6,328 research outputs found
A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy
Transmission spectroscopy, which consists of measuring the
wavelength-dependent absorption of starlight by a planet's atmosphere during a
transit, is a powerful probe of atmospheric composition. However, the expected
signal is typically orders of magnitude smaller than instrumental systematics,
and the results are crucially dependent on the treatment of the latter. In this
paper, we propose a new method to infer transit parameters in the presence of
systematic noise using Gaussian processes, a technique widely used in the
machine learning community for Bayesian regression and classification problems.
Our method makes use of auxiliary information about the state of the
instrument, but does so in a non-parametric manner, without imposing a specific
dependence of the systematics on the instrumental parameters, and naturally
allows for the correlated nature of the noise. We give an example application
of the method to archival NICMOS transmission spectroscopy of the hot Jupiter
HD 189733, which goes some way towards reconciling the controversy surrounding
this dataset in the literature. Finally, we provide an appendix giving a
general introduction to Gaussian processes for regression, in order to
encourage their application to a wider range of problems.Comment: 6 figures, 1 table, accepted for publication in MNRA
Specific configuration of dendritic degeneration in pyramidal neurons of the medial prefrontal cortex induced by differing corticosteroid regimens
We previously demonstrated that hypercorticalism induces pronounced
volumetric reductions in the rat medial prefrontal cortex
(mPFC) and that these structural changes correlate with deficits in
executive function. By applying 3-dimensional analysis of Golgi-
Cox--stained material, we now demonstrate that corticosteroids
can exert differential effects on dendritic arborizations of pyramidal
neurons in lamina II/III of the mPFC. Treatment with the glucocorticoid
receptor--selective agonist dexamethasone and with the
natural adrenosteroid, corticosterone (CORT), results in significant
reductions in the total length of apical dendrites in the pyramidal
neurons in lamina II/III of the anterior cingulate/prelimbic and
infralimbic cortices. Interestingly, although these treatments do not
affect the number of dendritic branches, they are associated with
impoverished arborizations in their distal portions and, in CORTtreated
animals, with increased branching in the middle portions of
the apical dendritic tree. Deprivation of corticosteroids by adrenalectomy
leads to decreases in total apical dendritic length and spine
number, but in this case, dendritic impoverishment was restricted
to the middle/proximal segments of the dendritic trees. None of
the treatments influenced the architecture of the basal dendrites.
These results add to our knowledge of the morphological substrates
through which corticosteroids may disrupt mPFC-dependent
behaviors
Physical consequences of PNP and the DMRG-annealing conjecture
Computational complexity theory contains a corpus of theorems and conjectures
regarding the time a Turing machine will need to solve certain types of
problems as a function of the input size. Nature {\em need not} be a Turing
machine and, thus, these theorems do not apply directly to it. But {\em
classical simulations} of physical processes are programs running on Turing
machines and, as such, are subject to them. In this work, computational
complexity theory is applied to classical simulations of systems performing an
adiabatic quantum computation (AQC), based on an annealed extension of the
density matrix renormalization group (DMRG). We conjecture that the
computational time required for those classical simulations is controlled
solely by the {\em maximal entanglement} found during the process. Thus, lower
bounds on the growth of entanglement with the system size can be provided. In
some cases, quantum phase transitions can be predicted to take place in certain
inhomogeneous systems. Concretely, physical conclusions are drawn from the
assumption that the complexity classes {\bf P} and {\bf NP} differ. As a
by-product, an alternative measure of entanglement is proposed which, via
Chebyshev's inequality, allows to establish strict bounds on the required
computational time.Comment: Accepted for publication in JSTA
Quasi-Adiabatic Continuation in Gapped Spin and Fermion Systems: Goldstone's Theorem and Flux Periodicity
We apply the technique of quasi-adiabatic continuation to study systems with
continuous symmetries. We first derive a general form of Goldstone's theorem
applicable to gapped nonrelativistic systems with continuous symmetries. We
then show that for a fermionic system with a spin gap, it is possible to insert
-flux into a cylinder with only exponentially small change in the energy
of the system, a scenario which covers several physically interesting cases
such as an s-wave superconductor or a resonating valence bond state.Comment: 19 pages, 2 figures, final version in press at JSTA
The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.: Data and preliminary Modeling
The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst
of the recurrent nova RS Oph was observed twice with Chandra and once with
XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after
outburst. We confirm a 35-sec period on day 54.0 and found that it originates
from the SSS emission and not from the shock. We discus the bound-free
absorption by neutral elements in the line of sight, resonance absorption lines
plus self-absorbed emission line components, collisionally excited emission
lines from the shock, He-like intersystem lines, and spectral changes during an
episode of high-amplitude variability. We find a decrease of the oxygen K-shell
absorption edge that can be explained by photoionization of oxygen. The
absorption component has average velocities of -1286+-267 km/s on day 39.7 and
of -771+-65 km/s on day 66.9. The wavelengths of the emission line components
are consistent with their rest wavelengths as confirmed by measurements of
non-self absorbed He-like intersystem lines. We have evidence that these lines
originate from the shock rather than the outer layers of the outflow and may be
photoexcited in addition to collisional excitations. We found collisionally
excited emission lines that are fading at wavelengths shorter than 15A that
originate from the radiatively cooling shock. On day 39.5 we find a systematic
blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i
ratios which indicates either high densities or significant UV radiation near
the plasma where the emission lines are formed. During the phase of strong
