35,736 research outputs found
A preliminary approach to intelligent x-ray imaging for baggage inspection at airports
Identifying explosives in baggage at airports relies on being able to characterize the materials that make up an X-ray image. If a suspicion is generated during the imaging process (step 1), the image data could be enhanced by adapting the scanning parameters (step 2). This paper addresses the first part of this problem and uses textural signatures to recognize and characterize materials and hence enabling system control. Directional Gabor-type filtering was applied to a series of different X-ray images. Images were processed in such a way as to simulate a line scanning geometry. Based on our experiments with images of industrial standards and our own samples it was found that different materials could be characterized in terms of the frequency range and orientation of the filters. It was also found that the signal strength generated by the filters could be used as an indicator of visibility and optimum imaging conditions predicted
VLBI imaging of a flare in the Crab Nebula: More than just a spot
We report on very long baseline interferometry (VLBI) observations of the
radio emission from the inner region of the Crab Nebula, made at 1.6 GHz and 5
GHz after a recent high-energy flare in this object. The 5 GHz data have
provided only upper limits of 0.4 milli-Jansky (mJy) on the flux density of the
pulsar and 0.4 mJy/beam on the brightness of the putative flaring region. The
1.6 GHz data have enabled imaging the inner regions of the nebula on scales of
up to ~40". The emission from the inner "wisps" is detected for the first time
with VLBI observations. A likely radio counterpart (designated "C1") of the
putative flaring region observed with Chandra and HST is detected in the radio
image, with an estimated flux density of \,mJy and a size of
0.2-0.6". Another compact feature ("C2") is also detected in the VLBI image
closer to the pulsar, with an estimated flux density of 0.4 +- 0.2 mJy and a
size smaller than 0{\farcs}2. Combined with the broad-band SED of the flare,
the radio properties of C1 yield a lower limit of ~0.5 mG for the magnetic
field and a total minimum energy of 1.2*10^41 ergs vested in the flare
(corresponding to using about 0.2% of the pulsar spin-down power). The 1.6 GHz
observations provide upper limits for the brightness (0.2 mJy/beam) and total
flux density (0.4 mJy) of the optical Knot 1 located at 0.6" from the pulsar.
The absolute position of the Crab pulsar is determined, and an estimate of the
pulsar proper motion is obtained.Comment: Astronomy & Astrophysics; accepted; 10 pages, 8 figure
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