Gamma
Repeater 1900+14, and of the apparent "braking glitch", \Delta P/P = l x 10^-4,
that was concurrent with the Aug. 27th giant flare. A radiation-hydrodynamical
outflow associated with the flare could impart the required torque, but only if
the dipole magnetic field is stronger than ~ 10^14 G and the outflow lasts
longer and/or is more energetic than the observed X-ray flare. A positive
period increment is also a natural consequence of a gradual, plastic
deformation of the neutron star crust by an intense magnetic field, which
forces the neutron superfluid to rotate more slowly than the crust. Sudden
unpinning of the neutron vortex lines during the August 27th event would then
induce a glitch opposite in sign to those observed in young pulsars, but of a
much larger magnitude as a result of the slower rotation.
The change in the persistent X-ray lightcurve following the August 27 event
is ascribed to continued particle heating in the active region of that
outburst. The enhanced X-ray output can be powered by a steady current flowing
through the magnetosphere, induced by the twisting motion of the crust. The
long term rate of spindown appears to be accelerated with respect to a simple
magnetic dipole torque. Accelerated spindown of a seismically-active magnetar
will occur when its persistent output of Alfven waves and particles exceeds its
spindown luminosity. We suggest that SGRs experience some episodes of relative
inactivity, with diminished spindown rates, and that such inactive magnetars
are observed as Anomalous X-ray Pulsars (AXPs). The rapid reappearence of
persistent X-ray emission following August 27 flare gives evidence against
accretion-powered models.Comment: 24 pages, no figure
Gravitational radiation from collapsing magnetized dust
In this article we study the influence of magnetic fields on the axial
gravitational waves emitted during the collapse of a homogeneous dust sphere.
We found that while the energy emitted depends weakly on the initial matter
perturbations it has strong dependence on the strength and the distribution of
the magnetic field perturbations. The gravitational wave output of such a
collapse can be up to an order of magnitude larger or smaller calling for
detailed numerical 3D studies of collapsing magnetized configurations
ORGANIZATION OF THE SALIVARY-GLAND CHROMOSOME AS REVEALED BY THE PATTERN OF INCORPORATION OF H3-THYMIDINE
Deformation of Nuclei Close to the Two-Neutron Drip Line in Mg Region
We perform the Hartree-Fock-Bogoliubov (HFB) calculations for ground states
of even Mg isotopes using the Skyrme force and a density-dependent zero-range
pairing force. The HFB equation is solved in a three-dimensional cartesian
mesh, and a convergence of deformation is carefully examined with respect to a
cut-off radius for a check of the calculations. We discuss systematics of the
two-neutron separation energy, deformation and root-mean-square radius. We have
found that 36,38,40Mg have appreciable static deformation, where 40Mg is a
two-neutron drip-line nucleus in our calculation, and the deformations of the
neutron and proton are different in these three nuclei. The deformation
property is analyzed on the basis of the single-particle diagram. It is shown
that N=28 is not a closed shell in Mg as well as Si.Comment: 13 pages, 8 Postscript figures, submitted to Nucl.Phy
Creative teaching and learning: towards a common discourse and practice
There has recently been a call for more pedagogic comparative research to counter the dominance of structural and policy led studies. At the same time there is also a necessity to provide alternative comparative research to that concerned with global standardising performance and performativity strategies. The research, on which this paper is based, fulfils both these aims by investigating creative teaching and learning in nine European countries at classroom level using ethnographic methods in a small number of sites for each partner. The research partners share a common discourse of pedagogy that we are calling creative teaching and learning, a common humanitarian discourse and the ethnographic methodology for the research was a strong framework to counter differing cultural approaches to research. The article analytically characterises some significant strategies used by teachers, the creative learning experienced and the meaning that the experiences had for the students involved. We conclude that this research has laid the basis for a common discourse for further research in a comparative approach that will investigate commonalities to build an understanding of international creative pedagogy and investigate differences to enhance the conceptualisation of it
A note on leapfrogging vortex rings
In this paper we provide examples, by numerical simulation using the Navier-Stokes equations for axisymmetric laminar flow, of the 'leapfrogging' motion of two, initially identical, vortex rings which share a common axis of symmetry. We show that the number of clear passes that each ring makes through the other increases with Reynolds number, and that as long as the configuration remains stable the two rings ultimately merge to form a single vortex ring
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