2,427 research outputs found
Multi-epoch, multi-frequency VLBI study of the parsec-scale jet in the blazar 3C 66A
We present the observational results of the Gamma-ray blazar, 3C 66A, at 2.3,
8.4, and 22 GHz at 4 epochs during 2004-05 with the VLBA. The resulting images
show an overall core-jet structure extending roughly to the south with two
intermediate breaks occurring in the region near the core. By model-fitting to
the visibility data, the northmost component, which is also the brightest, is
identified as the core according to its relatively flat spectrum and its
compactness. As combined with some previous results to investigate the proper
motions of the jet components, it is found the kinematics of 3C 66A is quite
complicated with components of inward and outward, subluminal and superluminal
motions all detected in the radio structure. The superluminal motions indicate
strong Doppler boosting exists in the jet. The apparent inward motions of the
innermost components last for at least 10 years and could not be caused by
new-born components. The possible reason could be non-stationarity of the core
due to opacity change.Comment: 24 pages, 4 figure
An hourglass model for the flare of HST-1 in M87
To explain the multi-wavelength light curves (from radio to X-ray) of HST-1
in the M87 jet, we propose an hourglass model that is a modified two-zone
system of Tavecchio & Ghisellini (hereafter TG08): a slow hourglass-shaped or
Laval nozzle-shaped layer connected by two revolving exponential surfaces
surrounding a fast spine, through which plasma blobs flow. Based on the
conservation of magnetic flux, the magnetic field changes along the axis of the
hourglass. We adopt the result of TG08---the high-energy emission from GeV to
TeV can be produced through inverse Compton by the two-zone system, and the
photons from radio to X-ray are mainly radiated by the fast inner zone system.
Here, we only discuss the light curves of the fast inner blob from radio to
X-ray. When a compressible blob travels down the axis of the first bulb in the
hourglass, because of magnetic flux conservation, its cross section experiences
an adiabatic compression process, which results in particle acceleration and
the brightening of HST-1. When the blob moves into the second bulb of the
hourglass, because of magnetic flux conservation, the dimming of the knot
occurs along with an adiabatic expansion of its cross section. A similar broken
exponential function could fit the TeV peaks in M87, which may imply a
correlation between the TeV flares of M87 and the light curves from radio to
X-ray in HST-1. The Very Large Array (VLA) 22 GHz radio light curve of HST-1
verifies our prediction based on the model fit to the main peak of the VLA 15
GHz radio light curve.Comment: 14 pages, 2 figures, accepted for publication in A
Lightweight Vision Transformer with Cross Feature Attention
Recent advances in vision transformers (ViTs) have achieved great performance
in visual recognition tasks. Convolutional neural networks (CNNs) exploit
spatial inductive bias to learn visual representations, but these networks are
spatially local. ViTs can learn global representations with their
self-attention mechanism, but they are usually heavy-weight and unsuitable for
mobile devices. In this paper, we propose cross feature attention (XFA) to
bring down computation cost for transformers, and combine efficient mobile CNNs
to form a novel efficient light-weight CNN-ViT hybrid model, XFormer, which can
serve as a general-purpose backbone to learn both global and local
representation. Experimental results show that XFormer outperforms numerous CNN
and ViT-based models across different tasks and datasets. On ImageNet1K
dataset, XFormer achieves top-1 accuracy of 78.5% with 5.5 million parameters,
which is 2.2% and 6.3% more accurate than EfficientNet-B0 (CNN-based) and DeiT
(ViT-based) for similar number of parameters. Our model also performs well when
transferring to object detection and semantic segmentation tasks. On MS COCO
dataset, XFormer exceeds MobileNetV2 by 10.5 AP (22.7 -> 33.2 AP) in YOLOv3
framework with only 6.3M parameters and 3.8G FLOPs. On Cityscapes dataset, with
only a simple all-MLP decoder, XFormer achieves mIoU of 78.5 and FPS of 15.3,
surpassing state-of-the-art lightweight segmentation networks.Comment: Technical Repor
(3R*)-Methyl 3-[(2S*)-4,6-dimethoxy-2-(4-methoxyphenyl)-3-oxo-2,3-dihydro-1-benzofuran-2-yl]-2-methoxycarbonyl-3-phenylpropionate
The title compound, C29H28O9, was isolated from the reaction of 4,6-dimethÂoxy-2-(4-methoxyÂphenÂyl)-3-benzofuran and α-methoxyÂcarbonylÂcinnaminate. The two aromatic rings form a dihedral angle of 22.7 (1)°. One methoxyÂcarbonyl group is disordered between two orientations in a 0.612 (4):0.388 (4) ratio. The crystal structure exhibits no significantly short interÂmolecular contacts
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