Astrophysics and Space Science最新文献

This work focuses on developing an analytical method for constructing images in gravitational lenses. Building on our previously proposed elimination method, which utilized algebraic geometry for constructing images from circular sources, this study emphasizes images from elliptical sources using axially symmetric gravitational lenses. We detail the rational parameterization of an arbitrarily located source and use our algorithm to derive an expression that determines all images for an arbitrary axially symmetric gravitational lens. To validate this expression, we considered several limiting cases leading to well-known results. Specifically, we examined two simple models: a single-point lens and a homogeneous disk lens. By placing an elliptical source at the origin of the source plane and reducing it to a circle, we reproduced all previously known images, such as the Einstein ring for a single-point lens, and a double Einstein ring for a disk lens. Additionally, we demonstrated the Construction of images in other arbitrary lens-source configurations.

The formation of the Magellanic Stream has puzzled astronomers for decades. In this review, we outline the history of our understanding of the Magellanic System highlighting key observations that have revolutionized thinking of its evolution. We also walk through the major models and theoretical advances that have led to our current paradigm − (1) the LMC and SMC have just had their first pericentric passage around the Milky Way, having approached recently as a bound pair; (2) the LMC and SMC have had several tidal interactions in which material has been stripped out into the Trailing Stream and Leading Arm; (3) the LMC hosted an ionized gas circumgalactic medium which envelops the Clouds and the neutral Stream today, providing the bulk of the associated mass; and (4) the Milky Way’s circumgalactic gas provides strong ram pressure and hydrodynamic forces to shape the morphology of the Magellanic System including the formation of a bow shock due to the LMC’s supersonic approach.

Pi2 magnetic pulsations have been observed to occur at low latitudes when the magnetosphere is in a near ground state and substorm activity would be expected to have ceased. In the present study, we report a global non-substorm Pi2 magnetic micro-pulsations occurred, for the first time, during a solar flare event. We investigate this Pi2 at two different locations: in the inner magnetosphere using Van Allen Probe A (VAP-A) satellite, and ground-based magnetic stations located in Japan-Korea, Africa, and America sectors. By comparing the horizontal component of ground stations with the electric and magnetic components within the magnetosphere, we report a global Pi2 starting nearby at 12:05 UT lasting to 12:11 UT during the ejection of a solar flare with X9.3-class that have had its peak flux at 12:02 UT on 6 September 2017. We study the Pi2 oscillations at Kakioka (KAK) station and VAP-A satellite through wavelet analysis. By analyzing the high-latitude stations, we find that the Pi2 event has a good correlation with a Poleward Boundary Intensification (PBI).

We provide highlights of research investigations over the last five decades in the area of resolved stellar populations. After discussing the early work on the state of blue straggler stars in Galactic globular clusters, we broaden our gaze to include all of the stellar systems in the Local Volume with the overall goal of understanding the formation and evolution of galaxies. More specifically, we focus on the stellar constituents (e.g. star clusters and RR Lyrae variables) of the Milky Way, the Magellanic Clouds, the Andromeda galaxy (M31), the Triangulum galaxy (M33), as well as several dwarf galaxies in the Local Group and two dwarfs in the Sculptor group. Because of the available instrumental capabilities, work on resolved stars in galaxies has been largely focused on stellar systems in the Local Group and nearby galaxy groups. The big picture of galaxy formation and evolution that emerges from an analysis of all these constituents is largely consistent with the dwarf galaxy fragmentation and accretion scenario first put forth by Leonard Searle and Robert Zinn in 1978.

In this study, we develop five deep learning models, a Convolutional Neural Network (CNN) model, a CNN model with Squeeze-and-Excitation Attention(CNN-SE), a CNN model with Convolutional Block Attention Module (CNN-CBAM), a CNN model with Efficient Channel Attention (CNN-ECA), and a Vision Transformer (ViT) model, for predicting whether ≥C or ≥M-class solar flares occurring within 24 hours. We build a real-time forecasting system using these five models, which can achieve classification and probability forecasting. The 10-fold cross-validation sets are generated in chronological order using the full-disk magnetograms provided by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager at 00:00 UT from May 1, 2010, to March 31, 2023. Then after training, validation, and testing our models, we compare the results with the true skill statistic (TSS) and Brier Skill Score (BSS) as assessment metrics. The major results are as follows: (1) There are no statistically significant differences in TSS and BSS performance between models with attention mechanisms and the CNN model. (2) For ≥C-class flare prediction, the Recall of the ViT model reaches 0.833, significantly better than that of the CNN model. For ≥M-class flare prediction, the Recall of the CNN-ECA and ViT models are 0.799 and 0.855, respectively, which are significantly higher than those of the CNN model. (3) We develop a full-disk solar flare prediction system that has been running since May 1, 2023. By December 31, all five models achieve a TSS of 0.984 for predicting ≥C-class flares, with the CNN-SE model demonstrating a BSS of 0.939. For ≥M-class flares, the CNN-SE model achieves a TSS of 0.304, while the BSS values for the CNN and CNN-SE models are 0.019 and 0.018, respectively. Additionally, the prediction performance for ≥M-class flares on the testing set without No-flare class samples, is similar to that of real-time predictions, validating the good generation performance of the model in real-time forecasting.

