8,970 research outputs found
The uncoupling limit of identical Hopf bifurcations with an application to perceptual bistability
We study the dynamics arising when two identical oscillators are coupled near
a Hopf bifurcation where we assume a parameter uncouples the system
at . Using a normal form for identical systems undergoing
Hopf bifurcation, we explore the dynamical properties. Matching the normal form
coefficients to a coupled Wilson-Cowan oscillator network gives an
understanding of different types of behaviour that arise in a model of
perceptual bistability. Notably, we find bistability between in-phase and
anti-phase solutions that demonstrates the feasibility for synchronisation to
act as the mechanism by which periodic inputs can be segregated (rather than
via strong inhibitory coupling, as in existing models). Using numerical
continuation we confirm our theoretical analysis for small coupling strength
and explore the bifurcation diagrams for large coupling strength, where the
normal form approximation breaks down
Is pulsar B0656+14 a very nearby RRAT source?
The recently discovered RRAT sources are characterized by very bright radio
bursts which, while being periodically related, occur infrequently. We find
bursts with the same characteristics for the known pulsar B0656+14. These
bursts represent pulses from the bright end of an extended smooth pulse-energy
distribution and are shown to be unlike giant pulses, giant micropulses or the
pulses of normal pulsars. The extreme peak-fluxes of the brightest of these
pulses indicates that PSR B0656+14, were it not so near, could only have been
discovered as an RRAT source. Longer observations of the RRATs may reveal that
they, like PSR B0656+14, emit weaker emission in addition to the bursts.Comment: 4 pages, 4 figures, accepted by ApJ
Stokes tomography of radio pulsar magnetospheres. I. Linear polarization
Polarimetric studies of pulsar radio emission traditionally concentrate on
how the Stokes vector (I, Q, U, V) varies with pulse longitude, with special
emphasis on the position angle (PA) swing of the linearly polarized component.
The interpretation of the PA swing in terms of the rotating vector model is
limited by the assumption of an axisymmetric magnetic field and the degeneracy
of the output with respect to the orientation and magnetic geometry of the
pulsar; different combinations of the latter two properties can produce similar
PA swings. This paper introduces Stokes phase portraits as a supplementary
diagnostic tool with which the orientation and magnetic geometry can be
inferred more accurately. The Stokes phase portraits feature unique patterns in
the I-Q, I-U, and Q-U planes, whose shapes depend sensitively on the magnetic
geometry, inclination angle, beam and polarization patterns, and emission
altitude. We construct look-up tables of Stokes phase portraits and PA swings
for pure and current-modified dipole fields, filled core and hollow cone beams,
and two empirical linear polarization models, L/I = \cos \theta_0 and L/I =
\sin \theta_0, where \theta_0 is the colatitude of the emission point. We
compare our look-up tables to the measured phase portraits of 24 pulsars in the
European Pulsar Network online database. We find evidence in 60% of the objects
that the radio emission region may depart significantly from low altitudes,
even when the PA swing is S-shaped and/or the pulse-width-period relation is
well satisfied. On the other hand, the data are explained adequately if the
emission altitude exceeds ~10% of the light cylinder radius. We conclude that
Stokes phase portraits should be analysed concurrently with the PA swing and
pulse profiles in future when interpreting radio pulsar polarization data.Comment: 60 pages, 58 figures, submitted to MNRAS, accepted 13 Oct 201
Prolonged FGF signaling is necessary for lung and liver induction in Xenopus
BACKGROUND: FGF signaling plays numerous roles during organogenesis of the embryonic gut tube. Mouse explant studies suggest that different thresholds of FGF signaling from the cardiogenic mesoderm induce lung, liver, and pancreas lineages from the ventral foregut progenitor cells. The mechanisms that regulate FGF dose in vivo are unknown. Here we use Xenopus embryos to examine the hypothesis that a prolonged duration of FGF signaling from the mesoderm is required to induce foregut organs. RESULTS: We show that both mesoderm and FGF signaling are required for liver and lung development in Xenopus; formally demonstrating that this important step in organ induction is conserved with other vertebrate species. Prolonged contact with the mesoderm and persistent FGF signaling through both MEK and PI3K over an extended period of time are required for liver and lung specification. Inhibition of FGF signaling results in reduced liver and lung development, with a modest expansion of the pancreas/duodenum progenitor domain. Hyper-activation of FGF signaling has the opposite effect expanding liver and lung gene expression and repressing pancreatic markers. We show that FGF signaling is cell autonomously required in the endoderm and that a dominant negative FGF receptor decreases the ability of ventral foregut progenitor cells to contribute to the lung and liver buds. CONCLUSIONS: These results suggest that the liver and lungs are specified at progressively later times in development requiring mesoderm contact for different lengths of time. Our data suggest that this is achieved at least in part through prolonged FGF signaling. In addition to providing a foundation for further mechanistic studies on foregut organogenesis using the experimental advantages of the Xenopus system, these data have implications for the directed differentiation of stem cells into foregut lineages
Coherently Dedispersed Polarimetry of Millisecond Pulsars
We present a large sample of high-precision, coherently-dedispersed
polarization profiles of millisecond pulsars (MSPs) at frequencies between 410
and 1414 MHz. These data include the first polarimetric observations of several
of the pulsars, and the first low-frequency polarization profiles for others.
