73,760 research outputs found
Chandra detection of extended X-ray emission from the recurrent nova RS Ophiuchi
Radio, infrared, and optical observations of the 2006 eruption of the
symbiotic recurrent nova RS Ophiuchi (RS Oph) showed that the explosion
produced non-spherical ejecta. Some of this ejected material was in the form of
bipolar jets to the east and west of the central source. Here we describe Xray
observations taken with the Chandra X-ray Observatory one and a half years
after the beginning of the outburst that reveal narrow, extended structure with
a position angle of approximately 300 degrees (east of north). Although the
orientation of the extended feature in the X-ray image is consistent with the
readout direction of the CCD detector, extensive testing suggests that the
feature is not an artifact. Assuming it is not an instrumental effect, the
extended X-ray structure shows hot plasma stretching more than 1,900 AU from
the central binary (taking a distance of 1.6 kpc). The X-ray emission is
elongated in the northwest direction - in line with the extended infrared
emission and some minor features in the published radio image. It is less
consistent with the orientation of the radio jets and the main bipolar optical
structure. Most of the photons in the extended X-ray structure have energies of
less than 0.8 keV. If the extended X-ray feature was produced when the nova
explosion occurred, then its 1".2 length as of 2007 August implies that it
expanded at an average rate of more than 2 mas/d, which corresponds to a flow
speed of greater than 6,000 km/s (d/1.6 kpc) in the plane of the sky. This
expansion rate is similar to the earliest measured expansion rates for the
radio jets.Comment: accepted in Ap
Viterbi Training for PCFGs: Hardness Results and Competitiveness of Uniform Initialization
We consider the search for a maximum likelihood assignment of hidden derivations and grammar weights for a probabilistic context-free grammar, the problem approximately solved by “Viterbi training.” We show that solving and even approximating Viterbi training for PCFGs is NP-hard. We motivate the use of uniformat-random initialization for Viterbi EM as an optimal initializer in absence of further information about the correct model parameters, providing an approximate bound on the log-likelihood.
Empirical Risk Minimization for Probabilistic Grammars: Sample Complexity and Hardness of Learning
Probabilistic grammars are generative statistical models that are useful for compositional and sequential structures. They are used ubiquitously in computational linguistics. We present a framework, reminiscent of structural risk minimization, for empirical risk minimization of probabilistic grammars using the log-loss. We derive sample complexity bounds in this framework that apply both to the supervised setting and the unsupervised setting. By making assumptions about the underlying distribution that are appropriate for natural language scenarios, we are able to derive distribution-dependent sample complexity bounds for probabilistic grammars. We also give simple algorithms for carrying out empirical risk minimization using this framework in both the supervised and unsupervised settings. In the unsupervised case, we show that the problem of minimizing empirical risk is NP-hard. We therefore suggest an approximate algorithm, similar to expectation-maximization, to minimize the empirical risk. Learning from data is central to contemporary computational linguistics. It is in common in such learning to estimate a model in a parametric family using the maximum likelihood principle. This principle applies in the supervised case (i.e., using annotate
Empirical Risk Minimization with Approximations of Probabilistic Grammars
Probabilistic grammars are generative statistical models that are useful for compositional and sequential structures. We present a framework, reminiscent of structural risk minimization, for empirical risk minimization of the parameters of a fixed probabilistic grammar using the log-loss. We derive sample complexity bounds in this framework that apply both to the supervised setting and the unsupervised setting.
The locust frontal ganglion: a central pattern generator network controlling foregut rhythmic motor patterns
The frontal ganglion (FG) is part of the insect
stomatogastric nervous system and is found in most insect
orders. Previous work has shown that in the desert locust,
Schistocerca gregaria, the FG constitutes a major source of
innervation to the foregut. In an in vitro preparation,
isolated from all descending and sensory inputs, the FG
spontaneously generated rhythmic multi-unit bursts of
action potentials that could be recorded from all its
efferent nerves. The consistent endogenous FG rhythmic
pattern indicates the presence of a central pattern
generator network. We found the appearance of in vitro
rhythmic activity to be strongly correlated with the
physiological state of the donor locust. A robust pattern
emerged only after a period of saline superfusion, if the
locust had a very full foregut and crop, or if the animal
was close to ecdysis. Accordingly, haemolymph collected
at these stages inhibited an ongoing rhythmic pattern
when applied onto the ganglion. We present this novel
central pattern generating system as a basis for future
work on the neural network characterisation and its role
in generating and controlling behaviour
Joint Morphological and Syntactic Disambiguation
In morphologically rich languages, should morphological and syntactic disambiguation be treated sequentially or as a single problem? We describe several efficient, probabilistically interpretable ways to apply joint inference to morphological and syntactic disambiguation using lattice parsing. Joint inference is shown to compare favorably to pipeline parsing methods across a variety of component models. State-of-the-art performance on Hebrew Treebank parsing is demonstrated using the new method. The benefits of joint inference are modest with the current component models, but appear to increase as components themselves improve
First Results of the 74 MHz VLA-Pie Town Link. Hercules A at Low Frequencies
We present the results of the first successful observations of the Pie Town
link with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement
in resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like
features seen at higher frequencies. We also present new high dynamic range
images of this powerful radio galaxy at 325 MHz. Our low frequency observations
confirm the multiple outburst interpretation of the spectral index differences
at high frequencies. Comparison between our radio and ROSAT X-ray data does not
reveal any association between the X-ray emission from the cluster and the
radio lobes. There are no extra regions of radio emission at 74 MHz.Comment: 9 pages, 7 figures, accepted for publication in MNRA
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