519,357 research outputs found
Towards Informative Path Planning for Acoustic SLAM
Acoustic scene mapping is a challenging task as microphone arrays can often localize sound sources only in terms of their directions. Spatial diversity can be exploited constructively to infer source-sensor range when using microphone arrays installed on moving platforms, such as robots. As the absolute location of a moving robot is often unknown in practice, Acoustic Simultaneous Localization And Mapping (a-SLAM) is required in order to localize the moving robot’s positions and jointly map the sound sources. Using a novel a-SLAM approach, this paper investigates the impact of the choice of robot paths on source mapping accuracy. Simulation results demonstrate that a-SLAM performance can be improved by informatively planning robot paths
Cluster Multi-spacecraft Determination of AKR Angular Beaming
Simultaneous observations of AKR emission using the four-spacecraft Cluster
array were used to make the first direct measurements of the angular beaming
patterns of individual bursts. By comparing the spacecraft locations and AKR
burst locations, the angular beaming pattern was found to be narrowly confined
to a plane containing the magnetic field vector at the source and tangent to a
circle of constant latitude. Most rays paths are confined within 15 deg of this
tangent plane, consistent with numerical simulations of AKR k-vector
orientation at maximum growth rate. The emission is also strongly directed
upward in the tangent plane, which we interpret as refraction of the rays as
they leave the auroral cavity. The narrow beaming pattern implies that an
observer located above the polar cap can detect AKR emission only from a small
fraction of the auroral oval at a given location. This has important
consequences for interpreting AKR visibility at a given location. It also helps
re-interpret previously published Cluster VLBI studies of AKR source locations,
which are now seen to be only a subset of all possible source locations. These
observations are inconsistent with either filled or hollow cone beaming models.Comment: 5 pages, 4 figures. Geophys. Res. Letters (accepted
Attenuation correction for TOF-PET with a limited number of stationary coincidence line-sources
INTRODUCTION Accurate attenuation correction remains a major issue in combined PET/MRI. We have previously presented a method to derive the attenuation map by performing a transmission scan using an annulus-shaped source placed close to the edge of the FOV of the scanner. With this method, simultaneous transmission and emission data acquisition is possible as transmission data can be extracted using Time-of-Flight (TOF) information. As this method is strongly influenced by photon scatter and dead time effects, its performance depends on the accuracy of the correction techniques for these effects. In this work we present a new approach in which the annulus source is replaced with a limited number of line-sources positioned at 35 cm from the center of the FOV. By including the location of the line sources into the algorithm, the extraction of true transmission data can be improved. The setup was validated with simulations studies and evaluated with a phantom study acquired on the LaBr3-based TOF-PET scanner installed at UPENN. MATERIALS AND METHODS First we performed GATE simulations using the digital NCAT phantom. The phantom was segmented into bone, lung and soft-tissue and injected with 6.5 Mbq/kg 18F-FDG. Simultaneous transmission/emission scans of 3 minutes were simulated using 6, 12 and 24 18F-FDG line sources with a total activity of 0.5 mCi. To obtain the attenuation map, the transmission data is first extracted using TOF information. To reduce misclassification of prompt emission data as transmission data, only events on LORs, which pass within a radial distance of 1 cm from at least one line source, are accepted. The attenuation map is then reconstructed using an iterative gradient descent approach. As a proof of concept, the method was evaluated on the LaBr3-based TOF PET scanner using an anthropomorphic torso phantom injected with 2mCi of 18F-FDG. 24 line-sources of 20μCi each were fixed to a wooden template at the back of the scanner. Simultaneous transmission/emission scans were acquired using 24 line sources. RESULTS Simulation results demonstrate that the fraction of scattered emission events classified as transmission data was reduced from 4.32% with the annulus source to 2.29%, 1.25% and 0.63% for the 24, 12 and 6 line sources respectively. The fraction of misclassified true emission events was reduced from 1.10% to 0.42%, 0.24% and 0.13% respectively. Only in case of 6 line sources, the attenuation maps showed severe artifacts. Compared to the classification solely based on TOF-information, preliminary experimental results indicate an improvement in the accuracy of the attenuation coefficients of 10.44%, 0.12% and 5.09% for soft-tissue, lung and bone tissue respectively. CONCLUSION The proposed method can be used for attenuation correction in sequential or simultaneous TOF-PET/MRI systems. The PET transmission and emission data are acquired simultaneously so no acquisition time for attenuation correction is lost in PET or MRI. Attenuation maps with higher accuracy can be obtained by including information about the location of the line-sources. However, at least 12 line sources are needed to avoid severe artifacts
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Sound Organization by Source Models in Humans and Machines
When extracting information from simultaneous sound sources, listeners successfully exploit many different factors spanning spatial location and source characteristics. I will argue that detailed constraints on the form of particular source signals are being employed, and that this therefore is an important direction for research into automatic sound organization systems, in applications ranging from speech separation to environmental sound classification to music understanding
V1647 Orionis (IRAS 05436-0007) : A New Look at McNeil's Nebula
We present a study of the newly discovered McNeil's nebula in Orion using the
JHKs-band simultaneous observations with the near-infrared (NIR) camera SIRIUS
on the IRSF 1.4m telescope. The cometary infrared nebula is clearly seen
extending toward north and south from the NIR source (V1647 Orionis) that
illuminates McNeil's nebula. The compact nebula has an apparent diameter of
about 70 arcsec. The nebula is blue (bright in J) and has a cavity structure
with two rims extending toward north-east and north-west. The north-east rim is
brighter and sharp, while the north-west rim is diffuse. The north-east rim can
be traced out to ~ 40 arcsec from the location of the NIR source. In contrast,
no cavity structure is seen toward the south, although diffuse nebula is
extended out to ~ 20 arcsec. New NIR photometric data show a significant
variation in the magnitudes (> 0.15 mag) of the source of McNeil's nebula
within a period of one week, that is possibly under the phase of eruptive
variables like FUors or EXors.Comment: 13 pages, 5 figures in JPEG format. Accepted for the publication in
PASJ Letter
An X-ray-UV correlation in Cen X-4 during quiescence
Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is
seen to be variable on timescales from hundreds of seconds to years, suggesting
that at least in this object, low-level accretion is important during
quiescence. Here we present results from recent XMM-Newton and Swift
observations of Cen X-4, where the X-ray flux (0.5 - 10 keV) varies by a factor
of 6.5 between the brightest and faintest states. We find a positive
correlation between the X-ray flux and the simultaneous near-UV flux, where as
there is no significant correlation between the X-ray and simultaneous optical
(V, B) fluxes. This suggests that while the X-ray and UV emitting regions are
somehow linked, the optical region originates elsewhere. Comparing the
luminosities, it is plausible that the UV emission originates due to
reprocessing of the X-ray flux by the accretion disk, with the hot inner region
of the disk being a possible location for the UV emitting region. The optical
emission, however, could be dominated by the donor star. The X-ray/UV
correlation does not favour the accretion stream-impact point as the source of
the UV emission.Comment: 8 pages, 3 figures, accepted for publication in MNRA
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