Miracle-dz/UniFAR-FTUs huggingface dataset:
Covariant perturbations of Schwarzschild black holes

We present a new covariant and gauge-invariant perturbation formalism for dealing with spacetimes having spherical symmetry (or some preferred spatial direction) in the background, and apply it to the case of gravitational wave propagation in a Schwarzschild black-hole spacetime. The 1 + 3 covariant approach is extended to a '1 + 1 + 2 covariant sheet' formalism by introducing a radial unit vector in addition to the timelike congruence, and decomposing all covariant quantities with respect to this. The background Schwarzschild solution is discussed and a covariant characterization is given. We give the full first-order system of linearized 1 + 1 + 2 covariant equations, and we show how, by introducing (time and spherical) harmonic functions, these may be reduced to a system of first-order ordinary differential equations and algebraic constraints for the 1 + 1 + 2 variables which may be solved straightforwardly. We show how both odd- and even-parity perturbations may be unified by the discovery of a covariant, frame- and gauge-invariant, transverse-traceless tensor describing gravitational waves, which satisfies a covariant wave equation equivalent to the Regge–Wheeler equation for both even- and odd-parity perturbations. We show how the Zerilli equation may be derived from this tensor, and derive a similar transverse-traceless tensor equation equivalent to this equation. The so-called special quasinormal modes with purely imaginary frequency emerge naturally. The significance of the degrees of freedom in the choice of the two frame vectors is discussed, and we demonstrate that, for a certain frame choice, the underlying dynamics is governed purely by the Regge–Wheeler tensor. The two transverse-traceless Weyl tensors which carry the curvature of gravitational waves are discussed, and we give the closed system of four first-order ordinary differential equations describing their propagation. Finally, we consider the extension of this work to the study of gravitational waves in other astrophysical situations.
Quark-number susceptibility, thermodynamic sum rule, and the hard thermal loop approximation

The quark number susceptibility, associated with the conserved quark number density, is closely related to the baryon and charge fluctuations in the quark-gluon plasma, which might serve as signature for the quark-gluon plasma formation in ultrarelativistic heavy-ion collisions. In addition to QCD lattice simulations, the quark number susceptibility has been calculated recently using a resummed perturbation theory (hard thermal loop resummation). In the present work we show, based on general arguments, that the computation of this quantity neglecting hard thermal loop vertices contradicts the Ward identity and violates the thermodynamic sum rule following from quark number conservation. We further show that the hard thermal loop perturbation theory is consistent with the thermodynamic sum rule.
Data assimilation in the laboratory using a rotating annulus experiment

The thermally driven rotating annulus is a laboratory experiment important for the study of the dynamics of planetary atmospheres under controllable and reproducible conditions. We use the analysis correction method to assimilate laboratory data into an annulus model. We analyze the 2S and 3AV regular flow regimes between rotation rates of 0.75 and 0.875 rad s−1 and the 3SV chaotic flow regime between rotation rates of 2.2 and 3.1 rad s−1. Our assimilated observations are irregularly distributed, which is more meteorologically realistic than gridded observations as used in recent applications of data assimilation to laboratory measurements. We demonstrate that data assimilation can be used successfully and accurately in this context. We examine a number of specific assimilation scenarios: a wave‐number transition between two regimes, information propagation from data‐rich to data‐poor regions, the response of the assimilation to a strong disturbance to the flow, and a vortex‐shedding instability phenomenon at high rotation rate. At the highest rotation rates we calculated the barotropic E‐vectors using unobserved variables such as temperature and the vertical structure of the velocity field that are only available via the assimilation. These showed that the mean flow is weakened by the action of eddies, going some way towards explaining why vortices are shed at the very highest rotation rates but not at lower rotation. Rossby‐wave stability theory suggests that the underlying instability leading to vortex shedding may be baroclinic in character.
On the Spatial Dependence of Low-Degree Solar p-Mode Frequency Shifts from Full-Disk and Resolved-Sun Observations

We have analyzed low angular degree (low-ℓ) solar p-mode frequency shifts extracted from two types of observations, both made by instruments on board the ESA/NASA Solar and Heliospheric Observatory satellite during a period that covers the rising phase of activity, cycle 23 (1995-2002). The first are full-disk (Sun-as-a-star) averages made in Doppler velocity by the Global Oscillations at Low Frequency spectrophotometer; the second are Doppler velocity observations made with high spatial resolution by the Michelson Doppler Imager. We compare the eigenfrequency shifts from both sets of data and find that they are consistent at the level of precision of the observations. Furthermore, the sizes of the shifts uncovered for different mode components are found to scale in proportion to the corresponding spherical harmonic components of the observed line-of-sight surface magnetic field, with the sectoral mode components showing (as expected) the largest shifts.
Adaptation of secondary flow in nearly-horizontal flow

