2,917 research outputs found
The Vela pulsar `jet': a companion-punctured bubble of fallback material
Markwardt and Oegelman (1995) used ROSAT to reveal a 12 by 45 arcmin
structure in 1 keV X rays around the Vela pulsar, which they interpret as a jet
emanating from the pulsar. We here present an alternative view of the nature of
this feature, namely that it consists of material from very deep inside the
exploding star, close to the mass cut between material that became part of the
neutron star and ejected material. The initial radial velocity of the inner
material was lower than the bulk of the ejecta, and formed a bubble of slow
material that started expanding again due to heating by the young pulsar's
spindown energy. The expansion is mainly in one direction, and to explain this
we speculate that the pre-supernova system was a binary. The explosion caused
the binary to unbind, and the pulsar's former companion carved a lower-density
channel into the main ejecta. The resulting puncture of the bubble's edge
greatly facilitated expansion along its path relative to other directions. If
this is the case, we can estimate the current speed of the former binary
companion and from this reconstruct the presupernova binary orbit. It follows
that the exploding star was a helium star, hence that the supernova was of type
Ib. Since the most likely binary companion is another neutron star, the
evolution of the Vela remnant and its surroundings has been rather more
complicated than the simple expansion of one supernova blast wave into
unperturbed interstellar material.Comment: submitted to MNRAS; 6 pages laTeX, 3 figures (1 postscript, 2 gif
files of images
Aharonov-Bohm effect for excitons in a semiconductor quantum ring dressed by circularly polarized light
We show theoretically that the strong coupling of circularly polarized
photons to an exciton in ring-like semiconductor nanostructures results in
physical nonequivalence of clockwise and counterclockwise exciton rotations in
the ring. As a consequence, the stationary energy splitting of exciton states
corresponding to these mutually opposite rotations appears. This excitonic
Aharonov-Bohm effect depends on the intensity and frequency of the circularly
polarized field and can be detected in state-of-the-art optical experiments.Comment: Published versio
The Survival Rate of Ejected Terrestrial Planets with Moons
During planet formation, a gas giant will interact with smaller protoplanets
that stray within its sphere of gravitational influence. We investigate the
outcome of interactions between gas giants and terrestrial-sized protoplanets
with lunar-sized companions. An interaction between a giant planet and a
protoplanet binary may have one of several consequences, including the delivery
of volatiles to the inner system, the capture of retrograde moons by the giant
planet, and the ejection of one or both of the protoplanets. We show that an
interesting fraction of terrestrial-sized planets with lunar sized companions
will likely be ejected into interstellar space with the companion bound to the
planet. The companion provides an additional source of heating for the planet
from tidal dissipation of orbital and spin angular momentum. This heat flux
typically is larger than the current radiogenic heating of the Earth for up to
the first few hundred million years of evolution. In combination with an
atmosphere of sufficient thickness and composition, the heating can provide the
conditions necesary for liquid water to persist on the surface of the
terrestrial mass planet, making it a potential site for life. We also determine
the possibility for directly detecting such systems through all-sky infrared
surveys or microlensing surveys. Microlensing surveys in particular will
directly measure the frequency of this phenomenon.Comment: 4 pages, 2 figures, Accepted to ApJ
Switching waves in multi-level incoherently driven polariton condensates
We show theoretically that an open-dissipative polariton condensate confined
within a trapping potential and driven by an incoherent pumping scheme gives
rise to bistability between odd and even modes of the potential. Switching from
one state to the other can be controlled via incoherent pulsing which becomes
an important step towards construction of low-powered opto-electronic devices.
The origin of the effect comes from modulational instability between odd and
even states of the trapping potential governed by the nonlinear
polariton-polariton interactions
Parity solitons in nonresonantly driven-dissipative condensate channels
We study analytically and numerically the condensation of a
driven-dissipative exciton-polariton system using symmetric nonresonant pumping
geometries. We show that the lowest condensation threshold solution carries a
definite parity as a consequence of the symmetric excitation profile. At higher
pump intensities competition between the two parities can result in critical
quenching of one and saturation of the other. Using long pump channels, we show
that the competition of the condensate parities gives rise to a different type
of topologically stable defect propagating indefinitely along the condensate.
The defects display repulsive interactions and are characterized by a sustained
wavepacket carrying a pair of opposite parity domain walls in the condensate
channel
Much Ado About Time: Exhaustive Annotation of Temporal Data
Large-scale annotated datasets allow AI systems to learn from and build upon
the knowledge of the crowd. Many crowdsourcing techniques have been developed
for collecting image annotations. These techniques often implicitly rely on the
fact that a new input image takes a negligible amount of time to perceive. In
contrast, we investigate and determine the most cost-effective way of obtaining
high-quality multi-label annotations for temporal data such as videos. Watching
even a short 30-second video clip requires a significant time investment from a
crowd worker; thus, requesting multiple annotations following a single viewing
is an important cost-saving strategy. But how many questions should we ask per
video? We conclude that the optimal strategy is to ask as many questions as
possible in a HIT (up to 52 binary questions after watching a 30-second video
clip in our experiments). We demonstrate that while workers may not correctly
answer all questions, the cost-benefit analysis nevertheless favors consensus
from multiple such cheap-yet-imperfect iterations over more complex
alternatives. When compared with a one-question-per-video baseline, our method
is able to achieve a 10% improvement in recall 76.7% ours versus 66.7%
baseline) at comparable precision (83.8% ours versus 83.0% baseline) in about
half the annotation time (3.8 minutes ours compared to 7.1 minutes baseline).
We demonstrate the effectiveness of our method by collecting multi-label
annotations of 157 human activities on 1,815 videos.Comment: HCOMP 2016 Camera Read
- …