分类： 信息科学与系统科学 >> 信息科学与系统科学基础学科 分类： 物理学 >> 电磁学、光学、声学、传热、经典力学和流体动力学 分类： 天文学 >> 天文学 提交时间： 2024-04-11
摘要： The electromagnetic fields of point sources with time varying charges moving in the vacuum are derived using the Liénard-Wiechert potentials. The properties of the propagation velocities and the Doppler effect are discussed based on their far fields. The results show that the velocity of the electromagnetic waves and the velocity of the sources cannot be added like vectors; the velocity of electromagnetic waves of moving sources are anisotropic in the vacuum; the transverse Doppler shift is intrinsically included in the fields of the moving sources and is not a pure relativity effect caused by time dilation. Since the fields are rigorous solutions of the Maxwell’s equations, the findings can help us to abort the long-standing misinterpretations concerning about the classic mechanics and the classic electromagnetic theory. Although it may violate the theory of the special relativity, we show mathematically that, when the sources move faster than the light in the vacuum, the electromagnetic barriers and the electromagnetic shock waves can be clearly predicted using the exact solutions. Since they cannot be detected by observers in the region outside their shock wave zones, an intuitive and reasonable hypothesis can be made that the superluminal sources may be considered as a kind of electromagnetic blackholes.
分类： 天文学 >> 天文学 提交时间： 2024-03-30
Niankun Yu
Zheng Zheng
Chao-Wei Tsai
Pei Zuo
Sara L. Ellison
David V. Stark
Di Li
Jingwen Wu
Karen L. Masters
Ting Xiao
Yinghui Zheng
Zongnan Li
Kai Zhang
Hongying Chen
Shu Liu
Sihan Jiao
Fanyi Meng
摘要： The atomic-to-molecular gas conversion is a critical step in the baryon cycle of galaxies, which sets the initial conditions for subsequent star formation and influences the multi-phase interstellar medium. We compiled a sample of 94 nearby galaxies with observations of multi-phase gas contents by utilizing public H I, CO, and optical IFU data from the MaNGA survey together with new FAST H I observations. In agreement with previous results, our sample shows that the global molecular-to-atomic gas ratio ($R_{\rm mol} \equiv$ log $M_{\rm H_2}/M_{\rm H\ I}$) is correlated with the global stellar mass surface density $\mu_*$ with a Kendall's $\tau$ coefficient of 0.25 and $p < 10^{-3}$, less tightly but still correlated with stellar mass and NUV$-$ r color, and not related to the specific star formation rate (sSFR). The cold gas distribution and kinematics inferred from the H I and CO global profile asymmetry and shape do not significantly rely on $R_{\rm mol}$. Thanks to the availability of kpc-scale observations of MaNGA, we decompose galaxies into H II, composite, and AGN-dominated regions by using the BPT diagrams. With increasing $R_{\rm mol}$, the fraction of H II regions within 1.5 effective radius decreases slightly; the density distribution in the spatially resolved BPT diagram also changes significantly, suggesting changes in metallicity and ionization states. Galaxies with high $R_{\rm mol}$ tend to have high oxygen abundance, both at one effective radius with a Kendall's $\tau$ coefficient of 0.37 ($p < 10^{-3}$) and their central regions. Among all parameters investigated here, the oxygen abundance at one effective radius has the strongest relation with global $R_{\rm mol}$, but the dependence of gas conversion on gas distribution and galaxy ionization states is weak.
