4,527 research outputs found
A transient digitiser for fast air shower events
Air shower structure are often measured on time scales of a few nanoseconds. Longitudinal disk structure near the core is of the order of meters in dimension, air Cerenkov pulses have full widths at half maximum of the order of tens of nanoseconds, and fast timing over typical arrays is usually measured to nanosecond accuracy. oscilloscopes can be used but have very limited dynamic range and are expensive if measurements down to a few nanoseconds are to be made. For the fast Cerenkov work, an instrument with better dynamic range than an oscilloscope and with a time resolution sufficient to allow measurements limited only by system risetime of a few nanoseconds is needed. A 16/32 channel, 8 bit, fast transient digitizer was designed and built which runs at sample intervals down to approx. 1 nanosecond per channel
The longitudinal thickness of air-shower fronts
Linsely (1983) has proposed a technique for the detection and analysis of air showers at large distances from the shower axis based on a measurement of the shower front thickness and the assumption that this thickness is closely related to the core distance. Some of the problems involved with realizing such a technique were investigated, and some related observations are reported. The practical problems of how consistent the measurements of the shower front would be, how one would use the measurement, and how the rate of triggered events would depend on the minimum pulse width required are studied
Simulation Study of TenTen: A new Multi-TeV IACT array
TenTen is a proposed array of Imaging Atmospheric Cherenkov Telescopes (IACT)
optimized for the gamma ray energy regime of 10 TeV to 100 TeV, but with a
threshold of ~1 to a few TeV. It will offer a collecting area of 10 km2 above
energies of 10 TeV. In the initial phase, a cell of 3 to 5 modest-sized
telescopes, each with 10-30 m2 mirror area, is suggested for an Australian
site. A possible expansion of the array could comprise many such cells. Here we
present work on configuration and technical issues from our simulation studies
of the array. Working topics include array layout, telescope size and optics,
camera field of view, telescope trigger system, electronics, and site surveys.Comment: 4 pages, 7 figures, submitted to Proceedings of the ICRC 2007, pdf
forma
TenTen: A New Array of Multi-TeV Imaging Cherenkov Telescopes
The exciting results from H.E.S.S. point to a new population of gamma-ray
sources at energies E > 10 TeV, paving the way for future studies and new
discoveries in the multi-TeV energy range. Connected with these energies is the
search for sources of PeV cosmic-rays (CRs) and the study of multi-TeV
gamma-ray production in a growing number of astrophysical environments. TenTen
is a proposed stereoscopic array (with a suggested site in Australia) of
modest-sized (10 to 30m^2) Cherenkov imaging telescopes with a wide field of
view (8 to 10deg diameter) optimised for the E~10 to 100 TeV range. TenTen will
achieve an effective area of ~10 km^2 at energies above 10 TeV. We outline here
the motivation for TenTen and summarise key performance parameters.Comment: 4 pages, 2 figures, proceedings of the 30th ICRC, Merida, Mexico,
200
First detection of ammonia in M82
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion
lines of ammonia (NH3) towards the south--western molecular lobe in M82. The
relative intensities of the ammonia lines are characterized by a rotational
temperature of T_rot=29+/-5 K which implies an average kinetic temperature of
T_kin~60 K. A Gaussian decomposition of the observed spectra indicates
increasing kinetic temperatures towards the nucleus of M82, consistent with
recent findings based on CO observations. The observations imply a very low NH3
abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a
decreasing NH3 abundance towards the central active regions in M82 and
interpret this abundance gradient in terms of photodissociation of NH3 in PDRs.
The low temperature derived here from NH3 also explains the apparent
underabundance of complex molecules like CH_3OH and HNCO, which has previously
been reported.Comment: 4 pages, 4 figures, accepted by ApJ
Timing analysis techniques at large core distances for multi-TeV gamma ray astronomy
We present an analysis technique that uses the timing information of
Cherenkov images from extensive air showers (EAS). Our emphasis is on distant,
or large core distance gamma-ray induced showers at multi-TeV energies.
Specifically, combining pixel timing information with an improved direction
reconstruction algorithm, leads to improvements in angular and core resolution
as large as ~40% and ~30%, respectively, when compared with the same algorithm
without the use of timing. Above 10 TeV, this results in an angular resolution
approaching 0.05 degrees, together with a core resolution better than ~15 m.
The off-axis post-cut gamma-ray acceptance is energy dependent and its full
width at half maximum ranges from 4 degrees to 8 degrees. For shower directions
that are up to ~6 degrees off-axis, the angular resolution achieved by using
timing information is comparable, around 100 TeV, to the on-axis angular
resolution. The telescope specifications and layout we describe here are geared
towards energies above 10 TeV. However, the methods can in principle be applied
to other energies, given suitable telescope parameters. The 5-telescope cell
investigated in this study could initially pave the way for a larger array of
sparsely spaced telescopes in an effort to push the collection area to >10 km2.
