Astrophysics and Space Science最新文献

The goal of this paper is to obtain an approximate solution of the restricted three-body problem in the case of small perturbations in the vicinity of, but not in exact resonance. In this paper, we study the restricted three-body problem known as planetary type (i.e., when the eccentricity of the test particle is small). A method of linearizing the equation of motion close to (but not in) resonance is proposed under the assumption of small perturbations. In other words, we study orbits when the resonant argument circles the resonance. In the practically interesting case of resonant perturbations we can restrict our study to a perturbation with a single frequency with the largest amplitude, and reduce the problem to the Mathieu equation. The model qualitatively describes the behavior of the perturbation in the vicinity of the resonance. It can be used to estimate the exact position of the resonance and the boundaries between neighboring resonances.

Arrays of precisely-timed millisecond pulsars are used to search for gravitational waves with periods of months to decades. Gravitational waves affect the path of radio pulses propagating from a pulsar to Earth, causing the arrival times of those pulses to deviate from expectations based on the physical characteristics of the pulsar system. By correlating these timing residuals in a pulsar timing array (PTA), one can search for a statistically isotropic background of gravitational waves by revealing evidence for a distinctive pattern predicted by General Relativity, known as the Hellings & Downs curve. On June 29 2023, five regional PTA collaborations announced the first evidence for GWs at light-year wavelengths, predicated on support for this correlation pattern with statistical significances ranging from (sim !2-4sigma ). The amplitude and shape of the recovered GW spectrum has also allowed many investigations of the expected source characteristics, ranging from a cosmic population of supermassive binary black holes to numerous processes in the early Universe. In the future, we expect to resolve signals from individual binary systems of supermassive black holes, and probe fundamental assumptions about the background, including its polarization, anisotropy, Gaussianity, and stationarity, all of which will aid efforts to discriminate its origin. In tandem with new facilities like DSA-2000 and the SKA, fueling further observations by regional PTAs and the International Pulsar Timing Array, PTAs have extraordinary potential to be engines of nanohertz GW discovery.

In this work, we analytically derive the caustic equation for a general (N)-point gravitational lens using methods from algebraic geometry and complex function theory. Based on this equation, we construct the full pre-image of the caustic in the lens plane. This pre-image includes not only the critical curve but also additional closed curves that partition the lens plane into regions mapped to the interior and exterior of the caustic in the source plane. These regions define topological domains within which the number of lensed images remains constant. Notably, when the source moves within a region that does not intersect the caustic, its corresponding images remain confined to specific regions of the lens plane. The effectiveness of the proposed approach is demonstrated using the example of a general binary gravitational lens system.

We present a physically motivated extension of the FP for elliptical galaxies, derived from the scalar virial theorem and calibrated using observational data. Starting from the basic equilibrium condition, we incorporate key physical parameters that govern galaxy structure and dynamics, namely stellar mass-to-light ratio, central dark matter fraction, and structural non-homology as traced by the Sérsic profile. The resulting model retains the original dependencies on velocity dispersion and surface brightness, but introduces physically interpretable corrections that significantly improve the fit to real data. Using a large galaxy sample, we demonstrate that this extended FP achieves a higher level of accuracy than the classical form, with all parameters showing strong statistical significance. Our results indicate that the observed FP can be understood as an empirical refinement of the virial prediction, once variations in stellar populations, dark matter content, and internal structure are taken into account. This work provides a unified framework that bridges theoretical expectations with observed scaling relations in elliptical systems.

