Astrophysics and Space Science最新文献

The dynamics of Langmuir modes, waves (LW), and shear Langmuir vortices (SLV) are studied in kinematically complex astrophysical plasma flows. It is found that they exhibit several peculiar, velocity shear-induced, asymptotically persistent phenomena, including efficient energy exchange with the background flow and various kinds of instabilities, leading to their exponential growth and echoing solutions with persistent wave-vortex-wave conversions. There is a remarkable similarity between these phenomena and those happening with compressible acoustic modes. The relevance and possible importance of these phenomena for different types of astrophysical plasma flow patterns with kinematic complexity are discussed. In particular, we argue that these physical processes may account for the persistent appearance of plasma oscillations in the heliosphere and interstellar plasma flows. In particular, we believe that the kinematically complex motion of plasma may naturally lead to the asymptotically persistent appearance of Langmuir modes that are born, grown, fed, sustained and maintained by these flows.

Efficient forecasting of solar flares is of significant importance for better risk prevention. Currently, there is relatively rare research on multi/four-classification of flares, and the influence of the number of time steps and data feature dimensions on the prediction performance of multi-class models has not been considered. In this study, we utilize the Space-weather HMI Active Region Patch (SHARP) data to develop two categories of models for multiclass flare prediction within 24 hr, including direct output four-classification models and four-classification models using a cascading scheme. The former encompasses Random Forest (RF) model, Long Short-Term Memory (LSTM) model, and Bidirectional LSTM (BLSTM) model, while the latter includes BLSTM Cascade (BLSTM-C) model and BLSTM Cascade with Attention Mechanism (BLSTM-C-A) model. These two categories of models are employed to contrast the impact of different numbers of time steps and the predictive performance in solar flare multi/four-classification. Additionally, we conduct, for the first time, feature importance analysis for multi/four-classification solar flare prediction using deep learning models. The main results are as follows: (1) As the number of time steps increases, the True Skill Statistic (TSS) scores of the four deep learning models improve, showing an overall upward trend in predictive performance. The models achieve their optimal performance when the number of time steps reaches 120. (2) Among the direct output four-class models, deep learning models (LSTM and BLSTM) outperform traditional machine learning model (RF). In both multi-class and binary-class predictions using deep learning, the BLSTM-C model performs better than other deep learning models (LSTM, BLSTM, and BLSTM-C-A). (3) In the feature importance analysis, the top-ranked important features include SAVNCPP and R_VALUE, while the least important features include SHRGT45 and MEANPOT.

In this paper, we report the detailed observation of the drift subpulse behavior of PSR J1514–4834 at a central frequency of 1369 MHz using the Parkes 64-m radio telescope. We have found that individual pulses of this pulsar exhibit distinct modulation behaviors for different profile components. The leading and middle components display periodic amplitude modulation with a period of (mathrm{P}_{3}=37.5pm 0.8, mathrm{P}), and a drifting sub-pulse phenomenon is detected in the phase region of trailing component with the measured drifting periods (mathrm{P}_{2}=7.0pm 0.4,mathrm{P}) and (mathrm{P}_{3}=37.5pm 0.8, mathrm{P}). Additionally, it was observed that the leading and trailing components of the pulsar have a clear correlation, the middle and trailing components have a clear anti-correlation, and there is no apparent correlation between the leading and middle components. Moreover, this pulsar deviates from the range of most amplitude-modulated pulsars in the (dot{mathrm{E}}-mathrm{P}_{3}) diagram, but it still falls within the category of subpulse drifting. PSR J1514–4834 exhibits periodic emission modulation and sub-pulse drifting simultaneously in different profile components, which is difficult to understand with the traditional carousel model. Our observational results will provide new observation evidence for theoretical studies of single-pulse emission mechanisms in pulsars.

Phenomenological relations linking thermodynamics and kinetic theory in condensed matter have been empirically verified in numerous systems, yet their theoretical derivation from first principles remains an open question. One such relation is the so-called “excess-entropy scaling”. Do N-body gravitational systems exhibit a similar relation? We provide an affirmative response to this question, albeit with some limitations. Our analysis relies on a well-established thermodynamical quasi-equilibrium model for the cosmological N-body problem, along with an appropriate diffusion model for gravitational interactions. By identifying a scaling region, we were able to estimate diffusion coefficients directly from observational two-particle correlation functions or counts-in-cells distributions in large-scale structures. Intriguingly, the phenomenon of excess-entropy scaling manifests primarily during the nonlinear, asymptotic clustering phase preceding a potential thermodynamic phase transition.