These results highlight the potential of a `sparse array' approach in
effectively opening up the energy range above 10 TeV.Comment: Published in Astroparticle Physic
Chandra Observations of SN 2004et and the X-ray Emission of Type IIp Supernovae
We report the X-ray detection of the Type II-plateau supernova SN 2004et in
the spiral galaxy NGC 6946, using the Chandra X-Ray Observatory. The position
of the X-ray source was found to agree with the optical position within ~0.4
arcsec. Chandra also surveyed the region before the 2004 event, finding no
X-ray emission at the location of the progenitor. For the post-explosion
observations, a total of 202, 151, and 158 photons were detected in three
pointings, each ~29 ks in length, on 2004 October 22, November 6, and December
3, respectively. The spectrum of the first observation is best fit by a thermal
model with a temperature of kT=1.3 keV and a line-of-sight absorption of
N_H=1.0 x 10^{22} cm^{-2}. The inferred unabsorbed luminosity (0.4-8 keV) is
~4x10^{38} erg/s, adopting a distance of 5.5 Mpc. A comparison between hard and
soft counts on the first and third epochs indicates a softening over this time,
although there is an insufficient number of photons to constrain the variation
of temperature and absorption by spectral fitting. We model the emission as
arising from the reverse shock region in the interaction between the supernova
ejecta and the progenitor wind. For a Type IIP supernova with an extended
progenitor, the cool shell formed at the time of shock wave breakout from the
star can affect the initial evolution of the interaction shell and the
absorption of radiation from the reverse shock. The observed spectral softening
might be due to decreasing shell absorption. We find a pre-supernova mass loss
rate of (2-2.5)x 10^{-6} M_{\odot} /yr for a wind velocity of 10 kms, which is
in line with expectations for a Type IIP supernova.Comment: total 19 pages including 7 figures. ApJ, in press. See
http://spider.ipac.caltech.edu/staff/rho/preprint/SN2004etms.ps for the paper
including full resolution image
The impact of lightning on tropospheric ozone chemistry using a new global lightning parametrisation
A lightning parametrisation based on upward cloud ice flux is implemented in a chemistry–climate model (CCM) for the first time. The UK Chemistry and Aerosols model is used to study the impact of these lightning nitric oxide (NO) emissions on ozone. Comparisons are then made between the new ice flux parametrisation and the commonly used, cloud-top height parametrisation. The ice flux approach improves the simulation of lightning and the temporal correlations with ozone sonde measurements in the middle and upper troposphere. Peak values of ozone in these regions are attributed to high lightning NO emissions. The ice flux approach reduces the overestimation of tropical lightning apparent in this CCM when using the cloud-top approach. This results in less NO emission in the tropical upper troposphere and more in the extratropics when using the ice flux scheme. In the tropical upper troposphere the reduction in ozone concentration is around 5–10 %. Surprisingly, there is only a small reduction in tropospheric ozone burden when using the ice flux approach. The greatest absolute change in ozone burden is found in the lower stratosphere, suggesting that much of the ozone produced in the upper troposphere is transported to higher altitudes. Major differences in the frequency distribution of flash rates for the two approaches are found. The cloud-top height scheme has lower maximum flash rates and more mid-range flash rates than the ice flux scheme. The initial Ox (odd oxygen species) production associated with the frequency distribution of continental lightning is analysed to show that higher flash rates are less efficient at producing Ox; low flash rates initially produce around 10 times more Ox per flash than high-end flash rates. We find that the newly implemented lightning scheme performs favourably compared to the cloud-top scheme with respect to simulation of lightning and tropospheric ozone. This alternative lightning scheme shows spatial and temporal differences in ozone chemistry which may have implications for comparison between models and observations, as well as for simulation of future changes in tropospheric ozone
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Our changing Sun
Stellar astronomy tells us much about the long-term evolution of our Sun while forensic evidence (for example, cosmic-ray products in ice cores) gives us indications of its fluctuations over the last millennium. However, such studies do not give us a sufficiently detailed understanding of solar change over the last century to allow us to detect and quantify any role that the Sun might have played in the observed rise in average surface temperatures on Earth. This paper describes recent research that has filled this gap by applying advances in our understanding of the effects and structure of the solar wind to historical data on the Earth's magnetic field
Hot Electron Capture Dissociation Distinguishes Leucine from Isoleucine in a Novel Hemoglobin Variant, Hb Askew, β54(D5)Val→Ile
Population migration has led to the global dispersion of human hemoglobinopathies and has precipitated a need for their identification. An effective mass spectrometry-based procedure involves analysis of the intact α- and β-globin chains to determine their mass, followed by location of the variant amino acid residue by direct analysis of the enzymatically digested chains and low-energy collision induced dissociation of the variant peptide. Using this procedure, a variant was identified as either β54Val→Leu or β54Val→Ile, since the amino acids leucine and isoleucine cannot be distinguished using low-energy collisions. Here, we describe how hot electron capture dissociation on a Fourier transform-ion cyclotron resonance mass spectrometer was used to distinguish isoleucine from leucine and identify the mutation as β54(D5)Val→Ile. This is a novel variant, and we have named it Hb Askew
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