Our observations support previous suggestions that the pulse shapes and
polarimetry of MSPs are more complex than those of their slower relatives. An
immediate conclusion is that polarimetry-based classification schemes proposed
for young pulsars are of only limited use when applied to millisecond pulsars.Comment: 28 pages, 10 figures. Text matches version that appeared in ApJS.
Full paper with high-resolution figures available at
ftp://ftp.jb.man.ac.uk/pub/psr/papers/msppolpton.ps.g
A Strong Upper Limit on the Pulsed Radio Luminosity of the Compact Object 1RXS J141256.0+792204
The ROSAT X-ray source 1RXS J141256.0+792204 has recently been identified as
a likely compact object whose properties suggest it could be a very nearby
radio millisecond pulsar at d = 80 - 260pc. We investigated this hypothesis by
searching for radio pulsations using the Westerbork Synthesis Radio Telescope.
We observed 1RXS J141256.0+792204 at 385 and 1380MHz, recording at high time
and frequency resolution in order to maintain sensitivity to millisecond
pulsations. These data were searched both for dispersed single pulses and using
Fourier techniques sensitive to constant and orbitally modulated periodicities.
No radio pulsations were detected in these observations, resulting in pulsed
radio luminosity limits of L_400 ~ 0.3 (d/250pc)^2 mJy kpc^2 and L_1400 ~ 0.03
(d/250pc)^2 mJy kpc^2 at 400 and 1400MHz respectively. The lack of detectable
radio pulsations from 1RXS J141256.0+792204 brings into question its
identification as a nearby radio pulsar, though, because the pulsar could be
beamed away from us, this hypothesis cannot be strictly ruled out.Comment: To appear in A&A. 3 page
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Complete Experimental Structure Determination of the p(3x2)pg Phase of Glycine on Cu{110}
We present a quantitative low energy electron diffraction (LEED) surface-crystallograpic
study of the complete adsorption geometry of glycine adsorbed on Cu{110} in the ordered
p(3×2) phase. The glycine molecules form bonds to the surface through the N atoms of the
amino group and the two O atoms of the de-protonated carboxylate group, each with separate
Cu atoms such that every Cu atom in the first layer is involved in a bond. Laterally, N atoms are
nearest to the atop site (displacement 0.41 Ã…). The O atoms are asymmetrically displaced from
the atop site by 0.54 Ã… and 1.18 Ã… with two very different O-Cu bond lengths of 1.93 Ã… and
2.18 Ã…. The atom positions of the upper-most Cu layers show small relaxations within 0.07 Ã…
of the bulk-truncated surface geometry. The unit cell of the adsorbate layer consists of two
glycine molecules, which are related by a glide-line symmetry operation. This study clearly
shows that a significant coverage of adsorbate structures without this glide-line symmetry must
be rejected, both on the grounds of the energy dependence of the spot intensities (LEED-IV
curves) and of systematic absences in the LEED pattern
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