The hyperfine transition of atomic hydrogen at a wavelength of about 21 cm is an essential tool for studies of interstellar gas. It has been argued that also fine-structure transitions of hydrogen could be detected in astronomical sources. Our aim is to detect the fine-structure transition (2^{2}P_{3/2}-2^{2}S_{1/2}) of hydrogen at ∼10 GHz in the radio spectrum of the Sun, with a spectral resolution which enables a detailed study of the line profile. The Sun was observed with the 13.7 m radio telescope at the Metsähovi Radio Observatory, in Finland. We detect emission from two of the three hyperfine components of the transition. The width of the components is ∼15 MHz, much less than the expected natural line width of ∼100 MHz (broadened solely by the uncertainty principle). At red-shifted Doppler velocities, the lines show enhanced emission and possibly self-absorption. If the absorption happens at the chromosphere, our observations challenge the traditional view that chromospheric temperature increases gradually towards higher altitudes. Our unconventional results have to be confirmed by further observations. This transition would be the only known spectral line in the Sun at radio frequencies.

We report 11 new radio continuum measurements of established planetary nebulae (PNe) in the Small Magellanic Cloud (SMC) that we observed at 5.5 and 9 GHz with the Australia Telescope Compact Array (ATCA). These new radio detections are PNe with catalogued names: SMP SMC 2, SMP SMC 3, SMP SMC 5, SMP SMC 8, SMP SMC 13, SMP SMC 14, SMP SMC 19, MGPN SMC 8, SMP SMC 22, SMP SMC 26 and SMP SMC 27. We supplement our data with available high-resolution radio observations from MeerKAT and construct the spectral energy distribution (SED) in the radio regime for each PN. We determine the angular diameters of four of the eleven PNe from radio flux density alone using SED modelling, which are compared to the corresponding Hubble Space Telescope (HST) optical diameters. Our results are in good agreement with the optically-derived angular diameters from independent HST observations. We plot our new diameter estimates against a larger sample of Galactic PNe and compare diameters obtained via the SED method to those found in the literature. Our sample diameters, when compared to the Galactic PNe, suggest that the angular diameter measurement methods are comparable independent of the distance.

Astronomical observatories require sites with high altitudes, a high number of clear nights, and minimal light pollution. This study utilizes Geographic Information Systems and Multi-Criteria Decision Analysis to evaluate the suitability of Balkan regions for establishing International Dark Sky Parks (IDSP) based on the criteria set by the International Dark Sky Association. Three scenarios (DSPI A, B and C) were formulated to assess suitability under different conditions using satellite data on light pollution, cloud cover, elevation and water bodies. Although no ‘Conservation Area’ or ‘International Dark Sky Park’ sites were found due to the prevalence of light pollution, promising ‘reserve areas’ and astronomical observatory sites were identified, mainly concentrated in the southern Balkans inside the Montenegro-Bulgaria-Greece triangle. The southern part of Macedonia has twice as many clear nights (an average of approximately 240 nights) compared to the north. The southern region of Macedonia exhibited a range of brightness levels, while the Rozhen National Astronomical Observatory in Bulgaria had the darkest recorded sky brightness (20.89 (mathrm{mag}_{textrm{SQM}}) arcsec⁻²) and the highest suitability score (0.69). The Peloponnese offers suitable locations for astronomical sites in all scenarios. Higher altitudes and lower latitudes have more favorable conditions. The Balkans contain a significant proportion of reserve areas (24.8% of the region), with Bulgaria having the largest share, despite the lack of ideal astronomical sites. It is important to note that long-term in-situ observations should be carried out after the site selection process has been completed.

Morphological evolution of expanding shells of fast-mode magnetohydrodynamic (MHD) waves through an inhomogeneous ISM is investigated in order to qualitatively understand the complicated morphology of shell-type supernova remnants (SNR). Interstellar clouds with high Alfvén velocity act as concave lenses to diverge the MHD waves, while those with slow Alfvén velocity act as convex lenses to converge the waves to the focal points. By combination of various types of clouds and fluctuations with different Alfvén velocities, sizes, or wavelengths, the MHD-wave shells attain various morphological structures, exhibiting filaments, arcs, loops, holes, and focal strings, mimicking old and deformed SNRs.