296 research outputs found
Upper Limits On Periodic, Pulsed Radio Emission from the X-Ray Point Source in Cassiopeia A
The Chandra X-ray Observatory recently discovered an X-ray point source near
the center of Cassiopeia A, the youngest known Galactic supernova remnant. We
have conducted a sensitive search for radio pulsations from this source with
the Very Large Array, taking advantage of the high angular resolution of the
array to resolve out the emission from the remnant itself. No convincing
signatures of a dispersed, periodic source or of isolated dispersed pulses were
found, whether for an isolated or a binary source. We derive upper limits of 30
and 1.3 mJy at 327 and 1435 MHz for the phase-averaged pulsed flux density from
this source. The corresponding luminosity limits are lower than those for any
pulsar with age less than 10^4 years. The sensitivities of our search to single
pulses were 25 and 1.0 Jy at 327 and 1435 MHz. For comparison, the Crab pulsar
emits roughly 80 pulses per minute with flux densities greater than 100 Jy at
327 MHz and 8 pulses per minute with flux densities greater than 50 Jy at 1435
MHz. These limits are consistent with the suggestion that the X-ray point
source in Cas A adds to the growing number of neutron stars which are not radio
pulsars.Comment: accepted by ApJ Letter
Polarimetric Properties of the Crab Pulsar between 1.4 and 8.4 GHz
New polarimetric observations of the Crab pulsar at frequencies between 1.4
and 8.4 GHz are presented. Additional pulse components discovered in earlier
observations (Moffett & Hankins 1996, astro-ph/9604163) are found to have high
levels of linear polarization, even at 8.4 GHz. No abrupt sweeps in position
angle are found within pulse components; however, the position angle and
rotational phase of the interpulse do change dramatically between 1.4 and 4.9
GHz. The multi-frequency profile morphology and polarization properties
indicate a non-standard origin of the emission. Several emission geometries are
discussed, but the one favored locates sites of emission both near the pulsar
surface and in the outer magnetosphere.Comment: 20 pages, 7 postscript figures, uses aaspp4 Latex style. To appear in
Volume 522 of The Astrophysical Journa
LUNASKA experiments using the Australia Telescope Compact Array to search for ultra-high energy neutrinos and develop technology for the lunar Cherenkov technique
We describe the design, performance, sensitivity and results of our recent
experiments using the Australia Telescope Compact Array (ATCA) for lunar
Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond
timing, including a limit on an isotropic neutrino flux. We also make a first
estimate of the effects of small-scale surface roughness on the effective
experimental aperture, finding that contrary to expectations, such roughness
will act to increase the detectability of near-surface events over the neutrino
energy-range at which our experiment is most sensitive (though distortions to
the time-domain pulse profile may make identification more difficult). The aim
of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array"
(LUNASKA) project is to develop the lunar Cherenkov technique of using
terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR)
and neutrino detection, and in particular to prepare for using the Square
Kilometer Array (SKA) and its path-finders such as the Australian SKA
Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov
experiments.Comment: 27 pages, 18 figures, 4 tables
Size of the Vela Pulsar's Emission Region at 13 cm Wavelength
We present measurements of the size of the Vela pulsar in 3 gates across the
pulse, from observations of the distribution of intensity. We calculate the
effects on this distribution of noise in the observing system, and measure and
remove it using observations of a strong continuum source. We also calculate
and remove the expected effects of averaging in time and frequency. We find
that effects of variations in pulsar flux density and instrumental gain,
self-noise, and one-bit digitization are undetectably small. Effects of
normalization of the correlation are detectable, but do not affect the fitted
size. The size of the pulsar declines from 440 +/- 90 km (FWHM of best-fitting
Gaussian distribution) to less than 200 km across the pulse. We discuss
implications of this size for theories of pulsar emission.Comment: 51 pages, 10 figures. To appear in ApJ. Also available at
http://www.physics.ucsb.edu/~cgwinn/pulsar/size_14.p
Radio-Frequency Measurements of Coherent Transition and Cherenkov Radiation: Implications for High-Energy Neutrino Detection
We report on measurements of 11-18 cm wavelength radio emission from
interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield
