984 research outputs found
VLA Observations of Candidate Supernova Remnants from the Clark Lake 30.9 MHz Galactic Plane Survey
We report the results of 1464 MHz continuum VLA observations of eight fields containing unidentified
small-diameter objects associated with candidate supernova remnants from the Clark Lake 30.9 MHz
galactic plane survey. The observations were made in the C configuration, giving a resolution of
-12-20 arcsec, and a sensitivity of typically <0.5 mJy per beam. Polarization measurements were
made as well. One of the 30.9 MHz candidates, G41.4+ 1.2, appears to be confirmed as a supernova
remnant by our observations. Of the remaining seven fields observed, three were found to contain
small-diameter objects which met some of the criteria for nonthermal origin, but will require further
study to evaluate whether they are associated with the candidate supernova remnants. Two of the fields
were found to contain groups of unresolved objects consistent with expectations for extragalactic
background sources. In these cases the 30.9 MHz observations, which could not resolve the individual
sources but would view them as a single extended source, may have mistakenly identified them as
possible supernova remnants. Finally, two fields contained bright H II region
Experimental Limit on the Cosmic Diffuse Ultra-high Energy Neutrino Flux
We report results from 120 hours of livetime with the Goldstone Lunar
Ultra-high energy neutrino Experiment (GLUE). The experiment searches for <10
ns microwave pulses from the lunar regolith, appearing in coincidence at two
large radio telescopes separated by 22 km and linked by optical fiber. Such
pulses would arise from subsurface electromagnetic cascades induced by
interactions of >= 100 EeV neutrinos in the lunar regolith. No candidates are
yet seen, and the implied limits constrain several current models for
ultra-high energy neutrino fluxes.Comment: 4 pages, 4 figures, revtex4 style. New intro section, Fig. 2, Fig 4;
in final PRL revie
Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High Energy Particle Cascades
We present the first direct experimental evidence for the charge excess in
high energy particle showers predicted nearly 40 years ago by Askaryan. We
directed bremsstrahlung photons from picosecond pulses of 28.5 GeV electrons at
the SLAC Final Focus Test Beam facility into a 3.5 ton silica sand target,
producing electromagnetic showers several meters long. A series of antennas
spanning 0.3 to 6 GHz were used to detect strong, sub-nanosecond radio
frequency pulses produced whenever a shower was present. The measured electric
field strengths are consistent with a completely coherent radiation process.
The pulses show 100% linear polarization, consistent with the expectations of
Cherenkov radiation. The field strength versus depth closely follows the
expected particle number density profile of the cascade, consistent with
emission from excess charge distributed along the shower. These measurements
therefore provide strong support for experiments designed to detect high energy
cosmic rays and neutrinos via coherent radio emission from their cascades.Comment: 10 pages, 4 figures. Submitted to Phys. Rev. Let
Picosecond timing of Microwave Cherenkov Impulses from High-Energy Particle Showers Using Dielectric-loaded Waveguides
We report on the first measurements of coherent microwave impulses from
high-energy particle-induced electromagnetic showers generated via the Askaryan
effect in a dielectric-loaded waveguide. Bunches of 12.16 GeV electrons with
total bunch energy of GeV were pre-showered in tungsten, and
then measured with WR-51 rectangular (12.6 mm by 6.3 mm) waveguide elements
loaded with solid alumina () bars. In the 5-8 GHz
single-mode band determined by the presence of the dielectric in the waveguide,
we observed band-limited microwave impulses with amplitude proportional to
bunch energy. Signals in different waveguide elements measuring the same shower
were used to estimate relative time differences with 2.3 picosecond precision.
These measurements establish a basis for using arrays of alumina-loaded
waveguide elements, with exceptional radiation hardness, as very high precision
timing planes for high-energy physics detectors.Comment: 16 pages, 15 figure
Search for gamma-rays above 400 GeV from Geminga
Observations of Geminga made at the Whipple Observatory using the atmospheric Cherenkov technique during the moonless periods of November 1983 to February 1984 and November 1984 till February 1985 were examined for evidence for the emission of gamma rays with energy in excess of approx 400 GeV. Evidence of either a steady flux or a flux pulsed with a period near 60 seconds were studied. In neither case was any significant effect observed, enabling the establishment 3 of sigma upper limits of 5.5 x 10 to the -11th power photons/sq cm/s and 2.0 x 10 to the -11th power photons/sq cm/s for the steady and pulsed emission respectively. The limit to the pulsed flux is approximately a factor of six below that predicted
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