To implement the generation and decay of secondary flow in steady or quasi-steady nearly-horizontal flow models an approximative method is proposed which takes account of the convection of momentum of secondary flow in streamwise direction. The method is employed both for the effects of Coriolis acceleration and curvature. It is shown that both effects are almost equivalent. The method is verified by comparison with results obtained in various flumes of rectangular or almost rectangular cross-section. The agreement between theory and experiments for the intensity of the secondary flow is quite satisfactory, although the theory presented cannot reproduce each detail.
Solar Radiation and Asteroidal Motion

Effects of solar wind and solar electromagnetic radiation on motion of asteroids are discussed. The results complete the statements presented in Vokrouhlick\'{y} and Milani (2000). As for the effect of electromagnetic radiation, the complete equation of motion is presented to the first order in $v/c$ -- the shape of asteroid (spherical body is explicitly presented) and surface distribution of albedo should be taken into account. Optical quantities must be calculated in proper frame of reference.
WR 20a: a massive cornerstone binary system comprising two extreme early-type stars ⋆

We analyse spectroscopic observations of WR 20a revealing that this star is a massive early-type binary system with a most probable orbital period of ∼3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If ∗ /WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 ± 4.0 and 68.8± 3.8 Mfor the two stars. These properties make WR 20a a cornerstone system for the study of massive star evolution.
Active disturbance rejection control for uncertain multivariable systems with time-delay

A new nonlinear control method for multivariable systems with time delay is presented. It is based on a unique active disturbance rejection concept. The proposed active disturbance rejection controller (ADRC) consists of the tracking differentiator, the extended state observer and the nonlinear proportional derivative (PD) controller. In this approach, the systems with time delay in the input are viewed as higher-order systems without time delay in the input, the approximation error between the nominal systems and real systems and other uncertainties, all of which are seen as `disturbance' by ADRC and are actively compensated. The techniques developed here can be effectively used in engineering systems, such as chemical processes, and this is demonstrated by an example.
The Keck/OSIRIS Nearby AGN Survey (KONA). I. The Nuclear K-band Properties of Nearby AGN

We introduce the Keck OSIRIS Nearby AGN survey (KONA), a new adaptive optics-assisted integral-field spectroscopic survey of Seyfert galaxies. KONA permits at ∼0. ″1 resolution a detailed study of the nuclear kinematic structure of gas and stars in a representative sample of 40 local bona fide active galactic nucleus (AGN). KONA seeks to characterize the physical processes responsible for the coevolution of supermassive black holes and galaxies, principally inflows and outflows. With these IFU data of the nuclear regions of 40 Seyfert galaxies, the KONA survey will be able to study, for the first time, a number of key topics with meaningful statistics. In this paper we study the nuclear K-band properties of nearby AGN. We find that the K-band (2.1 μm) luminosities of the compact Seyfert 1 nuclei are correlated with the hard X-ray luminosities, implying a non-stellar origin for the majority of the continuum emission. The best-fit correlation is log LK = 0.9log L2–10 keV + 4 over three orders of magnitude in both K-band and X-ray luminosities. We find no strong correlation between 2.1 μm luminosity and hard X-ray luminosity for the Seyfert 2 galaxies. The spatial extent and spectral slope of the Seyfert 2 galaxies indicate the presence of nuclear star formation and attenuating material (gas and dust), which in some cases is compact and in some galaxies extended. We detect coronal-line emission in 36 galaxies and for the first time in 5 galaxies. Finally, we find 4/20 galaxies that are usually classified as Seyfert 2 based on their optical spectra exhibit a broad component of Brγ emission, and one galaxy (NGC 7465) shows evidence of a double nucleus.
X-Ray Sources and Star Formation Activity in the Sagittarius B2 Cloud Observed with Chandra

We report the X-ray population study in the giant molecular cloud Sagittarius B2 (Sgr B2). More than a dozen X-ray cloud members (and candidates) are discovered with Chandra. Two bright X-ray sources are located near Sgr B2 Main, the most copious complex of the ultracompact H II sources. The X-ray spectra are fitted with a thin thermal plasma model of 5-10 keV temperature. The intrinsic luminosity, after correcting the absorption of ~5 × 1023 H cm-2, is ~1033 ergs s-1. Although these two X-ray sources are attributable to young stellar objects (YSOs) in the same H II complex, they are in sharp contrast; the one at the center of the H II complex exhibits strong K-shell transition lines of iron, while the other near the east has only weak lines. The other H II complexes, Sgr B2 North and South, also show hard and highly absorbed X-ray emissions due possibly to the star formation activity. The composite X-ray spectrum of the other cloud member X-ray sources is fitted with a thin thermal plasma of ~10 keV temperature with hydrogen column density (NH) of 1.3 × 1023 H cm-2 and individual X-ray luminosity of a few times 1031-1032 ergs s-1. These are likely to be a single YSO or cluster of YSOs, but neither radio nor infrared counterpart is found. An alternative scenario of isolated white dwarfs powered by Bondi-Hoyle accretion from the dense cloud gas is also discussed. The X-ray spectra exhibit an additional 6.4 keV line of neutral or low-ionization irons, which indicates that the environment gas is concentrated near the sources.
On the nonlinear stochastic realization problem

