19,533 research outputs found
The cuticle
The nematode cuticle is an extremely flexible and resilient exoskeleton that permits locomotion via
attachment to muscle, confers environmental protection and allows growth by molting. It is synthesised five
times, once in the embryo and subsequently at the end of each larval stage prior to molting. It is a highly
structured extra-cellular matrix (ECM), composed predominantly of cross-linked collagens, additional
insoluble proteins termed cuticlins, associated glycoproteins and lipids. The cuticle collagens are encoded by a large gene family that are subject to strict patterns of temporal regulation. Cuticle collagen biosynthesis
involves numerous co- and post-translational modification, processing, secretion and cross-linking steps that
in turn are catalysed by specific enzymes and chaperones. Mutations in individual collagen genes and their
biosynthetic pathway components can result in a range of defects from abnormal morphology (dumpy and
blister) to embryonic and larval death, confirming an essential role for this structure and highlighting its
potential as an ECM experimental model system
A signature of anisotropic bubble collisions
Our universe may have formed via bubble nucleation in an eternally-inflating
background. Furthermore, the background may have a compact dimension---the
modulus of which tunnels out of a metastable minimum during bubble
nucleation---which subsequently grows to become one of our three large spatial
dimensions. When in this scenario our bubble universe collides with other ones
like it, the collision geometry is constrained by the reduced symmetry of the
tunneling instanton. While the regions affected by such bubble collisions still
appear (to leading order) as disks in an observer's sky, the centers of these
disks all lie on a single great circle, providing a distinct signature of
anisotropic bubble nucleation.Comment: 10 pages, 5 figures; v2: crucial error corrected, conclusions revise
Speed of reaction diffusion in embryogenesis
Reaction diffusion systems have been proposed as mechanisms for patterning during many stages of embryonic development. While much attention has been focused on the study of the steady state patterns formed and the robustness of pattern selection, much less is known about the time scales required for pattern formation. Studies of gradient formation by the diffusion of a single morphogen from a localized source have shown that patterning can occur on realistic time scales over distances of a millimeter or less. Reaction diffusion has the potential to give rise to patterns on a faster time scale, since all points in the domain can act as sources of morphogen. However, the speed at which patterning can occur has hitherto not been explored in depth. In this paper, we investigate this issue in specific reaction diffusion models and address the question of whether patterning via reaction diffusion is fast enough to be applicable to morphogenesis
An XMM-Newton observation of the Narrow Line Seyfert 1 Galaxy, Markarian 896
XMM-Newton observations of the NLS1 Markarian 896 are presented. Over the
2-10 keV band, an iron emission line, close to 6.4 keV, is seen. The line is
just resolved and has an equivalent width of ~170 eV. The broad-band spectrum
is well modelled by a power law slope of gamma ~ 2.03, together with two
blackbody components to fit the soft X-ray excess. Using a more physical
two-temperature Comptonisation model, a good fit is obtained for an input
photon distribution of kT ~ 60eV and Comptonising electron temperatures of ~0.3
and 200 keV. The soft excess cannot be explained purely through the
reprocessing of a hard X-ray continuum by an ionised disc reflector.Comment: 6 pages, 4 figures, accepted by MNRA
Complex pattern formation in reaction diffusion systems with spatially-varying parameters
Spontaneous pattern formation in reaction–diffusion systems on a spatially homogeneous domain has been well studied. However, in embryonic development and elsewhere, pattern formation often takes place on a spatially heterogeneous background. We explore the effects of spatially varying parameters on pattern formation in one and two dimensions using the Gierer–Meinhardt reaction–diffusion model. We investigate the effect of the wavelength of a pre-pattern and demonstrate a novel form of moving pattern. We find that spatially heterogeneous parameters can both increase the range and complexity of possible patterns and enhance the robustness of pattern selection
A search for thermal X-ray signatures in Gamma-Ray Bursts I: Swift bursts with optical supernovae
The X-ray spectra of Gamma-Ray Bursts can generally be described by an
absorbed power law. The landmark discovery of thermal X-ray emission in
addition to the power law in the unusual GRB 060218, followed by a similar
discovery in GRB 100316D, showed that during the first thousand seconds after
trigger the soft X-ray spectra can be complex. Both the origin and prevalence
of such spectral components still evade understanding, particularly after the
discovery of thermal X-ray emission in the classical GRB 090618. Possibly most
