74,144 research outputs found
The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows
The energy requirements of gamma ray bursts have in past been poorly
constrained because of three major uncertainties: The distances to bursts, the
degree of burst beaming, and the efficiency of gamma ray production. The first
of these has been resolved, with both indirect evidence (the distribution of
bursts in flux and position) and direct evidence (redshifted absorption
features in the afterglow spectrum of GRB 970508) pointing to cosmological
distances. We now wish to address the second uncertainty. Afterglows allow a
statistical test of beaming, described in an earlier paper. In this paper, we
modify a standard fireball afterglow model to explore the effects of beaming on
burst remnant dynamics and afterglow emission. If the burst ejecta are beamed
into angle zeta, the burst remnant's evolution changes qualitatively once its
bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law
of radius, while afterwards, it declines exponentially. This change results in
a broken power law light curve whose late-time decay is faster than expected
for a purely spherical geometry. These predictions disagree with afterglow
observations of GRB 970508. We explored several variations on our model, but
none seems able to change this result. We therefore suggest that this burst is
unlikely to have been highly beamed, and that its energy requirements were near
those of isotropic models. More recent afterglows may offer the first practical
applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures.
Submitted to The Astrophysical Journal, 199
Nonabelian dark matter: models and constraints
Numerous experimental anomalies hint at the existence of a dark matter (DM)
multiplet chi_i with small mass splittings. We survey the simplest such models
which arise from DM in the low representations of a new SU(2) gauge symmetry,
whose gauge bosons have a small mass mu < 1 GeV. We identify preferred
parameters M_chi ~ 1 TeV, mu ~ 100 MeV, alpha_g ~ 0.04 and the chi chi -> 4e
annihilation channel, for explaining PAMELA, Fermi, and INTEGRAL/SPI lepton
excesses, while remaining consistent with constraints from relic density,
diffuse gamma rays and the CMB. This consistency is strengthened if DM
annihilations occur mainly in subhalos, while excitations (relevant to the
excited DM proposal to explain the 511 keV excess) occur in the galactic center
(GC), due to higher velocity dispersions in the GC, induced by baryons. We
derive new constraints and predictions which are generic to these models.
Notably, decays of excited DM states chi' -> chi gamma arise at one loop and
could provide a new signal for INTEGRAL/SPI; big bang nucleosynthesis (BBN)
constraints on the density of dark SU(2) gauge bosons imply a lower bound on
the mixing parameter epsilon between the SU(2) gauge bosons and photon. These
considerations rule out the possibility of the gauge bosons that decay into
e^+e^- being long-lived. We study in detail models of doublet, triplet and
quintuplet DM, showing that both normal and inverted mass hierarchies can
occur, with mass splittings that can be parametrically smaller, e.g., O(100)
keV, than the generic MeV scale of splittings. A systematic treatment of Z_2
symmetry which insures the stability of the intermediate DM state is given for
cases with inverted mass hierarchy, of interest for boosting the 511 keV signal
from the excited dark matter mechanism.Comment: 28 pages, 17 figures; v2. added brief comment, reference
Excited ionic and neutral fragments produced by dissociation of the N2(+)* H band
N I and N II fluorescent radiation was observed when N2 was irradiated by undispersed synchrotron radiation with an upper energy limit of approximately 200 eV. The excited fragments originate from dissociation of a band of excited ionic states of N2 lying between 34.7 and 44 eV
XPS and AFM study of interaction of organosilane and sizing with e-glass fibre surface
Organosilanes are often used in commercial sizings for glass fibres to provide wettability with the resin and promote strong interfacial adhesion to the matrix in a fibre reinforced polymer composite. The silane treatment is introduced as part of a complex deposition from an aqueous emulsion immediately at the spinaret and determines the optimum properties of the cured composite. To understand the interaction of organosilanes contained in sizings for glass surfaces, XPS was used to investigate the adsorption of γ-aminopropyltriethoxysilane (APS) from a simple sizing system containing a polyurethane (PU) film former. It has been found that both APS and the sizing (containing APS and PU) deposits on E-glass fibre surfaces contained components of differing hydrolytic stability. The differences observed in the AFM images of APS coated E-glass fibres before and after water extraction also confirmed that the APS deposit contained components with different water solubility
