SIDRA: a blind algorithm for signal detection in photometric surveys

We present the Signal Detection using Random-Forest Algorithm (SIDRA). SIDRA is a detection and classification algorithm based on the Machine Learning technique (Random Forest). The goal of this paper is to show the power of SIDRA for quick and accurate signal detection and classification. We first diagnose the power of the method with simulated light curves and try it on a subset of the Kepler space mission catalogue. We use five classes of simulated light curves (CONSTANT, TRANSIT, VARIABLE, MLENS and EB for constant light curves, transiting exoplanet, variable, microlensing events and eclipsing binaries, respectively) to analyse the power of the method. The algorithm uses four features in order to classify the light curves. The training sample contains 5000 light curves (1000 from each class) and 50000 random light curves for testing. The total SIDRA success ratio is $\geq 90\%$. Furthermore, the success ratio reaches 95 - 100$\%$ for the CONSTANT, VARIABLE, EB, and MLENS classes and 92$\%$ for the TRANSIT class with a decision probability of 60$\%$. Because the TRANSIT class is the one which fails the most, we run a simultaneous fit using SIDRA and a Box Least Square (BLS) based algorithm for searching for transiting exoplanets. As a result, our algorithm detects 7.5$\%$ more planets than a classic BLS algorithm, with better results for lower signal-to-noise light curves. SIDRA succeeds to catch 98$\%$ of the planet candidates in the Kepler sample and fails for 7$\%$ of the false alarms subset. SIDRA promises to be useful for developing a detection algorithm and/or classifier for large photometric surveys such as TESS and PLATO exoplanet future space missions.Comment: 8 pages, 9 figures, 2 Table

Electron-phonon interaction in Graphite Intercalation Compounds

Motivated by the recent discovery of superconductivity in Ca- and Yb-intercalated graphite (CaC$_{6}$ and YbC$_{6}$) and from the ongoing debate on the nature and role of the interlayer state in this class of compounds, in this work we critically study the electron-phonon properties of a simple model based on primitive graphite. We show that this model captures an essential feature of the electron-phonon properties of the Graphite Intercalation Compounds (GICs), namely, the existence of a strong dormant electron-phonon interaction between interlayer and $\pi ^{\ast}$ electrons, for which we provide a simple geometrical explanation in terms of NMTO Wannier-like functions. Our findings correct the oversimplified view that nearly-free-electron states cannot interact with the surrounding lattice, and explain the empirical correlation between the filling of the interlayer band and the occurrence of superconductivity in Graphite-Intercalation Compounds.Comment: 13 pages, 12 figures, submitted to Phys. Rev.

Difference image analysis: The interplay between the photometric scale factor and systematic photometric errors

Context: Understanding the source of systematic errors in photometry is essential for their calibration. Aims: We investigate how photometry performed on difference images can be influenced by errors in the photometric scale factor. Methods: We explore the equations for difference image analysis (DIA) and we derive an expression describing how errors in the difference flux, the photometric scale factor and the reference flux are propagated to the object photometry. Results: We find that the error in the photometric scale factor is important, and while a few studies have shown that it can be at a significant level, it is currently neglected by the vast majority of photometric surveys employing DIA. Conclusions: Minimising the error in the photometric scale factor, or compensating for it in a post-calibration model, is crucial for reducing the systematic errors in DIA photometry.Comment: Accepted A&

Pair-distribution functions of the two-dimensional electron gas

Based on its known exact properties and a new set of extensive fixed-node reptation quantum Monte Carlo simulations (both with and without backflow correlations, which in this case turn out to yield negligible improvements), we propose a new analytical representation of (i) the spin-summed pair-distribution function and (ii) the spin-resolved potential energy of the ideal two-dimensional interacting electron gas for a wide range of electron densities and spin polarization, plus (iii) the spin-resolved pair-distribution function of the unpolarized gas. These formulae provide an accurate reference for quantities previously not available in analytic form, and may be relevant to semiconductor heterostructures, metal-insulator transitions and quantum dots both directly, in terms of phase diagram and spin susceptibility, and indirectly, as key ingredients for the construction of new two-dimensional spin density functionals, beyond the local approximation.Comment: 12 pages, 10 figures; misprints correcte

Local-spin-density functional for multideterminant density functional theory

Based on exact limits and quantum Monte Carlo simulations, we obtain, at any density and spin polarization, an accurate estimate for the energy of a modified homogeneous electron gas where electrons repel each other only with a long-range coulombic tail. This allows us to construct an analytic local-spin-density exchange-correlation functional appropriate to new, multideterminantal versions of the density functional theory, where quantum chemistry and approximate exchange-correlation functionals are combined to optimally describe both long- and short-range electron correlations.Comment: revised version, ti appear in PR

Accurate mass measurements of 26^{26}Ne, 26−30^{26-30}Na, 29−33^{29-33}Mg performed with the {\sc Mistral} spectrometer

The minuteness of the nuclear binding energy requires that mass measurements be highly precise and accurate. Here we report on new measurements $^{29-33}$Mg and $^{26}$Na performed with the {\sc Mistral} mass spectrometer at {\sc Cern}'s {\sc Isolde} facility. Since mass measurements are prone to systematic errors, considerable effort has been devoted to their evaluation and elimination in order to achieve accuracy and not only precision. We have therefore conducted a campaign of measurements for calibration and error evaluation. As a result, we now have a satisfactory description of the {\sc Mistral} calibration laws and error budget. We have applied our new understanding to previous measurements of $^{26}$Ne, $^{26-30}$Na and $^{29,32}$Mg for which re-evaluated values are reported.Comment: submitted to Nuclear Physics

A local density functional for the short-range part of the electron-electron interaction

Motivated by recent suggestions --to split the electron-electron interaction into a short-range part, to be treated within the density functional theory, and a long-range part, to be handled by other techniques-- we compute, with a diffusion Monte Carlo method, the ground-state energy of a uniform electron gas with a modified, short-range-only electron-electron interaction \erfc(\mu r)/r, for different values of the cutoff parameter $\mu$ and of the electron density. After deriving some exact limits, we propose an analytic representation of the correlation energy which accurately fits our Monte Carlo data and also includes, by construction, these exact limits, thus providing a reliable ``short-range local-density functional''.Comment: 7 pages, 3 figure