Astrophysics and Space Science最新文献

Affected by the Earth’s atmosphere, the image of a primary and satellite system may appear unresolved, such as the dwarf planet Haumea system. It is found by experiments that neither the two-dimensional Gaussian nor modified moment centering algorithms can accurately measure the photocenter of an image of unresolved primary and satellite system observed. This work investigates a specific centering algorithm to accurately measure the photocenter, which would be helpful to derive some physical parameters (e.g. orbital parameters and mass). Taking the dwarf planet Haumea and its brighter satellite Hi’iaka as an example, we simulate the motion of the photocenter with different seeings. We find that the photocenter of system changes significantly with seeings (∼0.074″ with the different seeings of 1″ and 3″) when using the two-dimensional Gaussian centering algorithm. However, the modified moment centering algorithm can accurately measure the photocenter of system without noises, but when noises are added its accuracy will be greatly influenced by noises. In this work, a new centering algorithm is proposed, which can accurately measure the photocenter with less influence of seeings and noises. Observations of dwarf planet Haumea taken over 25 nights are used to test the effectiveness of our proposed method. Compared with using two-dimensional Gaussian centering algorithm, the fitted parameter is slightly more accurate with less positional fitting errors when using the proposed method in this work. This method can also be applied to the centering of binary stars.

We present here a simple hydrodynamic model based on a sequence of steady states of the inner sub-Keplerian accretion disc to understand its different spectral states. Correlations between different hydrodynamic steady states are studied with a goal to understand the origin of, e.g., the aperiodic variabilities. The plausible source of corona/outflow close to the central compact object is shown to be a consequence of steady state transition in the underlying accretion flow. We envisage that this phenomenological model can give insight on the influence of viscosity, efficiency of energy advection, nature of the background flow and environment on the evolution of the inner sub-Keplerian accretion disc.

We study two-parametric families of spatial orbits given in the analytic form (f(x,y,z)=c_{1}), (g(x,y,z)=c_{2}) ((c_{1}), (c_{2}) = const.) which are produced by three-dimensional potentials (V=V(x,y,z)) inside a material concentration. These potentials must verify two linear partial differential equations (PDEs) which are the basic equations of the 3D Inverse Problem of Newtonian Dynamics and the well-known Poisson’s equation. A suitable class of potentials for this case is the axisymmetric potentials (V=mathcal{B}(x^{2}+y^{2}, z)) which have applications in astrophysical problems. For the given density function (rho =rho (x, y, z)), (rho =rho _{0}=const)., or, (rho =rho (z)) and a pre-assigned family of orbits, three-dimensional potentials producing this family of orbits are found in each case. We focus our interest on the cored, logarithmic potentials and another one of fourth degree describing elliptical galaxies. The two-parametric families of straight lines in 3D space are also considered.

In this paper, two models of interest for Celestial Mechanics are presented and analysed, using both analytic and numerical techniques, from the point of view of the possible presence of regular and/or chaotic motion, as well as the stability of the considered orbits. The first model, presented in a Hamiltonian formalism, can be used to describe the motion of a satellite around Earth, taking into account both the non-spherical shape of our planet and the third-body gravitational influence of Sun and Moon. Using semi-analytical techniques coming from Normal Form and Nekhoroshev theories it is possible to provide stability estimates for the orbital elements of its geocentric motion. The second dynamical system presented can be used as a simplified model to describe the motion of a particle in an elliptic galaxy having a central massive core; it is constructed as a refraction billiard where an inner dynamics, induced by a Keplerian potential, is coupled with an external one, where a harmonic oscillator-type potential is considered. The investigation of the dynamics is carried on by using results of ODEs’ theory and is focused on studying the trajectories’ properties in terms of periodicity, stability and, possibly, chaoticity.

We analyzed the timing parameters (the rotational frequency (nu ), the first (left ( dot{nu } right )) and second (left ( ddot{nu } right )) time-derivatives of frequency) and the derived parameters of a sample of pulsars for which (ddot{nu } ) (470 pulsars) were recorded in the Australian Telescope National Facility (ATNF) pulsar catalog. We formed various subsamples, those with braking indices (n<0) and (n>0), and glitching and non-glitching pulsars. Our statistical analyses of the timing and derived parameters indicated some level of differences and similarities among the parameters analyzed. Glitching pulsars appear to have a higher rotational frequency than non-glitching pulsars, and pulsars with (n>0) appear to rotate faster than those with (n<0). Our results also suggest that glitching pulsars have lower values of (left vert n right vert ) (where (left vert n right vert ) is the absolute value of the braking index), and it is lower for the subsample with (n>0) than for the subsample with (n<0). We believe that the results obtained could be useful in understanding the evolution of pulsar spin.

In the current study, we have considered three different parameterizations of deceleration parameter to describe the cosmological dynamics of the accelerating universe in (f(Q)) gravity. The power law symmetric teleparallel gravity with a specific form (f(Q)= Q + n Q^{m}) is assumed for the modelling purpose. Here, (m) and (n) are constants and (Q) is the non-metricity term that describes the gravitational interaction in space time. We constructed the field equations depending on the power law (f(Q)) gravity and parameters are extracted using experimental observations. Latest observational datasets of BAO, (H(z)) and Pantheon are utilized to predict the best fit values of parameters and current value of Hubble constant. The Markov Chain Monte Carlo (MCMC) algorithm has been used to decide the best plausible values of parameters. We numerically represent the physical and geometrical features of the models and thoroughly explore their development. We analyzed our models using the jerk and Om diagnosis that depict the derived cosmic models are different from the (Lambda )CDM model expressing late time accelerated expansion of cosmos with phantom type of the universe. We also discussed the viability of models by the analysis of energy conditions.

