4,606 research outputs found
The Diffuse Source at the Center of LMC SNR 0509-67.5 is a Background Galaxy at z = 0.031
Type Ia supernovae (SNe Ia) are well-known for their use in the measurement
of cosmological distances, but our continuing lack of concrete knowledge about
their progenitor stars is both a matter of debate and a source of systematic
error. In our attempts to answer this question, we presented unambiguous
evidence that LMC SNR 0509-67.5, the remnant of an SN Ia that exploded in the
Large Magellanic Cloud 400 +/- 50 years ago, did not have any point sources
(stars) near the site of the original supernova explosion, from which we
concluded that this particular supernova must have had a progenitor system
consisting of two white dwarfs (Schaefer & Pagnotta 2012). There is, however,
evidence of nebulosity near the center of the remnant, which could have been
left over detritus from the less massive WD, or could have been a background
galaxy unrelated to the supernova explosion. We obtained long-slit spectra of
the central nebulous region using GMOS on Gemini South to determine which of
these two possibilities is correct. The spectra show H-alpha emission at a
redshift of z = 0.031, which implies that the nebulosity in the center of LMC
SNR 0509-67.5 is a background galaxy, unrelated to the supernova.Comment: 2 figures, accepted for publication in Ap
A Gamma Ray Burst with a 220 Microsecond Rise Time and a Sharp Spectral Cutoff
The Gamma Ray Burst GRB920229 has four extreme and unprecedented properties;
a rise in brightness with an e-folding time scale of , a fall
in brightness with an e-folding time scale of , a large
change in spectral shape over a time of , and a sharp spectral
cutoff to high energies with . The rapid changes occur
during a spike in the light curve which was seen 0.164 s after the start of the
burst. The spectrum has a peak at 200 keV with no significant
flux above 239 keV, although the cutoff energy shifts to less than 100 keV
during the decay of the spike. These numbers can be used to place severe limits
on fireball models of bursts. The thickness of the energy production region
must be smaller than , ejected shells must have a dispersion of the
Lorentz factor of less than roughly 1% along a particular radius, and the
angular size of the radiation emission region is of order 1 arc-minute as
viewed from the burst center. The physical mechanism that caused the sharp
spectral cutoff has not been determined.Comment: 20 pages, 3 figures, Submitted to ApJ
BEDROCK GEOLOGY AND TECTONIC EVOLUTION OF THE WESTERN CENTRAL MAINE ZONE, SOUTH CENTRAL MASSACHUSETTS
Bedrock geology of the East Brookfield quadrangle (EBQ), based on new 1:24000 scale mapping, consists of an interfolded sequence of Rangeley Fm. and Paxton Fm. metasediments, intruded by Early Mississippian tonalitic to granitic orthogneisses and underlain in the western half by folded orthogneisses of unknown age. Pervasive flattening strains have created planar, generally NNE-SSW striking and consistent moderately west dipping foliations. Generally strike-parallel stretching lineations and boudinage structures, with rarer dip parallel stretching lineations, reflect extrusion of CMZ metasediments and orthogneisses with a flattening component. Mapping in the EBQ demonstrates that a transition between deformational mechanisms of the southern Central Maine Zone is located there. Evidence for previously mapped faults was not observed during mapping. Alternatively, map scale folds terminating in the East Brookfield quadrangle likely accommodated the across-strike shortening and repetition of lithologic units of the CMZ during Acadian orogenesis. Aeromagnetic patterns and outcrop data approximate a map scale, east-verging and west-dipping, shallowly north-plunging isoclinal fold consisting of interlayered mafic and felsic gneisses. Constrictional and flattening strains documented in the deformation features of the East Brookfield quadrangle suggest that it is located on/near the eastern margin of the zone of transpression approximated by the Bronson Hill gneiss domes. Outcrop- to meter-scale folds with hinge axes sub-parallel to parallel with regional lineations suggest constrictional deformation in a transpressional regime. Deformation features associated with map-scale folding, extrusion/extension, and flattening of CMZ lithologies in the East Brookfield quadrangle support a regime of transpressional deformation caused by oblique convergence of Avalonian terrane with Laurentia. U-Pb ages and outcrop structural relationships suggest that regional flattening and folding was syn- to late-orogenic in nature and Carboniferous or younger in age. All previous structural fabrics are transposed by late regional flattening. Lithologies of the East Brookfield quadrangle exemplify the nature of the Acadian granulite-facies high, displaying a peak metamorphic assemblage of Crd + Grt + Sil + Kfs that has undergone retrograde reactions to produce Bt + Sil. Retrograde metamorphism and deformation occurred during strike-parallel extrusion of CMZ lithologies. Retrograde minerals define kinematic/shear sense indicators that record an overall top to the NNE or ENE asymmetry on vertical faces and dextral west side north on horizontal surfaces. Progressive deformation of CMZ lithologies through transpressional mechanisms is proposed as an alternative to the sequential development of Acadian deformational features in three separate stages
Non-destructive spatial heterodyne imaging of cold atoms
We demonstrate a new method for non-destructive imaging of laser-cooled
atoms. This spatial heterodyne technique forms a phase image by interfering a
strong carrier laser beam with a weak probe beam that passes through the cold
atom cloud. The figure of merit equals or exceeds that of phase-contrast
imaging, and the technique can be used over a wider range of spatial scales. We
show images of a dark spot MOT taken with imaging fluences as low as 61 pJ/cm^2
at a detuning of 11 linewidths, resulting in 0.0004 photons scattered per atom.Comment: text+3 figures, submitted to Optics Letter
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An international survey of mature students' uses of mobile devices in life and learning
The paper presents research concerned with learner-driven innovative practice with mobile technologies and the interface between formal and informal learning. We build on our previous work investigating student use of personal devices for learning, work, social interaction and entertainment. A recent phase of the research included an international survey focusing on students registered on selected Masters and doctoral programmes in the UK, Sweden, Portugal, Hong Kong and Australia. The research gives an account of everyday uses and more unusual deployments of personal technologies by students from departments of education and technology. It illuminates learner choices and preferences, attitudes towards work–life boundaries, evolving social and cultural practices, and the impacts of technological change
Creating a self-induced dark spontaneous-force optical trap for neutral atoms
This communication describes the observation of a new type of dark
spontaneous-force optical trap (dark SPOT) obtained without the use of a mask
blocking the central part of the repumper laser beam. We observe that loading a
magneto-optical trap (MOT) from a continuous and intense flux of slowed atoms
and by appropriately tuning the frequency of the repumper laser is possible to
achieve basically the same effect of the dark SPOT, using a simpler apparatus.
This work characterizes the new system through measurements of absorption and
fluorescence imaging of the atomic cloud and presents a very simple model to
explain the main features of our observations. We believe that this new
approach may simplify the current experiments to produce quantum degenerated
gases.Comment: 13 pages, 8 figures, Submitted to Optics Communications (30/10/2003),
accepted for publication (Feb/2004
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