813 research outputs found
High speed techniques for synthetic aperture radar image formation
One possible approach to high speed synthetic aperture radar signal reconstruction involves the utilization of two dimensional real time spatial light modulators as recyclable replacements for photographic film in the input transducer plane of a modified synthetic aperture radar (SAR) coherent optical processor. Leading candidate spatial light modulators include modified Pockels readout optical modulators (PROM), charge coupled devices (CCD) addressed liquid crystal light valves, and CCD addressed membrane light modulators. The fundamental physical limitations affecting SAR processor performance characteristics of such real time devices are under investigation. Current research on PROM is focused on the effects of device operatonal mode, device constitutive parameters, electro-optic crystal orientation, writing wavelength, frame rate/data overwrite/presuming, erasure completeness, and image retention on the overall quality of SAR image formation. Both modulated laser scanning and intensified CRT temporal to spatial input approaches are being examined
Convergence to a self-similar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state
by full general relativistic numerical simulations. For 0, it has been known that there exists a general relativistic counterpart
of the Larson-Penston self-similar Newtonian solution. The numerical
simulations strongly suggest that, in the neighborhood of the center, generic
collapse converges to this solution in an approach to a singularity and that
self-similar solutions other than this solution, including a ``critical
solution'' in the black hole critical behavior, are relevant only when the
parameters which parametrize initial data are fine-tuned. This result is
supported by a mode analysis on the pertinent self-similar solutions. Since a
naked singularity forms in the general relativistic Larson-Penston solution for
0, this will be the most serious known counterexample against
cosmic censorship. It also provides strong evidence for the self-similarity
hypothesis in general relativistic gravitational collapse. The direct
consequence is that critical phenomena will be observed in the collapse of
isothermal gas in Newton gravity, and the critical exponent will be
given by , though the order parameter cannot be the black
hole mass.Comment: 22 pages, 15 figures, accepted for publication in Physical Review D,
reference added, typos correcte
Centaurus A: multiple outbursts or bursting bubble?
We present new radio observations of the brighter region of the northern lobe
(the Northern Middle Lobe, NML) of Centaurus A obtained at 20 cm with the
Australia Telescope Compact Array. The angular resolutions are ~50 and ~130
arcsec, therefore much higher than for the previously available radio images of
this region. The most interesting feature detected is a large-scale jet that
connects the inner radio lobe and the NML and that is imaged for the first
time. The NML itself appears as diffuse emission with a relatively bright ridge
on the eastern side. The radio morphology of Centaurus A and, in particular,
its NML could be the result of a precessing jet that has undergone a strong
interaction with the environment at least in the northern side. The very big
drop in intensity between the inner jet and the large-scale jet can be
explained with a sequence of bursts of activity at different epochs in the life
of the source. Alternatively (or additionally) a ``bursting bubble'' model is
proposed which could also explain the good collimation of the large-scale jet.
In this model, the plasma accumulated in the inner lobe would be able to
``burst'' out only through one nozzle that would be the region where the
large-scale jet forms. From the comparison between the radio emission and the
regions of ionized gas we find that the inner optical filament falls about 2
arcmin (~2 kpc) away from the large-scale radio jet. Thus, this filament does
not seem to have experienced a direct interaction with the radio plasma. The
outer filaments appear to be, in projection, closer to the radio emission,
arguing for a direct interaction with the radio jet. However, also in this case
a more complicated interaction than assumed so far has to be occuring.Comment: To appear in MNRAS; 11 pages, LateX, 7 figures. Fig 1 is available at
http://www.ira.bo.cnr.it/~rmorgant/Centaurus
CANGAROO-III Search for Gamma Rays from SN 1987A and the Surrounding Field
Optical images of SN 1987A show a triple ring structure. The inner (dust)
ring has recently increased in brightness and in the number of hot spots
suggesting that the supernova shock wave has collided with the dense
pre-existing circumstellar medium, a scenario supported by radio and X-ray
observations. Such a shocked environment is widely expected to result in the
acceleration of charged particles, and the accompanying emission of very high
energy gamma-rays. Here, we report the results of observations made in 2004 and
2006 which yield upper limits on the TeV gamma-ray flux, which are compared
with a theoretical prediction. In addition, we set upper limits on the TeV flux
for four high energy objects which are located within the same field of view of
the observation: the super-bubble 30 Dor C, the Crab-like pulsar PSR
B054069, the X-ray binary LMC X-1, and the supernova remnant N157B.Comment: 5 pages, 5 figures, Accepted for publication in Ap
Observation of Very High Energy Gamma Rays from HESS J1804-216 with CANGAROO-III Telescopes
We observed the unidentified TeV gamma-ray source HESS J1804-216 with the
CANGAROO-III atmospheric Cerenkov telescopes from May to July in 2006. We
detected very high energy gamma rays above 600 GeV at the 10 sigma level in an
effective exposure of 76 hr. We obtained a differential flux of
(5.0+/-1.5_{stat}+/-1.6_{sys})\times 10^{-12}(E/1 TeV)^{-\alpha}
cm^{-2}s^{-1}TeV^{-1} with a photon index \alpha of 2.69 +/- 0.30_{stat} +/-
0.34_{sys}, which is consistent with that of the H.E.S.S. observation in 2004.
We also confirm the extended morphology of the source. By combining our result
with multi-wavelength observations, we discuss the possible counterparts of
HESS J1804-216 and the radiation mechanism based on leptonic and hadronic
processes for a supernova remnant and a pulsar wind nebula.Comment: 11 pages, 12 figures, Accepted in Ap
Design Study of CANGAROO-III, Stereoscopic Imaging Atmospheric Cherenkov Telescopes for sub-TeV Gamma-ray
CANGAROO-III is an Imaging Atmospheric Cherenkov Telescope (IACT) array of
four 10 m telescopes for very high energy (sub-TeV) gamma-ray astronomy. A
design study of the CANGAROO-III telescope system was carried out using the
Monte Carlo technique in order to optimize the pixel size and the telescope
spacing. Studies were also made of observations at low elevation angles.Comment: 17pages, 13 figures, elsart, to appear in Astro. Part. Phy
Detection of Gamma-rays around 1TeV from RX J0852.0-4622 by CANGAROO-II
We have detected gamma-ray emission at the 6sigma level at energies greater
than 500GeV from the supernova remnant RX J0852.0-4622 (G266.2-1.2) using the
CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). The flux was 0.12
times of that of Crab at 1TeV. The signal centroid is consistent with the peak
of the X-ray emission in the north-west rim of the remnant.Comment: 12pages, 4figures, to be published in ApJ
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