We present an alternative model of unusual type-IIP SN 2018gj. Despite the short plateau and early gamma-ray escape seeming to favor low-mass ejecta, our hydrodynamic model requires a large ejected mass (≈23 (M_{odot })). The high ejecta velocity, we find from hydrogen lines in early spectra, is among the crucial constraints on the hydrodynamic model. We recover the wind density that rules out a notable contribution of the circumstellar interaction to the bolometric luminosity. The early radioactive gamma-ray escape is found to be due to the high velocity of ⁵⁶Ni, whereas the asymmetry of the H(alpha ) emission is attributed to the asymmetry of the ⁵⁶Ni ejecta. The available sample of type-IIP supernovae studied hydrodynamically in a uniform way indicates that the asymmetry of the ⁵⁶Ni ejecta is probably their intrinsic property. Hydrogen lines in the early spectra of SN 2018gi and SN 2020jfo are found to imply a clumpy structure of the outer ejecta. With two already known similar cases of SN 2008in and SN 2012A we speculate that the clumpiness of the outer ejecta is inherent to type-IIP supernovae related to the red supergiant explosion.

The research on solar flare predicting holds significant practical and scientific value for safeguarding human activities. Current solar flare prediction models have not fully considered important factors such as time step length, nor have they conducted a comparative analysis of the physical features in multiple models or explored the consistency in the importance of features. In this work, based on SHARP data from SDO, we build 9 machine learning-based solar flare prediction models for binary “Yes” or “No” class prediction within the next 24 hours, and study the impact of different time steps and other factors on the forecasting performance. The main results are as follows. (1) The predictive performance of eight deep learning models shows an increasing trend as the time step length increases, and the models perform the best at the length of 40. (2) In predicting solar flares of ≥C class and ≥M class, the True Skill Statistic(TSS) of deep learning models consistently outperforms that of baseline model. For the same model, the TSS for predicting ≥M class flares generally exceeds that for predicting ≥C class flares. (3) The Brier Skill Score (BSS) of deep learning models significantly surpasses that of baseline model in predicting ≥C class flares. However, the BSS scores of the nine models are comparable for predicting ≥M class flares. For the same model, the BSS for predicting ≥C class flares is generally higher than that for predicting ≥M class flares. (4) Through feature importance analysis of multiple models, the common features that consistently rank at the top and bottom are identified.

This study reveals a new feature of many solar jets: a group height, which links their acceleration and velocity.

The acceleration and velocity ((a), (V)) for jets such as spicules, often displayed as scattergraphs, show a strong correlation. This can be represented empirically by the equation, (V = pa + q), where (p) and (q ) are two arbitrary non-zero constants.

This study reanalyses the ((a), (V)) data for nine different groups of jets, in order to test an alternative proposal that a simpler relationship directly links ((a), (V)) to the mean height for the group of jets, without needing the empirical constants (p ) and (q). A standard mathematical test – plotting log((a)) against log((V)), tests whether (V sim a^{n}) and if so, gives the value of n. When this is done for a wide range of jets the index (n) is consistently found to be close to 0.5

The nine groups of jets include spicules, macrospicules and dynamic fibrils. The result, (V sim a)^0.5, or equivalently (V^{2} = ka), with only one constant, provides as close a match to the data as the equation (V = pa + q), which requires two unknown constants. It is found that the constant (k), is a known quantity: just twice the mean height, (overline{s}), of the group of jets being analysed. This then gives the equation (V^{2} =2 a overline{s}), for the jets in the group. This more succinct relationship links the acceleration and maximum velocity of every jet in the group to a well-defined quantity – the mean height of the group of spicules, without needing extra constants

This paper presented an analysis of geomagnetic disturbance observed on the ground during geomagnetic storms with different intensities in 2015 using the meridian chain data at geomagnetic mid and low latitudes. Ground observation records superimpose varying types of space-current system and noise interference. Geomagnetic disturbance with variation of discontinuity and irregularities are difficult to identify and distinguish. We proposed a variational mode decomposition (VMD) algorithm for reconstructing geomagnetic horizontal ((H)) disturbance signals. We decomposed the geomagnetic signals into geomagnetic disturbance signals, diurnal variation signals, and noise disturbance signals using the VMD algorithm. Intrinsic mode functions (IMFs) were selected to form the reconstructed signal, which represented a geomagnetic disturbance during a geomagnetic storm. We investigated the decreased amplitude of (H) component obtained from the reconstructed signals during main phase of geomagnetic storms with different geomagnetic storms intensities and seasons at mid and low latitudes. The maximum values of gradient variation of (H ) component disturbance with geomagnetic latitude cosine are near magnetic latitude 30°N during geomagnetic storms with different intensities and seasons. Ionopheric structural changes in the low-to-mid latitude transition zone maybe the primary cause. The result provides a reference for the complex coupling relationship between the ionosphere and magnetosphere during geomagnetic storms.