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Axisymmetric magnetic activity on the Sun and Sun-like stars increases the frequencies of the modes of acoustic oscillation. However, it is unclear how a corotating patch of activity affects the oscillations, since such a perturbation is unsteady in the frame of the observer. In this paper we qualitatively describe the asteroseismic signature of a large active region in the power spectrum of the dipole and quadrupole p modes. In the corotating frame of the active region, the perturbations due to (differential) rotation and the active region completely lift the $(2\ell + 1)$-fold azimuthal degeneracy of the frequency spectrum of modes with harmonic degree $\ell$. In the frame of the observer, the unsteady nature of the perturbation leads to the appearance of $(2\ell+1)^2$ peaks in the power spectrum of a multiplet. These peaks blend into each other to form asymmetric line profiles. In the limit of a small active region, we approximate the power spectrum of a multiplet in terms of $2\times(2\ell+1)$ peaks, whose amplitudes and frequencies depend on the latitude of the active region and the inclination angle of the star's rotation axis. In order to check the results and to explore the nonlinear regime, we also perform numerical simulations using the 3D time-domain pseudo-spectral linear pulsation code GLASS., Comment: Accepted for publication in Frontiers in Astronomy and Space Sciences

Gravity can play a role in critical phenomena. Topological singularities induce ground state degeneracy and break the continuum symmetry of the vacuum. They also generate momenta oscillations about an average momentum and a positive gravitational susceptibility. Gravitational analogues of the laws of Curie and Bloch have been found for a one-dimensional model. The critical temperature for a change in phase from unbound to isolated vortices has also been calculated in a $XY$-model., Comment: 7 pages, one table. arXiv admin note: text overlap with arXiv:1902.09940

Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin-Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation (DNS) of decaying plasma turbulence driven by large-scale balanced Alfv\'enic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale) exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic (MHD) to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The validity of the Taylor hypothesis in the sub-ion spatial range suggests that the fluctuations at sub-ion scales have predominantly low frequencies, consistent with a kinetic Alfv\'en wave-like nature or with quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event in the magnetopause is an order of magnitude larger than in the ambient magnetosheath., Comment: 11 pages, 6 figures, submitted to The Astrophysical Journal

Current research in High Energy Cosmic Ray Physics touches on fundamental questions regarding the origin of cosmic rays, their composition, the acceleration mechanisms, and their production. Unambiguous measurements of the energy spectra and of the composition of cosmic rays at the "knee" region could provide some of the answers to the above questions. So far only ground based observations, which rely on sophisticated models describing high energy interactions in the earth's atmosphere, have been possible due to the extremely low particle rates at these energies. A calorimetry based space experiment that could provide not only flux measurements but also energy spectra and particle identification, would certainly overcome some of the uncertainties of ground based experiments. Given the expected particle fluxes, a very large acceptance is needed to collect a sufficient quantity of data, in a time compatible with the duration of a space mission. This in turn, contrasts with the lightness and compactness requirements for space based experiments. We present a novel idea in calorimetry which addresses these issues whilst limiting the mass and volume of the detector. In this paper we report on a four year R&D program where we investigated materials, coatings, photo-sensors, Front End electronics, and mechanical structures with the aim of designing a high performance, high granularity calorimeter with the largest possible acceptance. Details are given of the design choices, component characterisation, and of the construction of a sizeable prototype (Calocube) which has been used in various tests with particle beams.

This paper contains a study of the separable form $J_s(\cdot)$ of the classical Jung constant. We first establish, following Davis \cite{davis}, that a Banach space $X$ is $1$-separably injective if and only if $J_s(X)=1$. This characterization is then used for the understanding of new $1$-separably injective spaces. The last section establishes the inequality $\frac{1}{2}K(Y)J_s(X)\leq e(Y,X)$ connecting the separable Jung constant, Kottman's constant and the extension constant for Lipschitz maps, which is then used to obtain a simple proof of the equality $K(X,c_0)=e(X,c_0)$ of Kalton and a new characterization of $1$-separable injectivity.