Accelerator. The electrons were observed both in a configuration where they
produced primarily transition radiation from an aluminum foil, and in a
configuration designed for the electrons to produce Cherenkov radiation in a
silica sand target. Our aim was to emulate the large electron excess expected
to develop during an electromagnetic cascade initiated by an ultra high-energy
particle. Such charge asymmetries are predicted to produce strong coherent
radio pulses, which are the basis for several experiments to detect high-energy
neutrinos from the showers they induce in Antarctic ice and in the lunar
regolith. We detected coherent emission which we attribute both to transition
and possibly Cherenkov radiation at different levels depending on the
experimental conditions. We discuss implications for experiments relying on
radio emission for detection of electromagnetic cascades produced by ultra
high-energy neutrinos.Comment: updated figure 10; fixed typo in equation 2.2; accepted by PR
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
An Empirical Model for the Radio Emission from Pulsars
A model for slow radio pulsars is proposed which involves the entire
magnetosphere in the production of the observed radio emission. It is argued
that observations of pulsar profiles suggest that a feedback mechanism exists
between the star surface and the null charge surface, requiring particle flow
in both directions. In their flow to and from the surface the particles execute
an azimuthal drift around the magnetic pole, thereby creating a ring of
discrete `emission nodes' close to the surface. Motion of the nodes is observed
as the well-known subpulse `drift', but is interpreted here as a small residual
component of the real particle drift. The nodes can therefore move in either
direction, or even remain stationary. A precise fit is found for the pulsar
PSR0943+10. Azimuthal interactions between different regions of the
magnetosphere depend on the angle between the magnetic and rotation axes and
influence the conal type, as observed. The requirement of intermittent weak
pair-production in an outergap suggests a natural evolutionary link between
radio and gamma-ray pulsars.Comment: 17 pages 8 figure
Accelerator Measurements of the Askaryan effect in Rock Salt: A Roadmap Toward Teraton Underground Neutrino Detectors
We report on further SLAC measurements of the Askaryan effect: coherent radio
emission from charge asymmetry in electromagnetic cascades. We used synthetic
rock salt as the dielectric medium, with cascades produced by GeV
bremsstrahlung photons at the Final Focus Test Beam. We extend our prior
discovery measurements to a wider range of parameter space and explore the
effect in a dielectric medium of great potential interest to large scale
ultra-high energy neutrino detectors: rock salt (halite), which occurs
naturally in high purity formations containing in many cases hundreds of cubic
km of water-equivalent mass. We observed strong coherent pulsed radio emission
over a frequency band from 0.2-15 GHz. A grid of embedded dual-polarization
antennas was used to confirm the high degree of linear polarization and track
the change of direction of the electric-field vector with azimuth around the
shower. Coherence was observed over 4 orders of magnitude of shower energy. The
frequency dependence of the radiation was tested over two orders of magnitude
of UHF and microwave frequencies. We have also made the first observations of
coherent transition radiation from the Askaryan charge excess, and the result
agrees well with theoretical predictions. Based on these results we have
performed a detailed and conservative simulation of a realistic GZK neutrino
telescope array within a salt-dome, and we find it capable of detecting 10 or
more contained events per year from even the most conservative GZK neutrino
models.Comment: 15 pages, 16 figures, submitted to Phys. Rev.
Practical and accurate calculations of Askaryan radiation
An in-depth characterization of coherent radio Cherenkov pulses from particle
showers in dense dielectric media, referred to as the Askaryan effect, is
presented. The time-domain calculation developed in this article is based on a
form factor to account for the lateral dimensions of the shower. It is
computationally efficient and able to reproduce the results of detailed
particle shower simulations with high fidelity in most regions of practical
interest including Fresnel effects due to the longitudinal development of the
shower. In addition, an intuitive interpretation of the characteristics of the
Askaryan pulse is provided. We expect our approach to benefit the analysis of
radio pulses in experiments exploiting the radio technique.Comment: Replaced with version published Phys. Rev.
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