Classical attempts to construct a galaxy model, in a stationary and axisymmetric situation, consist of giving a gravitational field and injecting it into the collisionless Boltzmann equation to deduce the solution distribution function f. We will do exactly the opposite, by assimilating the galaxy to a self-gravitating point-mass system. The velocity distribution function is then the solution of an integrodifferential equation. Taking into account the Newtonian character of the potential, we can replace it with the system consisting of the Vlasov equation, written in terms of residual velocity, and the Poisson equation. We then give (ln(f)) the form of a polynomial of degree 2, such that one of the axes of the velocity ellipsoid points towards the center of the system. This single constraint gives the evolution of the axes in space, these being equal to the center of the galaxy (Maxwell-Boltzmann distribution). Moving away from the center, the axis pointing in this direction remains constant while the transverse axes tend to zero at infinity. We then construct the macroscopic velocity field by excluding any vortex structure. This field then tends towards a solid body rotation at the center. The velocity tends towards a remote plateau, which is then consistent with the observational data.

We conduct a systematic study on the effects of rapid rotation on predicted Be star observables. We use the three-dimensional Monte Carlo radiative transfer code, hdust, to model a comprehensive range of Be star subtypes at varying rotation rates. Using these models, we predict (V) magnitude and photometric color, H(alpha ) line profiles, and polarization at UV wavelengths as well as in the (V)-band for Be stars from B0 to B8. For each spectral subtype, we investigate the effects of disk density on the produced observables. We find that reddening and brightening effects of gravity darkening may cause rapidly-rotating stars to appear more evolved than they truly are. Rotational effects on the H(alpha ) line profile shape may reduce line intensity for Be stars viewed at low inclinations and increase line intensity for those viewed at high inclinations. Additionally, rapid rotation can significantly impact the measured equivalent width of the line produced by a star with a moderate to high density disk, especially at high inclinations. When the star-disk system is viewed near edge-on, gravity darkening can result in stronger H(alpha ) emission than would otherwise be expected for a disk of a given density. We also find that the competing effects of rapid rotation and H i opacity cause the slope of the polarized continuum (the polarization color) to be very sensitive to changes in the stellar rotation rate. This quantity offers a strong diagnostic for the rotation rate of Be stars.

Optical interferometry is an observational technique that provides the highest spatial resolutions available in the optical. By interfering light from separate telescopes, and measuring the properties of the resulting interference pattern, it is possible to retrieve information about the night sky at spatial resolutions equal to the separation of the telescopes, overcoming the diffraction limit of a single telescope. In long baseline amplitude optical interferometry, the beams of light from the telescopes are transported to a central location and physically interfered. The interference is achieved via an instrument known as a beam combiner. In this review, I discuss the functionality of a beam combiner. I begin with a mathematical explanation of how interference fringes are produced and what information these interference fringes contain. This is followed by a discussion of how interference fringes are generated and measured in practise for the most common beam combination schemes, for both pupil plane and image plane combination and how these schemes can be realised in bulk optics or integrated optics. I also provide a detailed summary of the various design considerations that can affect the functionality of a beam combiner. Finally, I discuss current and future work in long baseline amplitude optical interferometry.

Accurate prediction of solar activity and geomagnetic disturbances is essential for reducing the risks posed by space weather to technological systems. This study presents a hybrid deep learning model that integrates Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU) networks for simultaneous forecasting of sunspot numbers and geomagnetic indices (Ap and Kp). Using comprehensive datasets spanning solar cycles 20 to 24 (1964–2016), the model employs advanced preprocessing techniques including Savitzky-Golay filtering and normalization and a 60-day sliding window to capture complex temporal dependencies. Model performance, evaluated via 8-fold cross-validation, demonstrates high predictive accuracy, achieving R² values of 0.9943 for sunspot numbers, 0.970 for Kp, and 0.923 for Ap, with low RMSE values. Heatmaps highlighted low RMSE across most time segments, confirming model robustness. Our results confirm a strong correlation between high Ap-index values and increased sunspot activity. Extreme event analysis demonstrates reliable detection of high-intensity geomagnetic storms, with substantial improvements in probability of detection and false alarm rates relative to NOAA/SWPC benchmarks. Comparative assessments show that the hybrid LSTM-GRU model outperforms standalone deep learning and conventional approaches, offering both aggregate skill and operationally relevant performance, even in the presence of severe class imbalance. The hybrid LSTM–GRU model demonstrates clear advantages over standalone LSTM and GRU architectures, with correlation analysis confirming strong links between sunspot activity and the Ap-index, underscoring the model’s ability to capture solar-terrestrial interactions. The proposed LSTM-GRU model demonstrates significant potential for real time space weather forecasting and offers a scalable framework for extended solar-terrestrial predictive analysis.