We show that the Kottman constant $K(\cdot)$, together with its symmetric and finite variations, is continuous with respect to the Kadets metric, and they are log-convex, hence continuous, with respect to the interpolation parameter in a complex interpolation schema. Moreover, we show that $K(X)\cdot K(X^*)\geqslant 2$ for every infinite-dimensional Banach space $X$. We also consider the isomorphic Kottman constant (defined as the infimum of the Kottman constants taken over all renormings of the space) and solve the main problem left open in [CaGoPa17], namely that the isomorphic Kottman constant of a twisted-sum space is the maximum of the constants of the respective summands. Consequently, the Kalton--Peck space may be renormed to have Kottman's constant arbitrarily close to $\sqrt{2}$. For other classical parameters, such as the Whitley and the James constants, we prove the continuity with respect to the Kadets metric., Comment: 14 pp

Traditional model-based reinforcement learning approaches learn a model of the environment dynamics without explicitly considering how it will be used by the agent. In the presence of misspecified model classes, this can lead to poor estimates, as some relevant available information is ignored. In this paper, we introduce a novel model-based policy search approach that exploits the knowledge of the current agent policy to learn an approximate transition model, focusing on the portions of the environment that are most relevant for policy improvement. We leverage a weighting scheme, derived from the minimization of the error on the model-based policy gradient estimator, in order to define a suitable objective function that is optimized for learning the approximate transition model. Then, we integrate this procedure into a batch policy improvement algorithm, named Gradient-Aware Model-based Policy Search (GAMPS), which iteratively learns a transition model and uses it, together with the collected trajectories, to compute the new policy parameters. Finally, we empirically validate GAMPS on benchmark domains analyzing and discussing its properties.

The relationship between a decaying plasma turbulence and proton fire hose instabilities in a slowly expanding plasma is investigated using three-dimensional (3-D) hybrid expanding box simulations. We impose an initial ambient magnetic field along the radial direction, and we start with an isotropic spectrum of large-scale, linearly-polarized, random-phase Alfvenic fluctuations with zero cross-helicity. A turbulent cascade rapidly develops and leads to a weak proton heating that is not sufficient to overcome the expansion-driven perpendicular cooling. The plasma system eventually drives the parallel and oblique fire hose instabilities that generate quasi-monochromatic wave packets that reduce the proton temperature anisotropy. The fire hose wave activity has a low amplitude with wave vectors quasi-parallel/oblique with respect to the ambient magnetic field outside of the region dominated by the turbulent cascade and is discernible in one-dimensional power spectra taken only in the direction quasi-parallel/oblique with respect to the ambient magnetic field; at quasi-perpendicular angles the wave activity is hidden by the turbulent background. These waves are partly reabsorbed by protons and partly couple to and participate in the turbulent cascade. Their presence reduces kurtosis, a measure of intermittency, and the Shannon entropy but increases the Jensen-Shannon complexity of magnetic fluctuations; these changes are weak and anisotropic with respect to the ambient magnetic field and it's not clear if they can be used to indirectly discern the presence of instability-driven waves., Comment: 12 pages, 10 figures

We homogeneously analyse $\sim 3.2\times 10^5$ photometric measurements for $\sim 1100$ transit lightcurves belonging to $17$ exoplanet hosts. The photometric data cover $16$ years 2004--2019 and include amateur and professional observations. Old archival lightcurves were reprocessed using up-to-date exoplanetary parameters and empirically debiased limb-darkening models. We also derive self-consistent transit and radial-velocity fits for $13$ targets. We confirm the nonlinear TTV trend in the WASP-12 data at a high significance, and with a consistent magnitude. However, Doppler data reveal hints of a radial acceleration about $(-7.5\pm 2.2)$~m/s/yr, indicating the presence of unseen distant companions, and suggesting that roughly $10$ per cent of the observed TTV was induced via the light-travel (or Roemer) effect. For WASP-4, a similar TTV trend suspected after the recent TESS observations appears controversial and model-dependent. It is not supported by our homogeneus TTV sample, including $10$ ground-based EXPANSION lightcurves obtained in 2018 simultaneously with TESS. Even if the TTV trend itself does exist in WASP-4, its magnitude and tidal nature are uncertain. Doppler data cannot entirely rule out the Roemer effect induced by possible distant companions., Comment: 19 pages, 4 figures, 6 tables; revised manuscript submitted to MNRAS; online-only supplements are in the download archive

Mutual information has been successfully adopted in filter feature-selection methods to assess both the relevancy of a subset of features in predicting the target variable and the redundancy with respect to other variables. However, existing algorithms are mostly heuristic and do not offer any guarantee on the proposed solution. In this paper, we provide novel theoretical results showing that conditional mutual information naturally arises when bounding the ideal regression/classification errors achieved by different subsets of features. Leveraging on these insights, we propose a novel stopping condition for backward and forward greedy methods which ensures that the ideal prediction error using the selected feature subset remains bounded by a user-specified threshold. We provide numerical simulations to support our theoretical claims and compare to common heuristic methods., Comment: Accepted for presentation at the International Joint Conference on Neural Networks (IJCNN) 2019