The Effects of Individual Vessel Quotas in the British Columbia Halibut Fishery
Implementation of Individual vessel quotas (IVQs) in the British Columbia halibut fishery has provided a unique opportunity to examine the effects of this management technique on a previously intense "derby" fishery. This paper describes the changes that have occurred in the fishery since the introduction of individual vessel quotas in 1991. The results presented here are largely based on the findings of two surveys. In September 1993, we conducted in-depth interviews with most of the major halibut processors in British Columbia. These processors reported significant changes in the processors and marketing of halibut. In Spring 1994, we conducted a mail survey of all 435 licensed halibut fishermen. The survey consisted of several series of questions designed to measure changes in fishing operations (crew size, fishing practices, etc.). quota leasing activities, changes in fishing income, and opinions about the effects of IVQs. The results presented here provide important information about the effects of the British Columbia halibut IVQ program to date and will be useful for comparison to similar management programs implemented elsewhere.fishery management, ITQs, Pacific Halibut, Environmental Economics and Policy, International Relations/Trade, Resource /Energy Economics and Policy,
HilE regulates HilD by blocking DNA binding in Salmonella enterica serovar Typhimurium
ABSTRACT
The
Salmonella
type three secretion system (T3SS), encoded in the
Salmonella
pathogenicity island 1 (SPI1) locus, mediates the invasion of the host intestinal epithelium. SPI1 expression is dependent upon three AraC-like regulators: HilD, HilC, and RtsA. These regulators act in a complex feed-forward loop to activate each other and
hilA
, which encodes the activator of the T3SS structural genes. HilD has been shown to be the major integration point of most signals known to activate the expression of the SPI1 T3SS, acting as a switch to control induction of the system. HilE is a negative regulator that acts upon HilD. Here we provide genetic and biochemical data showing that HilE specifically binds to HilD but not to HilC or RtsA. This protein-protein interaction blocks the ability of HilD to bind DNA as shown by both an
in vivo
reporter system and an
in vitro
gel shift assay. HilE does not affect HilD dimerization, nor does it control the stability of the HilD protein. We also investigated the role of HilE during the infection of mice using competition assays. Although deletion of
hilE
does not confer a phenotype, the
hilE
mutation does suppress the invasion defect conferred by loss of FliZ, which acts as a positive signal controlling HilD protein activity. Together, these data suggest that HilE functions to restrict low-level HilD activity, preventing premature activation of SPI1 until positive inputs reach a threshold required to fully induce the system.
IMPORTANCE
Salmonella
is a leading cause of gastrointestinal and systemic disease throughout the world. The SPI1 T3SS is required for
Salmonella
to induce inflammatory diarrhea and to gain access to underlying tissue. A complex regulatory network controls expression of SPI1 in response to numerous physiological inputs. Most of these signals impinge primarily on HilD translation or activity. The system is triggered when HilD activity crosses a threshold that allows efficient activation of its own promoter. This threshold is set by HilE, which binds to HilD to prevent the inevitable minor fluctuations in HilD activity from inappropriately activating the system. The circuit also serves as a paradigm for systems that must integrate numerous environmental parameters to control regulatory output.
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Non- Mn-driven ferroelectricity in antiferromagnetic BaMnO
Using first-principles density functional theory we predict a ferroelectric
ground state -- driven by off-centering of the magnetic Mn ion -- in
perovskite-structure BaMnO.
Our finding is surprising, since the competition between energy-lowering
covalent bond formation, and energy-raising
Coulombic repulsions usually only favors off-centering on the perovskite
-site for non-magnetic ions.
We explain this tendency for ferroelectric off-centering by analyzing the
changes in electronic structure between the centrosymmetric and polar states,
and by calculating the Born effective charges; we find anomalously large values
for Mn and O consistent with our calculated polarization of 12.8 C/cm.
Finally, we suggest possible routes by which the perovskite phase may be
stabilized over the usual hexagonal phase, to enable a practical realization of
a single-phase multiferroic.Comment: 6 pages, 3 figure
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