Forbush decreases (FDs) are short-term reductions in the time-intensity flux of cosmic rays (CRs). Their spectacular and unpredictable intensity variations present their detection, timing, magnitude estimation, and cataloging as one of the most difficult tasks in space weather research. Due to the paucity of accurate event lists, new methods of event detection and FD catalogs continue to appear in the literature. But validation of either the old or new lists remains an open field. This work intends to remind the astrophysicist and space weather community that dramatic modifications of the age-long manual/semi-manual FD event cataloging are long overdue. In this present era of extremely fast and sophisticated computer algorithms, a complementary automated list of all the manually created FD catalogs, especially the IZMIRAN (Pushkin Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian Academy of Sciences, http://spaceweather.izmiran.ru/eng/dbs.html) (which is the currently available and widely used lists) should have been developed, the old lists expanded and new catalogs created. A Fourier decomposition technique, which guarantees the sinusoidal fidelity of the input and output signals, is presented. This highlights the possibility of a complete algorithm-based FD cataloging with the aim of stimulating other automated methodological approaches. The impossibility of creating accurate FD lists without first disentangling the contribution from diurnal CR anisotropy and the 11-year solar oscillation was qualitatively analyzed. Using a set of CR data from Moscow station, we automatically created four different FD catalogs − FD1 from the raw data without adjusting for the contribution from the 11-year solar cycle oscillation or CR anisotropy, FD2 after adjusting for the 11-year solar cycle effects, FD3 after adjusting for CR anisotropy and FD4 after adjusting for both the 11-year and CR anisotropy. This allows us to practically demonstrate the possible discrepancies among different FD catalogs and the attendant bias implications on FD-based space weather investigation. Given the detailed analyses performed, FD4 is the most accurate FD list. This serves as an evidence that accurate FD catalog is realizable. We also establish that several of the events in FD1, FD2 and FD3 catalogs may be intensity reductions/spurious events arising from other CR phenomena like anisotropies.

Stellar rotation has long been recognized as important to the evolution of stars, by virtue of the chemical mixing it can induce and how it interacts with binary mass transfer. Binary interaction and rapid rotation are both common in massive stars and involve processes of angular momentum distribution and transport. An important question is how this angular momentum transport leads to the creation of two important classes of rapidly rotating massive stars, Be stars defined by disklike emission lines, and Bn stars defined by rotationally broadened absorption lines. A related question is what limits this rotation places on how conservative the mass transfer can be. Central to addressing these issues is knowledge of how close to rotational break-up stars can get before they produce a disk. Here we calculate diagnostics of this rotational criticality using the continuum polarization arising from a combination of rotational stellar distortion (i.e., oblateness) and redistribution of stellar flux (i.e., gravity darkening), and compare polarizations produced in the von Zeipel approximation with the approach of Espinosa Lara & Rieutord (ELR). Both produce similar photospheric polarizations that rise significantly in the far ultraviolet (FUV) for B stars, with a stronger signal in the von Zeipel case. For early main-sequence and subgiant stars, it reaches a maximum of (sim 1)% at 140 nm for stars rotating at 98% of critical, when seen edge-on. Rotational rates above 80% critical result in polarizations of several tenths of a percent, at high inclination. Even at a low inclination of (i=40^{circ }), models at 98% critical show polarization in excess of 0.1% down to 200 nm. These predicted stable signal strengths indicate that determinations of near-critical rotations in B stars could be achieved with future spectropolarimetric instrumentation that can reach deep into the FUV, such as CASSTOR, the Polstar mission concept, or the POLLUX detector design.