importantly, these three objects are all associated with optical supernovae,
begging the question of whether the thermal X-ray components could be a result
of the GRB-SN connection, possibly in the shock breakout. We therefore
performed a search for blackbody components in the early Swift X-ray spectra of
11 GRBs that have or may have associated optical supernovae, accurately
recovering the thermal components reported in the literature for GRBs 060218,
090618 and 100316D. We present the discovery of a cooling blackbody in GRB
101219B/SN2010ma, and in four further GRB-SNe we find an improvement in the fit
with a blackbody which we deem possible blackbody candidates due to
case-specific caveats. All the possible new blackbody components we report lie
at the high end of the luminosity and radius distribution. GRB 101219B appears
to bridge the gap between the low-luminosity and the classical GRB-SNe with
thermal emission, and following the blackbody evolution we derive an expansion
velocity for this source of order 0.4c. We discuss potential origins for the
thermal X-ray emission in our sample, including a cocoon model which we find
can accommodate the more extreme physical parameters implied by many of our
model fits.Comment: 16 pages, 6 figures, accepted for MNRA
X-ray and UV observations of V751 Cyg in an optical high state
Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a
claim of a super-soft spectrum in the optical low state. Methods: A
serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray
and UV observations, have provided the best such data on this source so far.
These optical high-state datasets are used to study the flux and spectral
variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show
evidence for modulation of the X-rays for the first time at the known 3.467 hr
orbital period of V751 Cyg. In two Swift observations, taken ten days apart,
the mean X-ray flux remained unchanged, while the UV source brightened by half
a magnitude. The X-ray spectrum was not super-soft during the optical high
state, but rather due to multi-temperature optically thin emission, with
significant (10^{21-22} cm^-2) absorption, which was higher in the observation
by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum,
suggesting that the modulation is related to absorption, perhaps linked to the
azimuthally asymmetric wind absorption seen previously in H-alpha.Comment: 6 pages, 9 figures, accepted for publication in A&
Relativistic Effects on the Appearance of a Clothed Black Hole
For an accretion disk around a black hole, the strong relativistic effects
affect every aspect of the radiation from the disk, including its spectrum,
light-curve, and image. This work investigates in detail how the images of a
thin disk around a black hole will be distorted, and what the observer will see
from different viewing angles and in different energy bands.Comment: 4 pages, 5 figures. Based on the poster presented at the Sixth
Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17,
2002). Color versions of figures are given separatel
An Updated Ultraviolet Calibration for the Swift/UVOT
We present an updated calibration of the Swift/UVOT broadband ultraviolet
(uvw1, uvm2, and uvw2) filters. The new calibration accounts for the ~1% per
year decline in the UVOT sensitivity observed in all filters, and makes use of
additional calibration sources with a wider range of colours and with HST
spectrophotometry. In this paper we present the new effective area curves and
instrumental photometric zeropoints and compare with the previous calibration.Comment: 4 pages, 3 figures, 2 tables. Presented at GRB 2010 symposium,
Annapolis, November 2010 to be published in American Institute of Physics
Conference Serie
Resolving the large scale spectral variability of the luminous Seyfert 1 galaxy 1H 0419-577: Evidence for a new emission component and absorption by cold dense matter
An XMM-Newton observation of the luminous Seyfert 1 galaxy 1H 0419-577 in
September 2002, when the source was in an extreme low-flux state, found a very
hard X-ray spectrum at 1-10 keV with a strong soft excess below ~1 keV.
Comparison with an earlier XMM-Newton observation when 1H 0419-577 was `X-ray
bright' indicated the dominant spectral variability was due to a steep power
law or cool Comptonised thermal emission. Four further XMM-Newton observations,
with 1H 0419-577 in intermediate flux states, now support that conclusion,
while we also find the variable emission component in intermediate state
difference spectra to be strongly modified by absorption in low ionisation
matter. The variable `soft excess' then appears to be an artefact of absorption
of the underlying continuum while the `core' soft emission can be attributed to
recombination in an extended region of more highly ionised gas. We note the
wider implications of finding substantial cold dense matter overlying (or
embedded in) the X-ray continuum source in a luminous Seyfert 1 galaxy.Comment: 34 pages, 15 figures, submitted to Ap
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