6,062 research outputs found
Detection Techniques of Microsecond Gamma-Ray Bursts using Ground-Based Telescopes
Gamma-ray observations above 200 MeV are conventionally made by
satellite-based detectors. The EGRET detector on the Compton Gamma Ray
Observatory (CGRO) has provided good sensitivity for the detection of bursts
lasting for more than 200 ms. Theoretical predictions of high-energy gamma-ray
bursts produced by quantum-mechanical decay of primordial black holes (Hawking
1971) suggest the emission of bursts on shorter time scales. The final stage of
a primordial black hole results in a burst of gamma-rays, peaking around 250
MeV and lasting for a tenth of a microsecond or longer depending on particle
physics. In this work we show that there is an observational window using
ground-based imaging Cherenkov detectors to measure gamma-ray burst emission at
energies E greater than 200 MeV. This technique, with a sensitivity for bursts
lasting nanoseconds to several microseconds, is based on the detection of
multi-photon-initiated air showers.Comment: accepted for publication in the Astrophysical Journa
SGARFACE: A Novel Detector For Microsecond Gamma Ray Bursts
The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) is operated at
the Whipple Observatory utilizing the Whipple 10m gamma-ray telescope. SGARFACE
is sensitive to gamma-ray bursts of more than 100MeV with durations from 100ns
to 35us and provides a fluence sensitivity as low as 0.8 gamma-rays per m^2
above 200MeV (0.05 gamma-rays per m^2 above 2GeV) and allows to record the
burst time structure.Comment: 29 pages, 14 figures, accepted for publication in Astroparticle
Physic
Mass Loss, Destruction and Detection of Sun-grazing and -impacting Cometary Nuclei
[Abridged] Sun-grazing comets almost never re-emerge, but their sublimative
destruction near the sun has only recently been observed directly, while
chromospheric impacts have not yet been seen, nor impact theory developed.
Employing simple analytic models to describe comet destruction near the Sun and
to enable the estimation of observable signatures, we find analytic solutions
for the mass as a function of distance from the Sun, for insolation
sublimation, impact ablation and explosion. Sun-grazers are found to fall into
three regimes based on initial mass and perihelion: sublimation-, ablation-,
and explosion-dominated. Most sun-grazers are destroyed sublimatively, and our
analytic results are similar to numerical models. Larger masses (>10^11g) with
small perihelion (q<1.01Rsun) ablation dominates but results are sensitive to
nucleus strength, Pc, and entry angle to the vertical, phi.
Nuclei with initial mass >~10^10g (Pc/10^6 (dyne/cm^2) sec (phi))^3 are fully
ablated before exploding, though the hot wake itself explodes. For most
sun-impactors sec(phi)~1. For small perihelion the ablation regime applies to
moderate masses ~10^13-16 g impactors unless Pc is very low. For higher masses,
or smaller perihelia, nuclei reach higher densities where ram pressure causes
catastrophic explosion. For perihelion 10^11 g
nuclei are destroyed by ablation or explosion (depending on phi and Pc) in the
chromosphere, producing flare-like events with cometary abundance spectra. For
all plausible masses and physical parameters, nuclei are destroyed above the
photosphere.Comment: 12 pages, 5 figures, Accepted A&
UNLV College of Education Multicultural & Diversity Newsletter
Each morning I wound my way up the steep hill along the deeply rutted dirt path, exchanging daily maaa\u27s with five bleating sheep and shouting out, ¡Hola! in response to the children who gleefully identified me as ¡Gringa! Women and children, colorful bowls of cooked maize balanced atop their heads, sauntered to and from Maria Elena\u27s where their maize would be ground; at home the dough would be shaped and flattened into tortillas, the mainstay of every meal in the small Guatemalan village of San Juan
A Complete Expression Profile of Matrix-Degrading Metalloproteinases in Dupuytren’s Disease
Dupuytren’s disease (DD) is a common fibrotic condition of the palmar fascia, leading to deposition of collagen-rich cords and finger contractions. The metzincin superfamily contains key enzymes in the turnover of collagen and other extracellular matrix macromolecules. A number of broad-spectrum matrix metalloproteinase inhibitors, used in cancer clinical trials, caused side effects of DD-like contractures. We tested the hypothesis that changes in the expression of specific metalloproteinases underlie or contribute to the fibrosis and contracture seen in DD. We collected tissue from patients with DD and used normal palmar fascia as a control. We profiled the expression of the entire matrix metalloproteinase (MMP), tissue inhibitor of metalloproteinases (TIMP), and a disintegrin and metalloproteinase domain with thrombospondin motif (ADAMTS) gene families in these tissues using real-time reverse-transcription polymerase chain reaction. A number of metalloproteinases and inhibitors are regulated in DD. The expression of 3 key collagenases, MMP1, MMP13, and MMP14 is increased significantly in the DD nodule, as is the expression of the collagen biosynthetic enzyme ADAMTS14. The expression of MMP7, an enzyme with broad substrate specificity, is increased in the DD nodule and remains equally expressed in the DD cord. TIMP1 expression is increased significantly in DD nodule compared with normal palmar fascia. This study measured the expression of all MMP, ADAMTS, and TIMP genes in DD. Contraction and fibrosis may result from: (1) increased collagen biosynthesis mediated by increased ADAMTS-14; (2) an increased level of TIMP-1 blocking MMP-1– and MMP-13–mediated collagenolysis; and (3) contraction enabled by MMP-14–mediated pericellular collagenolysis (and potentially MMP-7), which may escape inhibition by TIMP-1. The complete expression profile will provide a knowledge-based approach to novel therapeutics targeting these genes
Cyclic Variability of the Circumstellar Disc of the Be Star Tau. II. Testing the 2D Global Disc Oscillation Model
Aims. In this paper we model, in a self-consistent way, polarimetric,
photometric, spectrophotometric and interferometric observations of the
classical Be star Tauri. Our primary goal is to conduct a critical
quantitative test of the global oscillation scenario. Methods. We have carried
out detailed three-dimensional, NLTE radiative transfer calculations using the
radiative transfer code HDUST. For the input for the code we have used the most
up-to-date research on Be stars to include a physically realistic description
for the central star and the circumstellar disc. We adopt a rotationally
deformed, gravity darkened central star, surrounded by a disc whose unperturbed
state is given by a steady-state viscous decretion disc model. We further
assume that disc is in vertical hydrostatic equilibrium. Results. By adopting a
viscous decretion disc model for Tauri and a rigorous solution of the
radiative transfer, we have obtained a very good fit of the time-average
properties of the disc. This provides strong theoretical evidence that the
viscous decretion disc model is the mechanism responsible for disc formation.
With the global oscillation model we have successfully fitted spatially
resolved VLTI/AMBER observations and the temporal V/R variations of the
H and Br lines. This result convincingly demonstrates that the
oscillation pattern in the disc is a one-armed spiral. Possible model
shortcomings, as well as suggestions for future improvements, are also
discussed.Comment: 14 pages, 9 figures, accepted to A&
Surveyor surface sampler instrument
Operational capabilities, methods of deployment command and control, areas of lunar operation, and television viewability of Surveyor lunar surface sampler instrumen
Models of turbulent dissipation regions in the diffuse interstellar medium
Supersonic turbulence is a large reservoir of suprathermal energy in the
interstellar medium. Its dissipation, because it is intermittent in space and
time, can deeply modify the chemistry of the gas. We further explore a hybrid
method to compute the chemical and thermal evolution of a magnetized
dissipative structure, under the energetic constraints provided by the observed
properties of turbulence in the cold neutral medium. For the first time, we
model a random line of sight by taking into account the relative duration of
the bursts with respect to the thermal and chemical relaxation timescales of
the gas. The key parameter is the turbulent rate of strain "a" due to the
ambient turbulence. With the gas density, it controls the size of the
dissipative structures, therefore the strength of the burst. For a large range
of rates of strain and densities, the models of turbulent dissipation regions
(TDR) reproduce the CH+ column densities observed in the diffuse medium and
their correlation with highly excited H2. They do so without producing an
excess of CH. As a natural consequence, they reproduce the abundance ratios of
HCO+/OH and HCO+/H2O, and their dynamic range of about one order of magnitude
observed in diffuse gas. Large C2H and CO abundances, also related to those of
HCO+, are another outcome of the TDR models that compare well with observed
values. The abundances and column densities computed for CN, HCN and HNC are
one order of magnitude above PDR model predictions, although still
significantly smaller than observed values
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