分类： 天文学 >> 天文学 提交时间： 2024-03-29
摘要： 针对长期有争议的万有引力问题,论述了空间物体从小、大至星体的发展是以物体的转动惯量(I)为特征的自然进化过程.用时间、路径、质量三个因素说明I这一特征:时间说明I物体从小至今的持续时长;路径说明I是一种自然匀速行进的惯性物体,但在行进中其惯性作用在空间产生了以空间介质为媒介的惯介力矩,该力矩通过空间介质与另一物体产生关联逐渐形成了椭圆轨道;质量说明了质量的变化情况,在I与其他物体关联后会渐渐地归并另外物体,使其质量增加，则I相应增强. 给出了两个在空间相关星体的惯量平衡方程,由方程解出两物体的空间平衡点,通过地球、月球系统的真实数据和平衡点数据获得双方对应的运行轨迹、用以测定两者的紧密程度的刚性惯力与惯介力的比值、和两个星体的加速度原理及各自加速度计算值,经比对证明惯动平衡方程的正确性.这一成果对进一步理解太阳系的惯性特征将会有很大的促进作用.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： To solve the problem of time-consuming measurement and correction of large antennas' reflector deformation, a new microwave holography methodology based on a Phased Array Feed (PAF) is proposed. Starting from the known expression of receiving signals in microwave holography, the theory of PAF holography is derived through Geometrical Optics. Reflector deformation, as well as pointing deviation and subreflector offset, can be calculated out by applying the derived equations. A measurement and correction system based on PAF holography is depicted, and two kinds of measurement methods are illustrated. The proposed measurement methodology is verified by numerical simulation, and its measurement error is analyzed. The results indicate that our proposed methodology is feasible, especially for Cassegrain antennas.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： We present an independent catalog (FRIIRGcat) of 45,241 Fanaroff–Riley Type II (FR-II) radio galaxies compiled from the Very Large Array Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey and employed the deep learning method. Among them, optical and/or infrared counterparts are identified for 41,425 FR-IIs. This catalog spans luminosities 2.63 × 1022 ≤ Lrad ≤ 6.76 × 1029 W Hz−1 and redshifts up to z = 5.01. The spectroscopic classification indicates that there are 1431 low-excitation radio galaxies and 260 high-excitation radio galaxies. Among the spectroscopically identified sources, black hole masses are estimated for 4837 FR-IIs, which are in 107.5 ≲ MBH ≲ 109.5M⊙. Interestingly, this catalog reveals a couple of giant radio galaxies (GRGs), which are already in the existing GRG catalog, confirming the efficiency of this FR-II catalog. Furthermore, 284 new GRGs are unveiled in this new FR-II sample; they have the largest projected sizes ranging from 701 to 1209 kpc and are located at redshifts 0.31 < z < 2.42. Finally, we explore the distribution of the jet position angle and it shows that the faint Images of the FIRST images are significantly affected by the systematic effect (the observing beams). The method presented in this work is expected to be applicable to the radio sky surveys that are currently being conducted because they have finely refined telescope arrays. On the other hand, we are expecting that further new methods will be dedicated to solving this problem.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Pulsar search is always the basis of pulsar navigation, gravitational wave detection and other research topics. Currently, the volume of pulsar candidates collected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) shows an explosive growth rate that has brought challenges for its pulsar candidate filtering system. Particularly, the multi-view heterogeneous data and class imbalance between true pulsars and non-pulsar candidates have negative effects on traditional single-modal supervised classification methods. In this study, a multi-modal and semi-supervised learning based on a pulsar candidate sifting algorithm is presented, which adopts a hybrid ensemble clustering scheme of density-based and partition-based methods combined with a feature-level fusion strategy for input data and a data partition strategy for parallelization. Experiments on both High Time Resolution Universe Survey II (HTRU2) and actual FAST observation data demonstrate that the proposed algorithm could excellently identify pulsars: On HTRU2, the precision and recall rates of its parallel mode reach 0.981 and 0.988 respectively. On FAST data, those of its parallel mode reach 0.891 and 0.961, meanwhile, the running time also significantly decreases with the increment of parallel nodes within limits. Thus, we can conclude that our algorithm could be a feasible idea for large scale pulsar candidate sifting for FAST drift scan observation.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Radiation pattern captures the electromagnetic performance of reflector antennas, which is significantly affected by the deformation of the primary reflector due to gravity and the displacement of the secondary reflector. During the design process of large reflector antennas, a substantial amount of time is often dedicated to iteratively adjusting structural parameters and validating electromagnetic performance. To improve the efficiency of the design process, we first propose an approximate calculation method of optical path difference (OPD) for the deformation of the primary reflector under gravity and the displacement of the secondary reflector. Then an OPD fitting function based on the modified Zernike polynomials is proposed to capture the phase difference of radiation over the aperture plane, based on which the radiation pattern will be obtained quickly by the aperture field integration method. Numerical experiments demonstrate the effectiveness of the proposed quick calculation method for analyzing the radiation pattern of a 10.4 m submillimeter telescope antenna at its highest operating frequency of 856 GHz. In comparison with the numerical simulation method based on GRASP (which is an antenna electromagnetic analysis tool combining physical optics (PO) and physical theory of diffraction (PTD)), the quick calculation method reduces the time for radiation pattern analysis from more than one hour to less than two minutes. Furthermore, the quick calculation method exhibits excellent accuracy for the figure of merit (FOM) of the radiation pattern. Therefore, the proposed quick calculation method can obtain the radiation pattern with high speed and accuracy. Compared to the time-consuming numerical simulation method (PO and PTD), it can be employed for quick analysis of the radiation pattern for the lateral displacement of the secondary reflector and the deformation of the primary reflector under gravity in the design process of a reflector antenna.