3,679 research outputs found
Hard X-ray emission and Ti line features of Tycho Supernova Remnant
A deep hard X-ray survey of the INTEGRAL satellite first detected the
non-thermal emission up to 90 keV in the Tycho supernova (SN) remnant. Its 3 --
100 keV spectrum is fitted with a thermal bremsstrahlung of keV plus a power-law model of . Based on the
diffusive shock acceleration theory, this non-thermal emission, together with
radio measurements, implies that Tycho remnant may not accelerate protons up to
PeV but hundreds TeV. Only heavier nuclei may be accelerated to the cosmic
ray spectral "knee". In addition, we search for soft gamma-ray lines at 67.9
and 78.4 keV coming from the decay of radioactive Ti in Tycho remnant by
INTEGRAL. A bump feature in the 60-90 keV energy band, potentially associated
with the Ti line emission, is found with a marginal significance level
of 2.6 . The corresponding 3 upper limit on the
Ti line flux amounts to 1.5 10 ph cm s.
Implications on the progenitor of Tycho SN, considered to be the prototype of
type Ia SN, are discussed.Comment: 15 pages, 4 figures, accepted for publication in Ap
Hard X-ray emissions from Cassiopeia A observed by INTEGRAL
Cassiopeia A (Cas A) as the nearby young remnant of a core-collapse supernova
is the best candidate for astrophysical studies in supernova explosion and its
environment. We studied hard X-ray emissions from Cas A using the ten-year data
of INTEGRAL observations, and first detected non-thermal continuum emission
from the source up to 220 keV. The Ti line emissions at 68 and 78 keV
are confirmed by our observations with a mean flux of ph cm s, corresponding to a Ti yield in Cas A of
\ms. The continuum emission from 3 -- 500 keV can
be fitted with a thermal bremsstrahlung of keV plus a
power-law model of . The non-thermal emission from
Cas A is well fitted with a power-law model without a cutoff up to 220 keV.
This radiation characteristic is inconsistent with the diffusive shock
acceleration models with the remnant shock velocity of only 5000km s.
The central compact object in Cas A cannot contribute to the emission above 80
keV significantly. Some possible physical origins of the non-thermal emission
above 80 keV from the remnant shock are discussed. We deduce that the
asymmetrical supernova explosion scenario of Cas A is a promising scenario to
produce high energy synchrotron radiation photons, where a part of ejecta with
the velocity of and opening angle of can account for
the 100-keV emission, consistent with the "jet" observed in Cas A.Comment: 20 pages, 6 figures, 2 tables; accepted for the publication in Ap
2D+3D Indoor Scene Understanding from a Single Monocular Image
Scene understanding, as a broad field encompassing many
subtopics, has gained great interest in recent years. Among these
subtopics, indoor scene understanding, having its own specific
attributes and challenges compared to outdoor scene under-
standing, has drawn a lot of attention. It has potential
applications in a wide variety of domains, such as robotic
navigation, object grasping for personal robotics, augmented
reality, etc. To our knowledge, existing research for indoor
scenes typically makes use of depth sensors, such as Kinect, that
is however not always available.
In this thesis, we focused on addressing the indoor scene
understanding tasks in a general case, where only a monocular
color image of the scene is available. Specifically, we first
studied the problem of estimating a detailed depth map from a
monocular image. Then, benefiting from deep-learning-based depth
estimation, we tackled the higher-level tasks of 3D box proposal
generation, and scene parsing with instance segmentation,
semantic labeling and support relationship inference from a
monocular image. Our research on indoor scene understanding
provides a comprehensive scene interpretation at various
perspectives and scales.
For monocular image depth estimation, previous approaches are
limited in that they only reason about depth locally on a single
scale, and do not utilize the important information of geometric
scene structures. Here, we developed a novel graphical model,
which reasons about detailed depth while leveraging geometric
scene structures at multiple scales.
For 3D box proposals, to our best knowledge, our approach
constitutes the first attempt to reason about class-independent
3D box proposals from a single monocular image. To this end, we
developed a novel integrated, differentiable framework that
estimates depth, extracts a volumetric scene representation and
generates 3D proposals. At the core of this framework lies a
novel residual, differentiable truncated signed distance function
module, which is able to handle the relatively low accuracy of
the predicted depth map.
