arXiv subjects

Martin Horvat

Publications and source records attributed to Martin Horvat.

At least 19 recordsLinked to original sources

arXiv2025

Physics Of Eclipsing Binaries. VII. Interferometric module

Interferometric measurements are essential to constrain models of stellar systems, by spatially resolving angular distances and diameters well below the classical diffraction limit. In this work, we describe the interferometric module of Phoebe, which could be used just for this purpose. Since binaries in Phoebe are represented by a triangular mesh, our complex model is based on the integration over triangles. Consequently, Roche distortion, rotation, non-synchronicity, misalignment, eclipses of components, darkening, reflection, or irradiation are all accurately accounted for. For comparison purposes, we provide a simplified model, where components are represented by circular disks. The key point of our approach is a possibility of combination with other datasets (light curves, radial velocities), which allows to construct robust models of stellar systems. This draft refers to a development version of Phoebe, available at https://github.com/miroslavbroz/phoebe2/tree/interferometry . It is not yet included in the official Phoebe repository!

astro-ph.SR↗

arXiv2024

Hamiltonian Lorenz-like models

The reduced-complexity models developed by Edward Lorenz are widely used in atmospheric and climate sciences to study nonlinear aspect of dynamics and to demonstrate new methods for numerical weather prediction. A set of inviscid Lorenz models describing the dynamics of a single variable in a zonally-periodic domain, without dissipation and forcing, conserve energy but are not Hamiltonian. In this paper, we start from a general continuous parent fluid model, from which we derive a family of Hamiltonian Lorenz-like models through a symplectic discretization of the associated Poisson bracket that preserves the Jacobi identity. A symplectic-split integrator is also formulated. These Hamiltonian models conserve energy and maintain the nearest-neighbor couplings inherent in the original Lorenz model. As a corollary, we find that the Lorenz-96 model can be seen as a result of a poor discretization of a Poisson bracket. Hamiltonian Lorenz-like models offer promising alternatives to the original Lorenz models, especially for the qualitative representation of non-Gaussian weather extremes and wave interactions, which are key factors in understanding many phenomena of the climate system.

physics.ao-ph↗

arXiv2023

Dynamical and statistical properties of estimated high-dimensional ODE models: The case of the Lorenz'05 type II model

The performance of estimated models is often evaluated in terms of their predictive capability. In this study, we investigate another important aspect of estimated model evaluation: the disparity between the statistical and dynamical properties of estimated models and their source system. Specifically, we focus on estimated models obtained via the regression method, sparse identification of nonlinear dynamics (SINDy), one of the promising algorithms for determining equations of motion from time series of dynamical systems. We chose our data source dynamical system to be a higher-dimensional instance of the Lorenz 2005 type II model, an important meteorological toy model. We examine how the dynamical and statistical properties of the estimated models are affected by the standard deviation of white Gaussian noise added to the numerical data on which the estimated models were fitted. Our results show that the dynamical properties of the estimated models match those of the source system reasonably well within a range of data-added noise levels, where the estimated models do not generate divergent (unbounded) trajectories. Additionally, we find that the dynamics of the estimated models become increasingly less chaotic as the data-added noise level increases. We also perform a variance analysis of the (SINDy) estimated model's free parameters, revealing strong correlations between parameters belonging to the same component of the estimated model's ordinary differential equation.

physics.comp-ph↗

arXiv2022

Dynamic mode decomposition as an analysis tool for time-dependent partial differential equations

The time-dependent fields obtained by solving partial differential equations in two and more dimensions quickly overwhelm the analytical capabilities of the human brain. A meaningful insight into the temporal behaviour can be obtained by using scalar reductions, which, however, come with a loss of spatial detail. Dynamic Mode Decomposition is a data-driven analysis method that solves this problem by identifying oscillating spatial structures and their corresponding frequencies. This paper presents the algorithm and provides a physical interpretation of the results by applying the decomposition method to a series of increasingly complex examples.