We consider rotating, electrically charged, supersymmetric AdS black holes in four, five, six and seven dimensions, and provide a derivation of the respective extremization principles stating that the Bekenstein-Hawking entropy is the Legendre transform of a homogeneous function of chemical potentials, subject to a complex constraint. Extending a recently proposed BPS limit, we start from finite temperature and reach extremality following a supersymmetric trajectory in the space of complexified solutions. We show that the entropy function is the supergravity on-shell action in this limit. Chemical potentials satisfying the extremization equations also emerge from the complexified solution., Comment: 51 pages; v3: new appendix on Legendre transform of the general entropy function (6.1), matches published version

Magnetic reconnection provides the primary source for explosive energy release, plasma heating and particle acceleration in many astrophysical environments. The last years witnessed a revival of interest in the MHD tearing instability as a driver for efficient reconnection. It has been established that, provided the current sheet aspect ratio becomes small enough ($a/L \sim S^{-1/3}$ for a given Lundquist number $S\gg 1$), reconnection occurs on ideal Alfv\'en timescales and becomes independent on $S$. Here we investigate, by means of two-dimensional simulations, the \emph{ideal} tearing instability in the Hall-MHD regime, which is appropriate when the width of the resistive layer $\delta$ becomes comparable to the ion inertial length $d_i$. Moreover, we study in detail the spontaneous development and reconnection of secondary current sheets, which for high $S$ naturally adjust to the ideal aspect ratio and hence their evolution proceeds very rapidly. For moderate low $S$, the aspect ratio tends to the Sweet-Parker scaling ($a/L \sim S^{-1/2}$), in order to fulfill the condition $\delta \ll a$ necessary for the onset of a tearing instability. When the Hall term is included, the reconnection rate of this secondary nonlinear phase is enhanced and, depending on the ratio $d_i/\delta$, can be twice with respect to the pure MHD case, and up to ten times larger than the linear phase. Therefore, the evolution of the tearing instability in thin current sheets in the Hall-MHD regime naturally leads to an explosive disruption of the reconnecting site and to energy release on super-Alfv\'enic timescales, as required to explain astrophysical observations., Comment: 14 pages, 9 figures, published on ApJ

In this paper, we present the analysis and results of a direct measurement of the cosmic-ray proton spectrum with the CALET instrument onboard the International Space Station, including the detailed assessment of systematic uncertainties. The observation period used in this analysis is from October 13, 2015 to August 31, 2018 (1054 days). We have achieved the very wide energy range necessary to carry out measurements of the spectrum from 50 GeV to 10 TeV covering, for the first time in space, with a single instrument the whole energy interval previously investigated in most cases in separate subranges by magnetic spectrometers (BESS-TeV, PAMELA, and AMS-02) and calorimetric instruments (ATIC, CREAM, and NUCLEON). The observed spectrum is consistent with AMS-02 but extends to nearly an order of magnitude higher energy, showing a very smooth transition of the power-law spectral index from -2.81 +- 0.03 (50--500 GeV) neglecting solar modulation effects (or -2.87 +- 0.06 including solar modulation effects in the lower energy region) to -2.56 +- 0.04 (1--10 TeV), thereby confirming the existence of spectral hardening and providing evidence of a deviation from a single power law by more than 3 sigma., Comment: main text: 8 pages, 5 figures, supplemental material: 12 pages, 7 figures, 1 table, marked as a PRL Editor's Suggestion

Policy Gradient (PG) algorithms are among the best candidates for the much-anticipated applications of reinforcement learning to real-world control tasks, such as robotics. However, the trial-and-error nature of these methods poses safety issues whenever the learning process itself must be performed on a physical system or involves any form of human-computer interaction. In this paper, we address a specific safety formulation, where both goals and dangers are encoded in a scalar reward signal and the learning agent is constrained to never worsen its performance, measured as the expected sum of rewards. By studying actor-only policy gradient from a stochastic optimization perspective, we establish improvement guarantees for a wide class of parametric policies, generalizing existing results on Gaussian policies. This, together with novel upper bounds on the variance of policy gradient estimators, allows us to identify meta-parameter schedules that guarantee monotonic improvement with high probability. The two key meta-parameters are the step size of the parameter updates and the batch size of the gradient estimates. Through a joint, adaptive selection of these meta-parameters, we obtain a policy gradient algorithm with monotonic improvement guarantees.