This paper traces the 37 years of my career dedicated to the development of integral field spectroscopy (IFS), highlighting significant milestones and advancements. This extensive journey encompasses three generations of IFS: the initial prototype TIGER at CFHT, the first generation including OASIS at CFHT and SAURON at WHT, the second generation with MUSE at VLT, and the potential third generation represented by the Wide-field Spectroscopic Telescope (WST) project. Throughout, I discuss the lessons learned at each stage and provide my perspective on the future of IFS.

Solar images are critically important for studying solar activities and features. Today, many observatories rely on CMOS sensors to acquire these images. However, these sensors often introduce column fixed-pattern noise (CFPN), seriously affecting image quality. Therefore, we proposed a two-way filtering algorithm to remove CFPN. Firstly, in the horizontal direction, we used the one-dimensional global weighted least squares filter and the efficient bilateral filter to obtain a coarse denoised image. Then, we utilized the weighted guided filter in the vertical direction to estimate the CFPN components, thereby obtaining a clean solar image. We selected three different solar observation images to compare and evaluate our results to those obtained by three comparative methods. The images are observed by the Solar Upper Transition Region Imager aboard the SATech-01 satellite. Additionally, we further used two quantitative metrics, photo response non-uniformity and mean relative deviation, to quantify the denoised results. The results demonstrate that our proposed method removes the CFPN better and preserves the image features in a more balanced way.

A total solar eclipse occurred on April 8, 2024, across the United States, with obscuration rates ranging from 20–100% at different locations. We study the ionospheric Total Electron Content (TEC) response to the eclipse using data from 51 ground-based GPS stations. We find a significant depletion in TEC of up to 65%, with larger depletion rates occurring along the path of totality. We also observe a time delay between the maximum TEC depletion and the time of maximum obscuration, ranging from 8 to 80 minutes, with longer delays occurring at stations with lower obscuration rates. Both time delay and TEC depletion rates are found to vary with latitude, with slightly longer delays at lower latitudes and higher TEC depletion rates at mid-latitudes. The TEC depletion rate also increased with local time, showing a higher decrement after noon compared to before. The spatiotemporal variation in TEC closely followed the eclipse’s path, indicating a positive correlation with the movement of the moon’s shadow. Monitoring rate of TEC change during such rare events is crucial for enhancing our understanding of ionospheric dynamics, which can be very helpful to improve the accuracy of global communication and navigation systems.

The unified model of jetted active galactic nuclei (AGNs) based on intrinsic properties presumes that the blazar subtypes of BL Lacertae objects (BLs) and flat-spectrum radio quasars (FSRQs) are aligned equivalents of Fanaroff–Riley type I (FR I) and Fanaroff–Riley type II (FR II) radio galaxies (RGs) from where their parent sources are obtained. Using compiled radio, X-ray and (gamma )-ray data of 397 Fermi Large Area Telescope (Fermi-LAT) blazars and 153 non-Fermi detected RGs, we computed the (gamma )-ray core-dominance parameter (R_γ) and (gamma )-ray emission components (beamed and unbeamed) to study the consequences of relativistic beaming and orientation effects on (gamma )-ray properties of radio RGs, BLs and FSRQs. Data analysis shows that the distributions of R_X and R_γ, on average, are consistent with the prediction of the unified scheme in which FSRQs and BLs are strongly beamed and inclined at smaller angles than RGs. Through statistical tests with chance probability, (p < 0.05), we find that the distributions of R_X and R_γ for radio galaxies and the blazar subtypes are not significantly different. There is a regular positive trend ((r > +0.60)) in the variation of radio core-dominance parameter – X-ray luminosity ((R - L_{mathrm{x}})) data from radio galaxies to FSRQs through BLs, which suggests that radio galaxies could be relativistically less beamed population of jetted AGNs. The results are consistent with the unified model for radio galaxies and the conventional blazar subtypes.

Stellar halos of galaxies retain crucial clues to their mass assembly history. It is in these galactic components that the remains of cannibalised galactic building blocks are deposited. For the case of the Milky Way, the opportunity to analyse the stellar halo’s structure on a star-by-star basis in a multi-faceted approach provides a basis from which to infer its past and assembly history in unrivalled detail. Moreover, the insights that can be gained about the formation of the Galaxy not only help constrain the evolution of our Milky Way, but may also help place constraints on the formation of other disc galaxies in the Universe. This paper includes a summary of work undertaken during a PhD thesis aiming to make progress toward answering the most fundamental question in the field of Galactic archaeology: “How did the Milky Way form?” Through the effort to answer this question, we summarise new insights into aspects of the history of assembly and evolution of our Galaxy and measurements of the structure of various of its Galactic components.