math.NA↗

arXiv2021

Detection of coupling in Duffing oscillator systems

In complex dynamical systems, the detection of coupling and its direction from observed time series is a challenging task. We study coupling in coupled Duffing oscillator systems in regular and chaotic dynamical regimes. By observing the conditional mutual information (CMI) based on the Shannon entropy, we successfully infer the direction of coupling for different system regimes. Moreover, we show that in the weak coupling limit the values of CMI can be used to infer the coupling parameters by computing the derivative of the conditional mutual information with respect to the coupling strength, called the information susceptibility. The complete numerical implementation is available at https://repo.ijs.si/mbresar/duffing-cmi.

nlin.AO↗

arXiv2021

Calculation of multiphoton ionization amplitudes and cross sections of few-electron atoms

We present a theoretical method for calculating multiphoton ionization amplitudes and cross sections of few-electron atoms. The present approach is based on an extraction of partial wave amplitudes from a scattering wave function, which is calculated by solving a system of driven Schroedinger equations. The extraction relies on a description of partial waves in terms of a small number of Coulomb waves with fixed wave numbers. The method can be used for photon energies below and above the ionization threshold and to treat resonance-enhanced multiphoton ionization. We use it to calculate two-, three-, and four-photon ionization cross sections of hydrogen and helium atoms for a wide range of photon energies and to determine the asymmetry parameters of photoelectron angular distributions for two-, three-, and four-photon ionization of the helium atom.

physics.atom-ph↗

arXiv2020

Physics of Eclipsing Binaries. IV. The impact of interstellar extinction on the light curves of eclipsing binaries

Traditionally, the effects of interstellar extinction on binary star light curves have been treated as a uniform reduction in the observed brightness of the system that is independent of orbital phase. However, unless the orbital plane of the system coincides with the plane of the sky, or if the two stars are completely identical and present with minimal mutual irradiation and tidal/rotational distortions, then this is unlikely to be an accurate representation of the effect of interstellar extinction. Here, we present an updated treatment of interstellar extinction as incorporated in the PHOEBE 2.2 release (publicly available from http://phoebe-project.org) and assess the importance of using such an approach in the modeling of different types of binary systems. We also present the incorporation of PHOENIX model atmospheres into the PHOEBE 2.2 release, providing increased fidelity on computed observables down to lower temperatures than previously available. The importance of these new code developments is then highlighted via an extincted toy model of the eclipsing white-dwarf-subdwarf binary SDSS~J235524.29+044855.7 -- demonstrating that, in the age of LSST as well as complementary space-based photometric missions, a proper accounting for extinction and as well as the use of realistic model atmospheres will be essential in deriving accurate binary parameters.

astro-ph.SR↗

arXiv2019

Bolometric treatment of irradiation effects: general discussion and application to binary stars

A general framework for dealing with irradiation effects in the bolometric sense --- specifically, reflection with heat absorption and the consequent redistribution of the absorbed heat, for systems of astrophysical bodies where the boundaries are used as support for the description of the processes, is presented. Discussed are its mathematical and physical properties, as well as its implementation approximations, with a focus on three plausible redistribution processes (uniform, latitudinal, and local redistribution). These are tested by extending PHOEBE 2.1 ( http://phoebe-project.org ), the open-source package for modeling eclipsing binaries, and applied to a toy model of the known two-body eclipsing systems.

astro-ph.SR↗

arXiv2019

Physics of Eclipsing Binaries. III. Spin-Orbit Misalignment

Binary systems where the axis of rotation (spin) of one or both components is tilted w.r.t. the axis of revolution are called misaligned binary systems. The angle of misalignment, obliquity, has been measured for a handful of stars and extrasolar planets to date. Here we present a mathematical framework for a complete and rigorous treatment of misalignment and introduce an extension to the public PHOEBE code that implements this framework. We discuss misalignment for the Roche geometry and introduce methods for computing stellar shapes, equilibrium (generalized Lagrange) points of the potential and minimal requirements for lobe existence. Efficient parametrization of misalignment is proposed in the plane-of-sky coordinates and implementation details in PHOEBE are given alongside the proof-of-concept toy model, comparison with a known misaligned binary DI Her, and comparison with a misaligned planetary system Kepler-13. We provide important mathematical details of the model in the Appendix. This paper accompanies the release of PHOEBE 2.1, which will be available soon from its website http://phoebe-project.org.

