16,857 research outputs found
Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory
The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been
used for the past 25 years as a reference source in TeV astronomy, for
calibration and verification of new TeV instruments. The High Altitude Water
Cherenkov Observatory (HAWC), completed in early 2015, has been used to observe
the Crab Nebula at high significance across nearly the full spectrum of
energies to which HAWC is sensitive. HAWC is unique for its wide field-of-view,
nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC's
sensitivity improves with the gamma-ray energy. Above 1 TeV the
sensitivity is driven by the best background rejection and angular resolution
ever achieved for a wide-field ground array.
We present a time-integrated analysis of the Crab using 507 live days of HAWC
data from 2014 November to 2016 June. The spectrum of the Crab is fit to a
function of the form . The data is well-fit with values of
, , and
log when
is fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the
systematic errors in this HAWC measurement is discussed and estimated to be
50\% in the photon flux between 1 and 37 TeV.
Confirmation of the Crab flux serves to establish the HAWC instrument's
sensitivity for surveys of the sky. The HAWC survey will exceed sensitivity of
current-generation observatories and open a new view of 2/3 of the sky above 10
TeV.Comment: Submitted 2017/01/06 to the Astrophysical Journa
Revealing networks from dynamics: an introduction
What can we learn from the collective dynamics of a complex network about its
interaction topology? Taking the perspective from nonlinear dynamics, we
briefly review recent progress on how to infer structural connectivity (direct
interactions) from accessing the dynamics of the units. Potential applications
range from interaction networks in physics, to chemical and metabolic
reactions, protein and gene regulatory networks as well as neural circuits in
biology and electric power grids or wireless sensor networks in engineering.
Moreover, we briefly mention some standard ways of inferring effective or
functional connectivity.Comment: Topical review, 48 pages, 7 figure
The Dantzig selector: Statistical estimation when is much larger than
In many important statistical applications, the number of variables or
parameters is much larger than the number of observations . Suppose then
that we have observations , where is a
parameter vector of interest, is a data matrix with possibly far fewer rows
than columns, , and the 's are i.i.d. . Is it
possible to estimate reliably based on the noisy data ? To estimate
, we introduce a new estimator--we call it the Dantzig selector--which
is a solution to the -regularization problem \min_{\tilde{\b
eta}\in\mathbf{R}^p}\|\tilde{\beta}\|_{\ell_1}\quad subject to\quad
\|X^*r\|_{\ell_{\infty}}\leq(1+t^{-1})\sqrt{2\log p}\cdot\sigma, where is
the residual vector and is a positive scalar. We show
that if obeys a uniform uncertainty principle (with unit-normed columns)
and if the true parameter vector is sufficiently sparse (which here
roughly guarantees that the model is identifiable), then with very large
probability, Our results are
nonasymptotic and we give values for the constant . Even though may be
much smaller than , our estimator achieves a loss within a logarithmic
factor of the ideal mean squared error one would achieve with an oracle which
would supply perfect information about which coordinates are nonzero, and which
were above the noise level. In multivariate regression and from a model
selection viewpoint, our result says that it is possible nearly to select the
best subset of variables by solving a very simple convex program, which, in
fact, can easily be recast as a convenient linear program (LP).Comment: This paper discussed in: [arXiv:0803.3124], [arXiv:0803.3126],
[arXiv:0803.3127], [arXiv:0803.3130], [arXiv:0803.3134], [arXiv:0803.3135].
Rejoinder in [arXiv:0803.3136]. Published in at
http://dx.doi.org/10.1214/009053606000001523 the Annals of Statistics
(http://www.imstat.org/aos/) by the Institute of Mathematical Statistics
(http://www.imstat.org
Improving the performance of translation wavelet transform using BMICA
Research has shown Wavelet Transform to be one of the best methods for denoising biosignals. Translation-Invariant
form of this method has been found to be the best performance. In this paper however we utilize this method and merger with our newly created Independent Component Analysis method â BMICA. Different EEG signals are used to verify the method within the MATLAB environment. Results are then compared with those of the actual Translation-Invariant algorithm and evaluated using the performance measures Mean Square Error (MSE), Peak Signal to Noise Ratio (PSNR), Signal to Distortion Ratio (SDR), and Signal to Interference Ratio (SIR). Experiments revealed that the BMICA Translation-Invariant Wavelet Transform out performed in all four measures. This indicates that it performed superior to the basic Translation- Invariant Wavelet Transform algorithm producing cleaner EEG signals which can influence diagnosis as well as clinical studies of the brain
Variational Bayesian Inference of Line Spectra
In this paper, we address the fundamental problem of line spectral estimation
in a Bayesian framework. We target model order and parameter estimation via
variational inference in a probabilistic model in which the frequencies are
continuous-valued, i.e., not restricted to a grid; and the coefficients are
governed by a Bernoulli-Gaussian prior model turning model order selection into
binary sequence detection. Unlike earlier works which retain only point
estimates of the frequencies, we undertake a more complete Bayesian treatment
by estimating the posterior probability density functions (pdfs) of the
frequencies and computing expectations over them. Thus, we additionally capture
and operate with the uncertainty of the frequency estimates. Aiming to maximize
the model evidence, variational optimization provides analytic approximations
of the posterior pdfs and also gives estimates of the additional parameters. We
propose an accurate representation of the pdfs of the frequencies by mixtures
of von Mises pdfs, which yields closed-form expectations. We define the
algorithm VALSE in which the estimates of the pdfs and parameters are
iteratively updated. VALSE is a gridless, convergent method, does not require
parameter tuning, can easily include prior knowledge about the frequencies and
provides approximate posterior pdfs based on which the uncertainty in line
spectral estimation can be quantified. Simulation results show that accounting
for the uncertainty of frequency estimates, rather than computing just point
estimates, significantly improves the performance. The performance of VALSE is
superior to that of state-of-the-art methods and closely approaches the
Cram\'er-Rao bound computed for the true model order.Comment: 15 pages, 8 figures, accepted for publication in IEEE Transactions on
Signal Processin
Learning Graphs from Linear Measurements: Fundamental Trade-offs and Applications
We consider a specific graph learning task: reconstructing a symmetric matrix that represents an underlying graph using linear measurements. We present a sparsity characterization for distributions of random graphs (that are allowed to contain high-degree nodes), based on which we study fundamental trade-offs between the number of measurements, the complexity of the graph class, and the probability of error. We first derive a necessary condition on the number of measurements. Then, by considering a three-stage recovery scheme, we give a sufficient condition for recovery. Furthermore, assuming the measurements are Gaussian IID, we prove upper and lower bounds on the (worst-case) sample complexity for both noisy and noiseless recovery. In the special cases of the uniform distribution on trees with n nodes and the ErdĆs-RĂ©nyi (n,p) class, the fundamental trade-offs are tight up to multiplicative factors with noiseless measurements. In addition, for practical applications, we design and implement a polynomial-time (in n ) algorithm based on the three-stage recovery scheme. Experiments show that the heuristic algorithm outperforms basis pursuit on star graphs. We apply the heuristic algorithm to learn admittance matrices in electric grids. Simulations for several canonical graph classes and IEEE power system test cases demonstrate the effectiveness and robustness of the proposed algorithm for parameter reconstruction
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