We analyse velocity fluctuations in the solar wind at magneto-fluid scales in two datasets, extracted from Wind data in the period 2005-2015, that are characterised by strong or weak expansion. Expansion affects measurements of anisotropy because it breaks axisymmetry around the mean magnetic field. Indeed, the small-scale three-dimensional local anisotropy of magnetic fluctuations ({\delta}B) as measured by structure functions (SF_B) is consistent with tube-like structures for strong expansion. When passing to weak expansion, structures become ribbon-like because of the flattening of SFB along one of the two perpendicular directions. The power-law index that is consistent with a spectral slope -5/3 for strong expansion now becomes closer to -3/2. This index is also characteristic of velocity fluctuations in the solar wind. We study velocity fluctuations ({\delta}V) to understand if the anisotropy of their structure functions (SF_V ) also changes with the strength of expansion and if the difference with the magnetic spectral index is washed out once anisotropy is accounted for. We find that SF_V is generally flatter than SF_B. When expansion passes from strong to weak, a further flattening of the perpendicular SF_V occurs and the small-scale anisotropy switches from tube-like to ribbon-like structures. These two types of anisotropy, common to SF_V and SF_B, are associated to distinct large-scale variance anisotropies of {\delta}B in the strong- and weak-expansion datasets. We conclude that SF_V shows anisotropic three-dimensional scaling similar to SF_B, with however systematic flatter scalings, reflecting the difference between global spectral slopes., Comment: accepted in MNRAS

By means of three dimensional high-resolution hybrid simulations we study the properties of the magnetic field spectral anisotropies near and beyond ion kinetic scales. By using both a Fourier analysis and a local analysis based on multi-point 2nd-order structure function techniques, we show that the anisotropy observed is less than what expected by standard wave normal modes turbulence theories although the non linear energy transfer is still in the perpendicular direction, only advected in the parallel direction as expected balancing the non-linear energy transfer time and the decorrelation time. Such result can be explained by a phenomenological model based on the formation of strong intermittent two-dimensional structures in the plane perpendicular to the local mean field that have some prescribed aspect ratio eventually depending on the scale. This model support the idea that small scales structures, such as reconnecting current sheets, contribute significantly to the formation of the turbulent cascade at kinetic scales., Comment: 9 pages, 4 figures

The CALorimetric Electron Telescope (CALET) is a high-energy astroparticle physics space experiment installed on the International Space Station (ISS), developed and operated by Japan in collaboration with Italy and the United States. The CALET mission goals include the investigation of possible nearby sources of high-energy electrons, of the details of galactic particle acceleration and propagation, and of potential signatures of dark matter. CALET measures the cosmic-ray electron + positron flux up to 20 TeV, gamma-rays up to 10 TeV, and nuclei with Z=1 to 40 up to 1,000 TeV for the more abundant elements during a long-term observation aboard the ISS. Starting science operation in mid-October 2015, CALET performed continuous observation without major interruption with close to 20 million triggered events over 10 GeV per month. Based on the data taken during the first two-years, we present an overview of CALET observations: uses w/o major interruption 1) Electron + positron energy spectrum, 2) Nuclei analysis, 3) Gamma-ray observation including a characterization of on-orbit performance. Results of the electromagnetic counterpart search for LIGO/Virgo gravitational wave events are discussed as well., Comment: 10 pages, 10 figures, a contribution to the proceedings of 26th Extended European Cosmic Ray Symposium, 6-10 July 2018, Russia, which summarizes our recent publications such as arXiv:1712.01711, arXiv:1712.01757, arXiv:1803.05834, arXiv:1806.09728, and arXiv:1807.01435

This paper briefly reviews a number of fundamental measurements that need to be made in order to characterize turbulence in space plasmas such as the solar wind. It has long been known that many of these quantities require simultaneous multipoint measurements to attain a proper characterization that would reveal the fundamental physics of plasma turbulence. The solar wind is an ideal plasma for such an investigation, and it now appears to be technologically feasible to carry out such an investigation, following the pioneering Cluster and MMS missions. Quantities that need to be measured using multipoint measurements include the two-point, two-time second correlation function of velocity, magnetic field and density, and higher order statistical objects such as third and fourth order structure functions. Some details of these requirements are given here, with a eye towards achieving closure on fundamental questions regarding the cascade rate, spectral anisotropy, characteristic coherent structures, intermittency, and dissipation mechanisms that describe plasma turbuelence, as well as its variability with plasma parameters in the solar wind. The motivation for this discussion is the current planning for a proposed Helioswarm mission that would be designed to make these measurements,leading to breakthrough understanding of the physics of space and astrophysical turbulence., Comment: White paper submitted to the PLASMA 2020 Decadal Survey Committee