For scene parsing, we tackled its three subtasks of instance
segmentation, se- mantic labeling, and the support relationship
inference on instances. Existing work typically reasons about
these individual subtasks independently. Here, we leverage the
fact that they bear strong connections, which can facilitate
addressing these sub- tasks if modeled properly. To this end, we
developed an integrated graphical model that reasons about the
mutual relationships of the above subtasks.
In summary, in this thesis, we introduced novel and effective
methodologies for each of three indoor scene understanding tasks,
i.e., depth estimation, 3D box proposal generation, and scene
parsing, and exploited the dependencies on depth estimates of the
latter two tasks. Evaluation on several benchmark datasets
demonstrated the effectiveness of our algorithms and the benefits
of utilizing depth estimates for higher-level tasks
A unified gas kinetic scheme for transport and collision effects in plasma
In this study, the Vlasov-Poisson equation with or without collision term for
plasma is solved by the unified gas kinetic scheme (UGKS). The Vlasov equation
is a differential equation describing time evolution of the distribution
function of plasma consisting of charged particles with long-range interaction.
The distribution function is discretized in discrete particle velocity space.
After the Vlasov equation is integrated in finite volumes of physical space,
the numerical flux across a cell interface and source term for particle
acceleration are computed to update the distribution function at next time
step. The flux is decided by Riemann problem and variation of distribution
function in discrete particle velocity space is evaluated with central
difference method. A electron-ion collision model is introduced in the Vlasov
equation. This finite volume method for the UGKS couples the free transport and
long-range interaction between particles. The electric field induced by charged
particles is controlled by the Poisson's equation. In this paper, the Poisson's
equation is solved using the Green's function for two dimensional plasma system
subjected to the symmetry or periodic boundary conditions. Two numerical tests
of the linear Landau damping and the Gaussian beam are carried out to validate
the proposed method. The linear electron plasma wave damping is simulated based
on electron-ion collision operator. Compared with previous methods, it is shown
that the current method is able to obtain accurate results of the
Vlasov-Poisson equation with a time step much larger than the particle
collision time. Highly non-equilibrium and rarefied plasma flows, such as
electron flows driven by electromagnetic field, can be simulated easily.Comment: 33 pages, 13 figure
Secondary-electron radiation accompanying hadronic GeV-TeV gamma-rays from supernova remnants
The synchrotron radiation from secondary electrons and positrons (SEPs)
generated by hadronic interactions in the shock of supernova remnant (SNR)
could be a distinct evidence of cosmic ray (CR) production in SNR shocks. Here
we provide a method where the observed gamma-ray flux from SNRs, created by
pion decays, is directly used to derive the SEP distribution and hence the
synchrotron spectrum. We apply the method to three gamma-ray bright SNRs. In
the young SNR RX J1713.7-3946, if the observed GeV-TeV gamma-rays are of
hadronic origin and the magnetic field in the SNR shock is mG,
the SEPs may produce a spectral bump at eV, exceeding the
predicted synchrotron component of the leptonic model, and a soft spectral tail
at keV, distinct from the hard spectral slope in the leptonic
model. In the middle-aged SNRs IC443 and W44, if the observed gamma-rays are of
hadronic origin, the SEP synchrotron radiation with G can
well account for the observed radio flux and spectral slopes, supporting the
hadronic origin of gamma-rays. Future microwave to far-infrared and hard X-ray
(>100keV) observations are encouraged to constraining the SEP radiation and the
gamma-ray origin in SNRs.Comment: 9 pages, 5 figures and 1 table, MNRAS accepte
Entanglement concentration for unknown atomic entangled states via entanglement swapping
An entanglement concentration scheme for unknown atomic entanglement states
is proposed via entanglement swapping in cavity QED. Because the interaction
used here is a large-detuned one between two driven atoms and a quantized
cavity mode, the effects of the cavity decay and thermal field have been
eliminated. These advantages can warrant the experimental feasibility of the
current scheme.Comment: 4 page
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