535 research outputs found
Diffuse Gamma-Ray Emission from Starburst Galaxies and M31
We present a search for high energy gamma-ray emission from 9 nearby
starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the
diffuse gamma-ray emission from starburst galaxies was suspected to be
detectable, we find no emission from NGC 253, M82 nor from the average of all 9
galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the
averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the
expected radio and gamma-ray emission, we find that the magnetic field in the
nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and
electron densities is < 400. The EGRET limits indicate that the rate of massive
star formation in the survey galaxies is only about an order of magnitude
higher than in the Milky Way. The upper limit to the gamma-ray flux above 100
MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way
flux would be over twice this value, indicating higher gamma-ray emissivities
in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher
than in M31, our null detection of M31 supports the theory of the supernova
origin of cosmic rays in galaxies.Comment: 17 pages, plus 1 Postscript figure, AAS Latex macros v4.0, accepted
for publication in ApJ Main Journa
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The antibacterial activity and release of quaternary ammonium compounds in an orthodontic primer
The aim of this study was to evaluate the impact of 10 wt% benzalkonium chloride (TB-BAC) or 10 wt% cetylpyridinium chloride (TB-CPC) on the antimicrobial properties of the orthodontic adhesive primer, Transbond XT™ (TB). Antimicrobial activity was assessed using a zone of inhibition diffusion test and the release of the antimicrobial compounds was monitored by high performance liquid chromatography (HPLC). Shear bond strength (SBS) was tested using bovine enamel. Control, TB, specimens failed to demonstrate intrinsic antibacterial activity at 1, 7 and 14 days; whereas, TB-BAC and TB-CPC showed antibacterial effects at all times. HPLC analysis indicated no significant differences in the release behaviour of TB-BAC and TB-CPC (t-test, p > 0.05), except for the 7-day release which was higher for TB-BAC (p 0.05)
EGRET Gamma-Ray Observations of the Crab P2/P1 Ratio
Recent observations of the Crab pulsar by the Energetic Gamma-Ray Experiment
Telescope (EGRET) on the Compton Gamma Ray Observatory show that the
high-energy gamma-ray light curve has changed little over the lifetime of the
instrument. Previous data collected by SAS-2 and COS-B in the years 1972-82,
along with earlier EGRET data, suggested a 14 year sinusoidal variation in the
flux ratio between the first and second peaks. The new data from EGRET indicate
that the flux ratio is constant.Comment: 6 pages, 3 figures. To be published in Ap
Shear thickening and jamming in densely packed suspensions of different particle shapes
We investigated the effects of particle shape on shear thickening in densely
packed suspensions. Rods of different aspect ratios and non-convex hooked rods
were fabricated. Viscosity curves and normal stresses were measured using a
rheometer for a wide range of packing fractions for each shape. Suspensions of
each shape exhibit qualitatively similar Discontinuous Shear Thickening. The
logarithmic slope of the stress/shear-rate relation increases dramatically with
packing fraction and diverges at a critical packing fraction phi_c which
depends on particle shape. The packing fraction dependence of the viscosity
curves for different convex shapes can be collapsed when the packing fraction
is normalized by phi_c. Intriguingly, viscosity curves for non-convex particles
do not collapse on the same set as convex particles, showing strong shear
thickening over a wider range of packing fraction. The value of phi_c is found
to coincide with the onset of a yield stress at the jamming transition,
suggesting the jamming transition also controls shear thickening. The yield
stress is found to correspond with trapped air in the suspensions, and the
scale of the stress can be attributed to interfacial tension forces which
dramatically increase above phi_c due to the geometric constraints of jamming.