astro-ph.SR↗

arXiv2018

COBAIN: generalized 3D radiative transfer code for contact binary atmospheres

Contact binary stars have been known to have a peculiar and somewhat mysterious hydro- and thermodynamical structure since their discovery, which directly affects the radiation distribution in their atmospheres. Over the past several decades, however, observational data of contact binaries have been modeled through a simplified approach, involving the artificial concatenation of the two components of the contact envelope and populating their respective surfaces with either blackbody atmospheres or plane-parallel model atmospheres of single stars. We show the implications this approach has on the reliability of the system parameter values and propose a method to overcome these issues with a new generalized radiative transfer code, COBAIN (COntact Binary Atmospheres with INterpolation). The basic principles of COBAIN are outlined and their application to different geometries and polytropic stellar structures is discussed. We present initial tests on single non-rotating, uniformly rotating and differentially rotating stars, as well as on simplified polytropic structural models of contact binaries. We briefly discuss the final goal of this ambitious project, which is the computation of model atmosphere tables under the correct assumptions for contact binary stars, to be used in modern binary star analysis codes.

astro-ph.SR↗

arXiv2018

The Effects of Barycentric and Asymmetric Transverse Velocities on Eclipse and Transit Times

It has long been recognized that the finite speed of light can affect the observed time of an event. For example, as a source moves radially toward or away from an observer, the path length and therefore the light travel time to the observer decreases or increases, causing the event to appear earlier or later than otherwise expected, respectively. This light travel time effect (LTTE) has been applied to transits and eclipses for a variety of purposes, including studies of eclipse timing variations (ETVs) and transit timing variations (TTVs) that reveal the presence of additional bodies in the system. Here we highlight another non-relativistic effect on eclipse or transit times arising from the finite speed of light---caused by an asymmetry in the transverse velocity of the two eclipsing objects, relative to the observer. This asymmetry can be due to a non-unity mass ratio or to the presence of external barycentric motion. Although usually constant, this barycentric and asymmetric transverse velocities (BATV) effect can vary between sequential eclipses if either the path length between the two objects or the barycentric transverse velocity varies in time. We discuss this BATV effect and estimate its magnitude for both time-dependent and time-independent cases. For the time-dependent cases, we consider binaries that experience a change in orbital inclination, eccentric systems with and without apsidal motion, and hierarchical triple systems. We also consider the time-independent case which, by affecting the primary and secondary eclipses differently, can influence the inferred system parameters, such as the orbital eccentricity.

astro-ph.SR↗

arXiv2016

Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity

The precision of photometric and spectroscopic observations has been systematically improved in the last decade, mostly thanks to space-borne photometric missions and ground-based spectrographs dedicated to finding exoplanets. The field of eclipsing binary stars strongly benefited from this development. Eclipsing binaries serve as critical tools for determining fundamental stellar properties (masses, radii, temperatures and luminosities), yet the models are not capable of reproducing observed data well either because of the missing physics or because of insufficient precision. This led to a predicament where radiative and dynamical effects, insofar buried in noise, started showing up routinely in the data, but were not accounted for in the models. PHOEBE (PHysics Of Eclipsing BinariEs; http://phoebe-project.org) is an open source modeling code for computing theoretical light and radial velocity curves that addresses both problems by incorporating missing physics and by increasing the computational fidelity. In particular, we discuss triangulation as a superior surface discretization algorithm, meshing of rotating single stars, light time travel effect, advanced phase computation, volume conservation in eccentric orbits, and improved computation of local intensity across the stellar surfaces that includes photon-weighted mode, enhanced limb darkening treatment, better reflection treatment and Doppler boosting. Here we present the concepts on which PHOEBE is built on and proofs of concept that demonstrate the increased model fidelity.

