127 research outputs found
Relational Prior Knowledge Graphs for Detection and Instance Segmentation
Humans have a remarkable ability to perceive and reason about the world around them by understanding the relationships between objects. In this paper, we investigate the effectiveness of using such relationships for object detection and instance segmentation. To this end, we propose a Relational Prior-based Feature Enhancement Model (RP-FEM), a graph transformer that enhances object proposal features using relational priors. The proposed architecture operates on top of scene graphs obtained from initial proposals and aims to concurrently learn relational context modeling for object detection and instance segmentation.Experimental evaluations on COCO show that the utilization of scene graphs, augmented with relational priors, offer benefits for object detection and instance segmentation. RP-FEM demonstrates its capacity to suppress improbable class predictions within the image while also preventing the model from generating duplicate predictions, leading to improvements over the baseline model on which it is built.</p
Gamma-ray bursts: optical afterglows in the deep Newtonian phase
Gamma-ray burst remnants become trans-relativistic typically in days to tens
of days, and they enter the deep Newtonian phase in tens of days to months,
during which the majority of shock-accelerated electrons will no longer be
highly relativistic. However, a small portion of electrons are still
accelerated to ultra-relativistic speeds and capable of emitting synchrotron
radiation. The distribution function for electrons is re-derived here so that
synchrotron emission from these relativistic electrons can be calculated. Based
on the revised model, optical afterglows from both isotropic fireballs and
highly collimated jets are studied numerically, and compared to analytical
results. In the beamed cases, it is found that, in addition to the steepening
due to the edge effect and the lateral expansion effect, the light curves are
universally characterized by a flattening during the deep Newtonian phase.Comment: MNRAS in press (originally submitted in October 2002), 8 pages with 8
eps figures embedded, references update
Variation of microphysics in wind bubbles: an alternative mechanism for explaining the rebrightenings in GRB afterglows
Conventionally, long GRBs are thought to be caused by the core collapses of
massive stars. During the lifetime of a massive star, a stellar wind bubble
environment should be produced. Furthermore, the microphysics shock parameters
may vary along with the evolution of the fireball. Here we investigate the
variation of the microphysics shock parameters under the condition of wind
bubble environment, and allow the microphysics shock parameters to be
discontinuous at shocks in the ambient medium. It is found that our model can
acceptably reproduce the rebrightenings observed in GRB afterglows, at least in
some cases. The effects of various model parameters on rebrightenings are
investigated. The rebrightenings observed in both the R-band and X-ray
afterglow light curves of GRB 060206, GRB 070311 and GRB 071010A are reproduced
in this model.Comment: 9 figures, accepted by MNRA
Gamma-Ray Bursts: Progress, Problems & Prospects
The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad
observational facts and empirical phenomenological relations of the GRB prompt
emission and afterglow are outlined. A well-tested, successful fireball shock
model is introduced in a pedagogical manner. Several important uncertainties in
the current understanding of the phenomenon are reviewed, and prospects of how
future experiments and extensive observational and theoretical efforts may
address these problems are discussed.Comment: 86 pages, 17 figures, 566 references, an invited review for
International Journal of Modern Physics A, in pres
Relativistic Mass Ejecta from Phase-transition-induced Collapse of Neutron Stars
We study the dynamical evolution of a phase-transition-induced collapse
neutron star to a hybrid star, which consists of a mixture of hadronic matter
and strange quark matter. The collapse is triggered by a sudden change of
equation of state, which result in a large amplitude stellar oscillation. The
evolution of the system is simulated by using a 3D Newtonian hydrodynamic code
with a high resolution shock capture scheme. We find that both the temperature
and the density at the neutrinosphere are oscillating with acoustic frequency.
However, they are nearly 180 out of phase. Consequently, extremely
intense, pulsating neutrino/antineutrino fluxes will be emitted periodically.