The relationship between shear and normal stresses is found to be linear in
both the shear thickening and jammed regimes, indicating that the shear
stresses come from friction. In the limit of zero shear rate, normal stresses
pull the rheometer plates together due to the surface tension of the liquid
below phi_c, but push the rheometer plates apart due to jamming above phi_c.Comment: 13 pages, 13 figures. published in Physical Review
The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field
We have used deep V and R images acquired at the ESO Very Large Telescope to
identify the optical companion to the binary pulsar PSR J0610-2100, one of the
black-widow millisecond pulsars recently detected by the Fermi Gamma-ray
Telescope in the Galactic plane. We found a faint star (V~26.7) nearly
coincident (\delta r ~0".28) with the pulsar nominal position. This star is
visible only in half of the available images, while it disappears in the
deepest ones (those acquired under the best seeing conditions), thus indicating
that it is variable. Although our observations do not sample the entire orbital
period (P=0.28 d) of the pulsar, we found that the optical modulation of the
variable star nicely correlates with the pulsar orbital period and describes a
well defined peak (R~25.6) at \Phi=0.75, suggesting a modulation due to the
pulsar heating. We tentatively conclude that the companion to PSR J0610-2100 is
a heavily ablated very low mass star (~ 0.02Msun) that completely filled its
Roche Lobe.Comment: 17 pages, 5 figures - Accepted for pubblication in Ap
Effect of gallic acid addition on some mechanical properties of self-adhesive resin cements
Self-adhesive resin cements (RCs) activate matrix metalloproteinase
(MMP) and cathepsin-related collagen degradation, and gallic acid (GA)
inhibits the activity of both MMPs and cysteine cathepsins. The purpose
of this study was to evaluate the setting time, biaxial flexural
strength, and Vickers hardness of self-adhesive RCs after the addition
of two different concentrations of GA. RelyX U200 (3M ESPE) and Panavia
SA (Kuraray) were modified with 0.5 and 1 wt% GA. The setting time of
five samples in each RC group was assessed using a thermocouple
apparatus as described in the ISO 4049 test. Biaxial flexure strength
was measured using a universal testing machine until failure. Vickers
hardness was measured with three randomized indentations on the surface
of each resin disc. RCs without GA were used as control. Data were
analyzed using a one-way analysis of variance and Tukey’s HSD test (α =
0.05). The setting times ranged from 2.4 to 4.6 min for RelyX and from
4.9 to 6.0 min for Panavia. The biaxial flexure strength ranged from
76.5 to 109.7 MPa for RelyX and from 73.3 to 108.2 MPa for Panavia.
Vickers hardness values ranged from 41.6 to 58.6 for RelyX and 27.2 to
33.6 for Panavia. The addition of 0.5 and 1 wt% GA to improve durability
of resin-dentin bonds had no adverse effects on setting time, whereas
the biaxial flexure strength and Vickers hardness values for the tested
materials were significantly reduced.</p
The Dark Side of ROTSE-III Prompt GRB Observations
We present several cases of optical observations during gamma-ray bursts
(GRBs) which resulted in prompt limits but no detection of optical emission.
These limits constrain the prompt optical flux densities and the optical
brightness relative to the gamma-ray emission. The derived constraints fall
within the range of properties observed in GRBs with prompt optical detections,
though at the faint end of optical/gamma flux ratios. The presently accessible
prompt optical limits do not require a different set of intrinsic or
environmental GRB properties, relative to the events with prompt optical
detections.Comment: ApJ accepted. 20 pages in draft manuscript form, which includes 6
pages of tables and 2 figure
Status of the ROTSE-III telescope network
ROTSE-III is a homogeneous worldwide array of 4 robotic telescopes. They were designed to provide optical observations of γ-ray burst (GRB) afterglows as close as possible to the start of γ-ray emission. ROTSE-III is fulfilling its potential for GRB science, and provides optical observations for a variety of astrophysical sources in the interim between GRB events
Prompt Optical Detection of GRB 050401 with ROTSE-IIIa
The ROTSE-IIIa telescope at Siding Spring Observatory, Australia, detected
prompt optical emission from Swift GRB 050401. In this letter, we present
observations of the early optical afterglow, first detected by the ROTSE-IIIa
telescope 33 s after the start of gamma-ray emission, contemporaneous with the
brightest peak of this emission. This GRB was neither exceptionally long nor
bright. This is the first prompt optical detection of a GRB of typical duration
and luminosity. We find that the early afterglow decay does not deviate
significantly from the power-law decay observable at later times, and is
uncorrelated with the prompt gamma-ray emission. We compare this detection with
the other two GRBs with prompt observations, GRB 990123 and GRB 041219a. All
three bursts exhibit quite different behavior at early times.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
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