We report on the behaviour of two-level quantum systems, or qubits, in the background of rotating and non-rotating metrics and provide a method to derive the related spin currents and motions. The calculations are performed in the external field approximation., Comment: 16 pages. arXiv admin note: text overlap with arXiv:0709.0819, arXiv:gr-qc/0503027

Context. Pluto's tenuous nitrogen (N2) atmosphere undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has been recently (July 2015) observed by the New Horizons spacecraft. Goals are (i) construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) constrain the structure of the lower atmosphere using a central flash observed in 2015. Method: eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between $\sim$5 km and $\sim$380 km altitude levels (i.e. pressures from about 10 microbar to 10 nanobar). Results: (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived; (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia and/or (b) hazes with tangential optical depth of about 0.3 are present at 4-7 km altitude levels and/or (c) the nominal REX density values are overestimated by an implausibly large factor of about 20% and/or (d) higher terrains block part of the flash in the Charon facing hemisphere., Comment: 21 pages, 11 figures

We present a new family of asymptotically locally AdS$_5$ squashed supersymmetric black hole solutions of Fayet-Iliopoulos gauged ${\cal N}=2$, $D=5$ supergravity with two vector multiplets that have a natural uplift to type IIB supergravity. Our new family of black holes is characterized by three parameters, of which two control the horizon geometry while the latter regulates the squashing at the boundary. We evaluate the main physical properties of the family of solutions using holographic renormalization and find that the entropy is independent on the squashing and it is reproduced by using the angular momentum and the Page charges. In previously known solutions Page and holographic charges are equal, due to the vanishing of the Chern-Simons term that here, instead, is relevant. This result suggests that for asymptotically locally AdS$_5$ solutions we should refer to the Page charges to describe the thermodynamics of the system., Comment: 38+20 pages, 5 figures, 2 appendices; v2: version matching the published one on EPJ-C; references added; minor comment about the numerical procedure added; corrected minor typos

Gravity induced condensation takes the form of momentum alignment in an ensemble of identical particles. Use is made of a one-dimensional Ising model to calculate the alignment per particle and the correlation length as a function of the temperature. These parameters indicate that momentum alignment is possible in the proximity of some astrophysical objects and in earth, or near earth laboratories. Momenta oscillations behave as known spin oscillations and obey identical dispersion relations., Comment: 7 pages, 0 figures

Given a smooth function $U(t,x)$, $T$-periodic in the first variable and satisfying $U(t,x) = \mathcal{O}(\vert x \vert^{\alpha})$ for some $\alpha \in (0,2)$ as $\vert x \vert \to \infty$, we prove that the forced Kepler problem $$ \ddot x = - \dfrac{x}{|x|^3} + \nabla_x U(t,x),\qquad x\in {\mathbb{R}}^2, $$ has a generalized $T$-periodic solution, according to the definition given in the paper [Boscaggin, Ortega, Zhao, \emph{Periodic solutions and regularization of a Kepler problem with time-dependent perturbation}, Trans. Amer. Math. Soc, 2018]. The proof relies on variational arguments.

PDS 110 is a young disk-hosting star in the Orion OB1A association. Two dimming events of similar depth and duration were seen in 2008 (WASP) and 2011 (KELT), consistent with an object in a closed periodic orbit. In this paper we present data from a ground-based observing campaign designed to measure the star both photometrically and spectroscopically during the time of predicted eclipse in September 2017. Despite high-quality photometry, the predicted eclipse did not occur, although coherent structure is present suggesting variable amounts of stellar flux or dust obscuration. We also searched for RV oscillations caused by any hypothetical companion and can rule out close binaries to 0.1 $M_\odot$. A search of Sonneberg plate archive data also enabled us to extend the photometric baseline of this star back more than 50 years, and similarly does not re-detect any deep eclipses. Taken together, they suggest that the eclipses seen in WASP and KELT photometry were due to aperiodic events. It would seem that PDS 110 undergoes stochastic dimmings that are shallower and shorter-duration than those of UX Ori variables, but may have a similar mechanism., Comment: Accepted to MNRAS; 12 pages, 7 figures; Supplementary photometric data in zipped latex source as all_photometry.csv