In this paper we construct a first solution of the stochastic realization problem in a nonlinear setting. The great bulk of previous work on stochastic realization has been in the linear Gaussian setting. Such Markovian representations are used e.g., to apply certain filtering and stochastic control techniques. Our methods consist of an amalgamation of methods introduced by Nelson with the Lax-Phillips type geometric approach to linear Gaussian stochastic realization which has been developed by Lindquist and Picci and by Ruckebusch. The result of this that we are able to realize any purely nondeterministic process satisfying suitable continuity conditions as an instantaneous function of a Markov process.
Smectic Liquid Crystals in Random Environments

We study smectic liquid crystals in random environments, e.g., aerogel. A low temperature analysis reveals that even arbitrarily weak quenched disorder (i.e., arbitrarily low aerogel density) destroys translational (smectic) order. A harmonic approximation to the elastic energy suggests that there is no ``smectic Bragg glass'' phase in this system: even at zero temperature, it is riddled with dislocation loops induced by the quenched disorder. This result implies the destruction of orientational (nematic) order as well, and that the thermodynamically sharp Nematic-Smectic-A transition is destroyed by disorder, in agreement with recent experimental results. We also show that the anharmonic elastic terms neglected in the above treatment are important (i.e., are ``relevant'' in the renormalization group sense); whether they alter the above conclusions about the smectic Bragg glass, orientational disorder, and the existence of sharp transitions, remains an open question. However, they do not alter our conclusion that translational (smectic) order is always destroyed. In contrast, we expect that weak annealed disorder should have no qualitative effects on the smectic order.
Rapid Interstellar Scintillation of PKS 1257–326: Two-Station Pattern Time Delays and Constraints on Scattering and Microarcsecond Source Structure

We report measurements of time delays of up to 8 minutes in the centimeter-wavelength variability patterns of the intrahour scintillating quasar PKS 1257-326, as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intrahour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasar's intensity-scintillation pattern, as well as to attempt to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >10 : 1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major-axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc.
Quantized acoustoelectric current transport through a static quantum dot using a surface acoustic wave

We present a detailed study of the surface acoustic wave mediated quantized transport of electrons through a split-gate device containing an impurity potential defined quantum dot within the split-gate channel, A regime of quantized transport is observed at low rf powers where the surface acoustic wave amplitude is comparable to the quantum dot charging energy. In this regime resonant transport through the single-electron dot state occurs which we interpret as turnstile-like operation in which the traveling wave amplitude modulates the entrance and exit barriers of the quantum dot in a cyclic fashion at GHz frequencies. For high rf powers, where the amplitude of the surface acoustic wave is much larger than the quantum dot energies, the quantized acoustoelectric current transport shows behavior consistent with previously reported results. However, in this regime, the number of quantized current plateaus observed and the plateau widths are determined by the properties of the quantum dot, demonstrating that the microscopic detail of the potential landscape in the split-gate channel has a profound influence on the quantized acoustoelectric current transport.
Decorrelating the errors of the galaxy correlation function with compact transformation matrices

Covariance matrix estimation is a persistent challenge for cosmology, often requiring a large number of synthetic mock catalogues. The off-diagonal components of the covariance matrix also make it difficult to show representative error bars on the 2-point correlation function (2PCF) since errors computed from the diagonal values of the covariance matrix greatly underestimate the uncertainties. We develop a routine for decorrelating the projected and anisotropic 2PCF with simple and scale-compact transformations on the 2PCF. These transformation matrices are modelled after the Cholesky decomposition and the symmetric square root of the Fisher matrix. Using mock catalogues, we show that the transformed projected and anisotropic 2PCF recover the same structure as the original 2PCF while producing largely decorrelated error bars. Specifically, we propose simple Cholesky-based transformation matrices that suppress the off-diagonal covariances on the projected 2PCF by ${\sim } 95{{\ \rm per\ cent}}$ and that on the anisotropic 2PCF by ${\sim } 87{{\ \rm per\ cent}}$. These transformations also serve as highly regularized models of the Fisher matrix, compressing the degrees of freedom so that one can fit for the Fisher matrix with a much smaller number of mocks.
Interactions between coherent twin boundaries and phase transition of iron under dynamic loading and unloading