variability the spectral hardness light curve overlies the total light curve
when shifted by 1000sec. This can be explained by photoionization of neutral
oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.Comment: 16 pages, 10 figures, 4 tables. Accepted by ApJ; v2: Co-author
Woodward adde
Enhancement of the Binding Energy of Charged Excitons in Disordered Quantum Wires
Negatively and positively charged excitons are identified in the
spatially-resolved photoluminescence spectra of quantum wires. We demonstrate
that charged excitons are weakly localized in disordered quantum wires. As a
consequence, the enhancement of the "binding energy" of a charged exciton is
caused, for a significant part, by the recoil energy transferred to the
remaining charged carrier during its radiative recombination. We discover that
the Coulomb correlation energy is not the sole origin of the "binding energy",
in contrast to charged excitons confined in quantum dots.Comment: 4 Fig
Swift follow-up of IceCube triggers, and implications for the Advanced-LIGO era
Between 2011 March and 2014 August Swift responded to 20 triggers from the
IceCube neutrino observatory, observing the IceCube 50% confidence error circle
in X-rays, typically within 5 hours of the trigger. No confirmed counterpart
has been detected. We describe the Swift follow up strategy and data analysis
and present the results of the campaign. We discuss the challenges of
distinguishing the X-ray counterpart to a neutrino trigger from serendipitous
uncatalogued X-ray sources in the error circle, and consider the implications
of our results for future strategies for multi-messenger astronomy, with
particular reference to the follow up of gravitational wave triggers from the
advanced-era detectors.Comment: Accepted for publication in MNRAS. 18 pages, including 8 figures and
4 tables; two of which are landscape-oriente
Continuous variable tangle, monogamy inequality, and entanglement sharing in Gaussian states of continuous variable systems
For continuous-variable systems, we introduce a measure of entanglement, the
continuous variable tangle ({\em contangle}), with the purpose of quantifying
the distributed (shared) entanglement in multimode, multipartite Gaussian
states. This is achieved by a proper convex roof extension of the squared
logarithmic negativity. We prove that the contangle satisfies the
Coffman-Kundu-Wootters monogamy inequality in all three--mode Gaussian states,
and in all fully symmetric --mode Gaussian states, for arbitrary . For
three--mode pure states we prove that the residual entanglement is a genuine
tripartite entanglement monotone under Gaussian local operations and classical
communication. We show that pure, symmetric three--mode Gaussian states allow a
promiscuous entanglement sharing, having both maximum tripartite residual
entanglement and maximum couplewise entanglement between any pair of modes.
These states are thus simultaneous continuous-variable analogs of both the GHZ
and the states of three qubits: in continuous-variable systems monogamy
does not prevent promiscuity, and the inequivalence between different classes
of maximally entangled states, holding for systems of three or more qubits, is
removed.Comment: 13 pages, 1 figure. Replaced with published versio
Stimulation of shrimp (Penaeus monodon) hemocytes by lipopolysaccharide-like molecules derived from Novacqâą
Immune stimulation through feed additives is a promising strategy that can help to combat disease in shrimp farming and reduce the use of antibiotics and other chemotherapeutics. The present study investigated the in vitro immunostimulatory effects of lipopolysaccharide (LPS)-like molecules isolated from the microbial based feed additive Novacqâą (N-LPS). The presence of LPS-like molecules was confirmed and quantified Novacqâą using a HEK-TLR4 reporter cell line. Primary hemocytes isolated from adult Penaeus monodon were used to measure the immunostimulatory of N-LPS compared with the control group that were treated with E. coli derived LPS (E-LPS). The N-LPS stimulated a rapid and significant induction of the phenoloxidase (PO) response in the hemocytes. The PO response increased with exposure time and LPS concentration and was significantly higher compared with an E. coli LPS (E-LPS) control. In addition, using gene expression data, we quantified the transcriptome response of the hemocytes at 15, 30 and 60 mins post stimulation. Compared with the controls, the N-LPS treated hemocytes had a significant up-regulation of genes involved in the immune system modulation and control at all time-points. Most noteworthy was the significant induction of transcripts that function as serine protease inhibitors (namely SERPINs), that regulate the overexpression of the PO system. Transcription factors from the Notch family which directly regulate the expression of many immune genes were also induced within the hemocytes. Additionally, we also saw a strong up-regulation of crustacean hyperglycemic hormone (CHH) transcripts, an important neuropeptide involved in immune function. Overall, the transcriptome profile of the hemocytes suggests that the LPS component of Novacqâą is highly immunostimulatory and generates a strong PO response in vitro. The subsequent transcriptional response appears to be directed towards preventing further activation of the PO system most likely in an attempt to limit cytoxicity to the host. Our study highlights the immunostimulatory ability of Novacqâą and provides further evidence of the positive health benefits this microbial based feed additive can have in shrimp.</p
Polynomial-time algorithm for simulation of weakly interacting quantum spin systems
We describe an algorithm that computes the ground state energy and
correlation functions for 2-local Hamiltonians in which interactions between
qubits are weak compared to single-qubit terms. The running time of the
algorithm is polynomial in the number of qubits and the required precision.
Specifically, we consider Hamiltonians of the form , where
H_0 describes non-interacting qubits, V is a perturbation that involves
arbitrary two-qubit interactions on a graph of bounded degree, and
is a small parameter. The algorithm works if is below a certain
threshold value that depends only upon the spectral gap of H_0, the maximal
degree of the graph, and the maximal norm of the two-qubit interactions. The
main technical ingredient of the algorithm is a generalized Kirkwood-Thomas
ansatz for the ground state. The parameters of the ansatz are computed using
perturbative expansions in powers of . Our algorithm is closely
related to the coupled cluster method used in quantum chemistry.Comment: 27 page
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