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： In today's society, there is a wide demand for high-precision and high-stability time service in the fields of electric power, communication, transportation and finance. At present, the time standard in various countries is mainly based on atomic clocks, but the frequency drift of atomic clocks will affect the long-term stability performance. Compared with atomic clocks, millisecond pulsars have better long-term stability and can complement with the excellent short-term stability of atomic clocks. In order to improve the long-term stability of the atomic timescale, and then improve the timing accuracy, this paper proposes an algorithm for steering the atomic clock ensemble (ACE) by ensemble pulsar time (EPT) based on digital phase locked loop (DPLL). First, the ACE and EPT are generated by the ALGOS algorithm, then the ACE is steered by EPT based on DPLL to calibrate the long-term frequency drift of the atomic clock, so that the generated steered atomic time follows both the short-term stability characteristics of ACE and the long-term stability characteristics of EPT, and finally, the steered atomic time is used to calibrate the local cesium clock. The experimental results show that the long-term stability of atomic time after steering is improved by 2 orders of magnitude compared with that before steering, and the daily drift of a local cesium clock after calibration is less than 9.47 ns in 3 yr, 3 orders of magnitude higher than that before calibration on accuracy.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： The 2.5 m wide-field and high-resolution solar telescope (WeHoST) is currently under developing for solar observations. WeHoST aims to achieve high-resolution observations over a super-wide field of view (FOV) of 5' × 5', and a desired resolution of 0.3''. To meet the scientific requirements of WeHoST, the ground-layer adaptive optics (GLAO) with a specially designed wave front sensing system is as the primary consideration. We introduce the GLAO configuration, particularly the wave front sensing scheme. Utilizing analytic method, we simulate the performance of both classical AO and GLAO systems, optimize the wave front sensing system, and evaluate GLAO performance in terms of PSF uniformity and correction improvement across whole FOV. The results indicate that, the classical AO will achieve diffraction-limited resolution; the suggested GLAO configuration will uniformly improve the seeing across the full 5' × 5' FOV, reducing the FWHM across the axis FOV to less than 0.3'' (λ ≥ 705 nm, r0 ≥ 11 cm), which is more than two times improvement. The specially designed wave front sensor schedule offers new potential for WeHoST's GLAO, particularly the multi-FOV GLAO and the flexibility to select the detected area. These capabilities will significantly enhance the scientific output of the telescope.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： The development of spectroscopic survey telescopes like Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), Apache Point Observatory Galactic Evolution Experiment and Sloan Digital Sky Survey has opened up unprecedented opportunities for stellar classification. Specific types of stars, such as early-type emission-line stars and those with stellar winds, can be distinguished by the profiles of their spectral lines. In this paper, we introduce a method based on derivative spectroscopy (DS) designed to detect signals within complex backgrounds and provide a preliminary estimation of curve profiles. This method exhibits a unique advantage in identifying weak signals and unusual spectral line profiles when compared to other popular line detection methods. We validated our approach using synthesis spectra, demonstrating that DS can detect emission signals three times fainter than Gaussian fitting. Furthermore, we applied our method to 579,680 co-added spectra from LAMOST Medium-Resolution Spectroscopic Survey, identifying 16,629 spectra with emission peaks around the Hα line from 10,963 stars. These spectra were classified into three distinct morphological groups, resulting in nine subclasses as follows. (1) Emission peak above the pseudo-continuum line (single peak, double peaks, emission peak situated within an absorption line, P Cygni profile, Inverse P Cygni profile); (2) Emission peak below the pseudo-continuum line (sharp emission peak, double absorption peaks, emission peak shifted to one side of the absorption line); (3) Emission peak between the pseudo-continuum line.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Solar activities have a great impact on modern high-tech systems, such as human aerospace activities, satellite communication and navigation, deep space exploration, and related scientific research. Therefore, studying the long-term evolution trend of solar activity and accurately predicting the future solar cycles are highly anticipated. Based on the wavelet transform and empirical function fitting of the longest recorded data of the annual average relative sunspot number (ASN) series of 323 yr to date, this work decisively verifies the existence of the solar century cycles and confirms that its length is about 104.0 yr, and the magnitude has a slightly increasing trend on the timescale of several hundred years. Based on this long-term evolutionary trend, we predict solar cycles 25 and 26 by using phase similar prediction methods. As for solar cycle 25, its maximum ASN will be about 146.7 ± 33.40, obviously stronger than solar cycle 24. The peak year will occur approximately in 2024, and the period will be about 11 ± 1 yr. As for solar cycle 26, it will start around 2030, and reach its maximum between 2035 and 2036, with maximum ASN of about 133.0 ± 3.200, and period of about 10 yr.