Phase transitions and deformation twins are constantly reported in many BCC metals under high pressure, whose interactions are of fundamental importance to understand the strengthening mechanism of these metals under extreme conditions. However, the interactions between twins and phase transition in BCC metals remain largely unexplored. In this work, interactions between coherent twin boundaries and α ↔ e phase transition of iron are investigated using both non-equilibrium molecular dynamics simulations and the nudged elastic band method. Mechanisms of both twin-assisted phase transition and reverse phase transition are studied, and orientation relationships between BCC and HCP phases are found to be 111¯BCC||1¯21¯0HCP and 11¯0BCC||0001HCP for both cases. The twin boundary corresponds to 101¯0HCP after the phase transition. It is amazing that the reverse transition seems to be able to “memorize” and recover the initial BCC twins. The memory would be partly lost when plastic slips take place in the HCP pha.
Applying electric and magnetic field bias in a 3D superconducting waveguide cavity with high quality factor

Three-dimensional microwave waveguide cavities are essential tools for many cavity quantum electrodynamics experiments. However, the need to control quantum emitters with dc magnetic fields inside the cavity often limits such experiments to normal-conducting cavities with relatively low quality factors of about 104. Similarly, controlling quantum emitters with dc electric fields in normal- and superconducting waveguide cavities has so far been difficult, because the insertion of dc electrodes has strongly limited the quality factor. Here, we present a method to apply dc electric fields within a superconducting waveguide cavity, which is based on the insertion of dc electrodes at the nodes of the microwave electric field. Moreover, we present a method to apply dc magnetic fields within the same cavity by trapping the magnetic flux in holes positioned in facing walls of the cavity. We demonstrate that the TE301 mode of such a superconducting, rectangular cavity made from niobium maintains a high internal quality factor of Q int ≃ 1.7 × 10 6 at the few photon level and a base temperature of 3 K. A cloud of Rydberg atoms coupled to the microwave electric field of the cavity is used to probe the applied dc electric and magnetic fields via the quadratic Stark effect and the Zeeman effect, respectively.
Domestic corpuscular inflaton

The aim of this paper is to provide a more precise description of the paradigm of corpuscular slow-roll inflation, which was previously introduced by Casadio et al. in [Corpuscular slow-roll inflation, Phys. Rev. D 97 (2018) 024041]. Specifically, we start by expanding the Starobinsky theory on a curved background and then infer the number and nature of the propagating degrees of freedom, both in the true inflationary phase and in a quasi-de Sitter approximation. We correctly find that the particle spectrum contains a transverse trace-free mode and a scalar one. The scalar mode displays a tachyonic nature during the slow-roll phase, due to the instability of the system, whereas it acquires the appropriate oscillatory behavior as the background approaches a critical value of the curvature. These results confirm the fact that the Einstein–Hilbert term acts as a perturbation to the quadratic one, and is responsible for driving the early Universe out of the inflationary phase, thus realizing the inflaton field in terms of pure (corpuscular) gravity.
Thin-film nonlinear optical diode

We present results of a theoretical investigation into a nonlinear thin‐film multilayer device that exhibits passive anisotropic optical transmission—the analogue of the electronic diode. This optical diode is a nonlinear, asymmetric, distributed Bragg reflector. Material parameters for a nonlinear polymer (polydiacetylene 9‐BCMU) and rutile are used in alternating layers to model a realistic device. The diode exhibits more than five times as much transmittance in one direction as in the opposite direction. It has a thickness of only 2 μm and is polarization insensitive.
Contrasting the Ultraviolet and X-Ray O VI Column Density Inferred for the Outflow in NGC 5548

We compare X-ray and UV spectroscopic observations of NGC 5548. Both data sets show O VI absorption troughs associated with the active galactic nuclei outflow from this galaxy. We find that the robust lower limit on the column density of the O VI X-ray trough is 7 times larger than the column density found in a study of the O VI UV troughs. This discrepancy suggests that column densities inferred for UV troughs of Seyfert galaxy outflows are often severely underestimated. We identify the physical limitations of the UV Gaussian modeling as the probable explanation of the O VI column density discrepancy. Specifically, Gaussian modeling cannot account for a velocity-dependent covering fraction, and it is a poor representation for absorption associated with a dynamical outflow. Analysis techniques that use a single covering fraction value for each absorption component suffer from similar limitations. We conclude by suggesting ways to improve the UV analysis.
Aspects of the BTZ black hole interacting with fields

We show that there is no superradiance for the Dirac field in the rotating BTZ black hole if the field vanishes at infinity. Then we outline the calculation of the expression for the renormalized energy–momentum tensor, the effective action as well as the heat kernel for the Dirac field for the BTZ black hole. Finally, we point out how to construct the Hartle–Hawking–Israel state for the real scalar field in the non-rotating BTZ black hole in two and three dimensions.