astro-ph.SR↗

arXiv2015

Relativistic Positioning System in Perturbed Space-time

We present a variant of a Global Navigation Satellite System called a Relativistic Positioning System (RPS), which is based on emission coordinates. We modelled the RPS dynamics in a space-time around Earth, described by a perturbed Schwarzschild metric, where we included the perturbations due to Earth multipoles (up to the 6th), the Moon, the Sun, Venus, Jupiter, solid tide, ocean tide, and Kerr rotation effect. The exchange of signals between the satellites and a user was calculated using a ray-tracing method in the Schwarzschild space-time. We find that positioning in a perturbed space-time is feasible and is highly accurate already with standard numerical procedures: the positioning algorithms used to transform between the emission and the Schwarzschild coordinates of the user are very accurate and time efficient -- on a laptop it takes 0.04 s to determine the user's spatial and time coordinates with a relative accuracy of $10^{-28}-10^{-26}$ and $10^{-32}-10^{-30}$, respectively.

gr-qc↗

arXiv2014

An Autonomous Reference Frame for Relativistic GNSS

Current GNSS systems rely on global reference frames which are fixed to the Earth (via the ground stations) so their precision and stability in time are limited by our knowledge of the Earth dynamics. These drawbacks could be avoided by giving to the constellation of satellites the possibility of constituting by itself a primary and autonomous positioning system, without any a priori realization of a terrestrial reference frame. Our work shows that it is possible to construct such a system, an Autonomous Basis of Coordinates, via emission coordinates. Here we present the idea of the Autonomous Basis of Coordinates and its implementation in the perturbed space-time of Earth, where the motion of satellites, light propagation, and gravitational perturbations are treated in the formalism of general relativity.

gr-qc↗

arXiv2013

Transport in a disordered tight-binding chain with dephasing

We study transport properties of a disordered tight-binding model (XX spin chain) in the presence of dephasing. Focusing on diffusive behavior in the thermodynamic limit at high energies, we analytically derive the dependence of conductivity on dephasing and disorder strengths. As a function of dephasing, conductivity exhibits a single maximum at the optimal dephasing strength. The scaling of the position of this maximum with disorder strength is different for small and large disorder. In addition, we study periodic disorder for which we find a resonance phenomenon, with conductivity having two maxima as a function of dephasing strength. If disorder is nonzero only at a random fraction of all sites, conductivity is approximately the same as in the case of a disorder on all sites but with a rescaled disorder strength.

cond-mat.str-el↗

arXiv2012

Railway switch transport model

We propose a simple model of coupled heat and particle transport based on a zero-dimensional classical deterministic dynamics which is reminiscent of a railway switch whose action is only a function of the particle's energy. It is shown that already in the minimal three-terminal model, where the second terminal is considered as a probe with zero net particle and heat currents, one can find extremely asymmetric Onsager matrices as a consequence of time-reversal symmetry breaking of the model. This minimalistic transport model provides a better understanding of thermoelectric heat engines in the presence of time-reversal symmetry breaking.

cond-mat.mes-hall↗

arXiv2011

Nanocoolers

We present a simple kinematic model of a non-equilibrium steady state device, which can operate either as a heat engine or as a refrigerator. The model is composed of two or more scattering channels where the motion is fully described by deterministic classical dynamics, which connect a pair of stochastic (infinite) heat and particle baths at unequal temperatures. We discuss precise kinematic conditions under which our model may approach Carnot's optimal efficiency in different situations.

cond-mat.stat-mech↗

arXiv2009

A hybrid method for calculation of Ruelle-Pollicott resonances

We present a numerical method for calculation of Ruelle-Pollicott resonances of dynamical systems. It constructs an effective coarse-grained propagator by considering the correlations of multiple observables over multiple timesteps. The method is compared to the usual approaches on the example of the perturbed cat map and is shown to be numerically efficient and robust.

nlin.CD↗