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Galaxy morphology classifications based on machine learning are a typical technique to handle enormous amounts of astronomical observation data, but the key challenge is how to provide enough training data for the machine learning models. Therefore this article proposes an image data augmentation method that combines few-shot learning and generative adversarial networks. The Galaxy10 DECaLs data set is selected for the experiments with consistency, variance, and augmentation effects being evaluated. Three popular networks, including AlexNet, VGG, and ResNet, are used as examples to study the effectiveness of different augmentation methods on galaxy morphology classifications. Experiment results show that the proposed method can generate galaxy images and can be used for expanding the classification model's training set. According to comparative studies, the best enhancement effect on model performance is obtained by generating a data set that is 0.5–1 time larger than the original data set. Meanwhile, different augmentation strategies have considerably varied effects on different types of galaxies. FSL-GAN achieved the best classification performance on the ResNet network for In-between Round Smooth Galaxies and Unbarred Loose Spiral Galaxies, with F1 Scores of 89.54% and 63.18%, respectively. Experimental comparison reveals that various data augmentation techniques have varied effects on different categories of galaxy morphology and machine learning models. Finally, the best augmentation strategies for each galaxy category are suggested.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Twenty-five typical massive white dwarfs (WDs) are selected and the proton decay reaction catalyzed by magnetic monopoles (MMs) for these WDs is discussed. A velocity-dependent correction factor strongly affects the cross-section. We find that a strong suppression controls the monopole catalysis of nucleon decay by the correction factor. The maximum number of MMs is captured and the luminosity can be 2.235 × 1021 and 1.7859 × 1032erg s−1 (e.g., for the O+Ne core mass WD J055631.17+130639.78). The luminosities of most massive WDs agree well with the observations at relatively low temperatures (e.g., T6 = 0.1), but can be three and two orders of magnitude higher than those of the observations for model (I) and (II) at relatively high temperatures (e.g., T6 = 10), respectively. The luminosities of model (I) are about one order of magnitude higher than those of model (II). Since we consider the effect of the number of MMs captured on the mass–radius relation and the suppression of the proton decay by the correction factor, the study by model (II) may be an improved estimation.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： In gamma-ray burst prompt emission, there is still no consistent conclusion if the precursor and main burst share the same origin. In this paper, we try to study this issue based on the relationship between pulse width and energy of the precursor and main burst. We systematically search the light curve data observed by Swift/BAT and Fermi/GBM, and find 13 long bursts with well-structured precursors and main bursts. After fitting the precursor light curve of each different energy channel with the Norris function, we find that there is not only a power-law relationship between precursor width and energy, but also a power-law relationship between the ratio of the rising width to the decaying width and energy. By comparing the relationship between the precursors and the main burst pulses, we find that the distribution of the precursors and the relationship between the power-law indices are roughly the same as those of the main burst. In addition, it is found that the precursor width distribution as well as the upper limit of the pulse width ratio does not exceed 1 and both are asymmetric, which are also consistent with the main burst. These indicate that the precursor and the main burst are indistinguishable, and the precursor and the main burst may have the same physical origin.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： We report the confirmation of a sub-Saturn-size exoplanet, TOI-1194 b, with a mass of about MJ, and a very low mass companion star with a mass of about 96.5 ± 1.5 MJ, TOI-1251 B. Exoplanet candidates provided by the Transiting Exoplanet Survey Satellite (TESS) are suitable for further follow-up observations by ground-based telescopes with small and medium apertures. The analysis is performed based on data from several telescopes worldwide, including telescopes in the Sino-German multiband photometric campaign, which aimed at confirming TESS Objects of Interest (TOIs) using ground-based small-aperture and medium-aperture telescopes, especially for long-period targets. TOI-1194 b is confirmed based on the consistent periodic transit depths from the multiband photometric data. We measure an orbital period of 2.310644 ± 0.000001 days, the radius is RJ and the amplitude of the RV curve is m s−1. TOI-1251 B is confirmed based on the multiband photometric and high-resolution spectroscopic data, whose orbital period is days, radius is RJ and amplitude of the RV curve is m s−1.