Since the energy and density of neutrinos at the peaks of the pulsating fluxes
are much higher than the non-oscillating case, the electron/positron pair
creation rate can be enhanced dramatically. Some mass layers on the stellar
surface can be ejected by absorbing energy of neutrinos and pairs. These mass
ejecta can be further accelerated to relativistic speeds by absorbing
electron/positron pairs, created by the neutrino and antineutrino annihilation
outside the stellar surface. The possible connection between this process and
the cosmological Gamma-ray Bursts is discussed.Comment: 40 pages, 11 figures, accepted for publication in JCA
High energy cosmic-rays: puzzles, models, and giga-ton neutrino telescopes
The existence of cosmic rays of energies exceeding 10^20 eV is one of the
mysteries of high energy astrophysics. The spectrum and the high energy to
which it extends rule out almost all suggested source models. The challenges
posed by observations to models for the origin of high energy cosmic rays are
reviewed, and the implications of recent new experimental results are
discussed. Large area high energy cosmic ray detectors and large volume high
energy neutrino detectors currently under construction may resolve the high
energy cosmic ray puzzle, and shed light on the identity and physics of the
most powerful accelerators in the universe.Comment: 12 pages, 7 figures; Summary of review talk, PASCOS 03 (Mumbai,
India
X-Ray Flares of Gamma-Ray Bursts: Quakes of Solid Quark Stars?
We propose a star-quake model to understand X-ray flares of both long and
short Gamma-ray bursts (GRBs) in a solid quark star regime. Two kinds of
central engines for GRBs are available if pulsar-like stars are actually
(solid) quark stars, i.e., the SNE-type GRBs and the SGR-type GRBs. It is found
that a quark star could be solidified about 10^3 to 10^6 s later after its
birth if the critical temperature of phase transition is a few MeV, and then a
new source of free energy (i.e., elastic and gravitational ones, rather than
rotational or magnetic energy) could be possible to power GRB X-ray flares.Comment: 8 pages, latex file. 2 figures. To appear in Science in China Series
The Broad-lined Type Ic SN 2003jd
The results of a world-wide coordinated observational campaign on the
broad-lined Type Ic SN 2003jd are presented. In total, 74 photometric data
points and 26 spectra were collected using 11 different telescopes. SN 2003jd
is one of the most luminous SN Ic ever observed. A comparison with other Type
Ic supernovae (SNe Ic) confirms that SN 2003jd represents an intermediate case
between broad-line events (2002ap, 2006aj), and highly energetic SNe (1997ef,
1998bw, 2003dh, 2003lw), with an ejected mass of M_{ej} = 3.0 +/- 1 Mo and a
kinetic energy of E_{k}(tot) = 7_{-2}^{+3} 10^{51} erg. SN 2003jd is similar to
SN 1998bw in terms of overall luminosity, but it is closer to SNe 2006aj and
2002ap in terms of light-curve shape and spectral evolution. The comparison
with other SNe Ic, suggests that the V-band light curves of SNe Ic can be
partially homogenized by introducing a time stretch factor. Finally, due to the
similarity of SN 2003jd to the SN 2006aj/XRF 060218 event, we discuss the
possible connection of SN 2003jd with a GRB.Comment: 19 pages, 19 figures, Accepted for publication in MNRA
Hypernovae and Other Black-Hole-Forming Supernovae
During the last few years, a number of exceptional core-collapse supernovae
(SNe) have been discovered. Their kinetic energy of the explosions are larger
by more than an order of magnitude than the typical values for this type of
SNe, so that these SNe have been called `Hypernovae'. We first describe how the
basic properties of hypernovae can be derived from observations and modeling.
These hypernovae seem to come from rather massive stars, thus forming black
holes. On the other hand, there are some examples of massive SNe with only a
small kinetic energy. We suggest that stars with non-rotating black holes are
likely to collapse "quietly" ejecting a small amount of heavy elements (Faint
supernovae). In contrast, stars with rotating black holes are likely to give
rise to very energetic supernovae (Hypernovae). We present distinct
nucleosynthesis features of these two types of "black-hole-forming" supernovae.
Hypernova nucleosynthesis is characterized by larger abundance ratios
(Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe. Nucleosynthesis in Faint supernovae is
characterized by a large amount of fall-back. We show that the abundance
pattern of the most Fe deficient star, HE0107-5240, and other extremely
metal-poor carbon-rich stars are in good accord with those of
black-hole-forming supernovae, but not pair-instability supernovae. This
suggests that black-hole-forming supernovae made important contributions to the
early Galactic (and cosmic) chemical evolution.Comment: 49 pages, to be published in "Stellar Collapse" (Astrophysics and
Space Science; Kluwer) ed. C. L. Fryer (2003
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