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： A spectral analysis of the vertical positions and velocities of 374 open star clusters (OSCs) was carried out. We took these OSCs from the Hunt and Reffert catalog; they have an average age of about 10 million years, and are located on the galactic plane XY in a narrow zone inclined by 25° to the galactic axis Y. The following estimates of the parameters of the Radcliffe wave were obtained: (a) the maximum value in periodic perturbations of vertical coordinates pc with the wavelength of these perturbations λz = 4.82 ± 0.09 kpc; (b) the maximum value of the velocity of vertical disturbanceskm s−1 with disturbance wavelength λW = 1.78 ± 0.02 kpc. Note that the results of the vertical velocity analysis are first-class in accuracy and completely new.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： In source detection in the Tianlai project, locating the interferometric fringe in visibility data accurately will influence downstream tasks drastically, such as physical parameter estimation and weak source exploration. Considering that traditional locating methods are time-consuming and supervised methods require a great quantity of expensive labeled data, in this paper, we first investigate characteristics of interferometric fringes in the simulation and real scenario separately, and integrate an almost parameter-free unsupervised clustering method and seeding filling or eraser algorithm to propose a hierarchical plug and play method to improve location accuracy. Then, we apply our method to locate single and multiple sources' interferometric fringes in simulation data. Next, we apply our method to real data taken from the Tianlai radio telescope array. Finally, we compare with unsupervised methods that are state of the art. These results show that our method has robustness in different scenarios and can improve location measurement accuracy effectively.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： New analyses of earlier ALMA observations of oxygen-rich AGB star EP Aquarii are presented, which complete a previously published analysis and offer a different interpretation of the morpho-kinematics of the circumstellar envelope. The birth of the equatorial density enhancement (EDE) is shown to occur very close to the star where evidence for rotation has been obtained. Close to the star and where outflows have been observed: their interaction with the gas of the nascent EDE is seen to play an important role in the development of the wind and the evolution of its radial velocity from 8 to 10 km s−1 on the polar symmetry axis to ∼2 km s−1 at the equator. It implies complex morpho-kinematics: making reliable interpretations with reasonable confidence is difficult. In particular, it questions an earlier interpretation implying the presence of a white dwarf companion orbiting the star at an angular distance of ∼04 from its center. It proposes instead an interpretation in terms of a standard mass ejection associated with a shock wave leaving a void of emission in its wake. High Doppler velocity wings are seen to consist of two components, the upper velocity end of the global wind, reaching above ±12 km s−1, and an effective line broadening, confined within 200 mas from the center of the star, reaching above ±20 km s−1 and interpreted as caused by the pattern of shock waves resulting from the interaction between stellar pulsation and convective cell partition.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Dwarf irregular galaxies (dIrrs), as rotationally supported systems, have more reliable J-factor measurements than dwarf spheroidal galaxies and have received attention as targets for dark matter detection in recent years. In this paper, we use 10 yr of IceCube muon-track data and an unbinned maximum-likelihood-ratio method to search for neutrino signals beyond the background from the directions of seven dIrrs, aiming to detect neutrinos produced by heavy annihilation dark matter. We do not detect any significant signal. Based on such null results, we calculate the upper limits on the velocity-averaged annihilation cross section for 1 TeV–10 PeV dark matter. Our limits, although weaker than the strictest constraints in the literature in this mass range, are also a good complement to the existing results considering the more reliable J-factor measurements of dIrrs.
分类： 天文学 >> 天文学 提交时间： 2024-03-29 合作期刊: 《Research in Astronomy and Astrophysics》
摘要： Ground-based arrays of imaging atmospheric Cherenkov telescopes (IACTs) are the most sensitive γ-ray detectors for energies of approximately 100 GeV and above. One such IACT is the High Altitude Detection of Astronomical Radiation (HADAR) experiment, which uses a large aperture refractive water lens system to capture atmospheric Cherenkov photons (i.e., the imaging atmospheric Cherenkov technique). The telescope array has a low threshold energy and large field of view, and can continuously scan the area of the sky being observed, which is conducive to monitoring and promptly responding to transient phenomena. The process of γ-hadron separation is essential in very-high-energy (>30 GeV) γ-ray astronomy and is a key factor for the successful utilization of IACTs. In this study, Monte Carlo simulations were carried out to model the response of cosmic rays within the HADAR detectors. By analyzing the Hillas parameters and the distance between the event core and the telescope, the distinction between air showers initiated by γ-rays and those initiated by cosmic rays was determined. Additionally, a Quality Factor was introduced to assess the telescope's ability to suppress the background and to provide a more effective characterization of its performance.
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