分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：In 2016, an exposure meter was installed on the Lijiang Fiber-fed High-Resolution Spectrograph to monitor the coupling of starlight to the science fiber during observations. Based on it, we investigated a method to estimate the exposure flux of the CCD in real time by using the counts of the photomultiplier tubes (PMT) of the exposure meter, and developed a software to optimize the control of the exposure time. First, by using flat-field lamp observations, we determined that there is a linear and proportional relationship between the total counts of the PMT and the exposure flux of the CCD. Second, using historical observations of different spectral types, the corresponding relational conversion factors were determined and obtained separately. Third, the method was validated using actual observation data, which showed that all values of the coefficient of determination were greater than 0.92. Finally, software was developed to display the counts of the PMT and the estimated exposure flux of the CCD in real-time during the observation, providing a visual reference for optimizing the exposure time control.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：Machine learning has become a crucial technique for classifying the morphology of galaxies as a result of the meteoric development of galactic data. Unfortunately, traditional supervised learning has significant learning costs since it needs a lot of labeled data to be effective. FixMatch, a semi-supervised learning algorithm that serves as a good method, is now a key tool for using large amounts of unlabeled data. Nevertheless, the performance degrades significantly when dealing with large, imbalanced datasets since FixMatch uses a fixed threshold to filter pseudo labels. Therefore, this study proposes a dynamic threshold alignment (DTA) algorithm based on the FixMatch model. First, the class with the highest amount has its reliable pseudo label ratio determined, and the remaining classes' reliable pseudo label ratios are approximated in accordance. Second, based on the predicted reliable pseudo label ratio for each category, dynamically calculate the threshold for choosing pseudo labels. By employing this dynamic threshold, the accuracy bias of each category is decreased and the learning of classes with less samples is improved. Experimental results show that in galaxy morphology classification tasks, compared with supervised learning, the proposed algorithm significantly improves performance. When the amount of labeled data is 100, the accuracy and F1-score are improved by 12.8% and 12.6%, respectively. Compared with popular semi-supervised algorithms such as FixMatch and MixMatch, the proposed algorithm has better classification performance, greatly reducing the accuracy bias of each category. When the amount of labeled data is 1000, the accuracy of the cigar-shaped smooth galaxy with the least samples is improved by 25.87% compared to FixMatch.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：We use the distance sum rule (DSR) method to constrain the spatial curvature of the Universe with a large sample of 161 strong gravitational lensing (SGL) systems, whose distances are calibrated from the Pantheon compilation of type Ia supernovae (SNe Ia) using deep learning. To investigate the possible influence of mass model of the lens galaxy on constraining the curvature parameter $ Omega_k$, we consider three different lens models. Results show that a flat Universe is supported in the singular isothermal sphere (SIS) model with the parameter $ Omega_k=0.049^{+0.147}_{-0.125}$. While in the power-law (PL) model, a closed Universe is preferred at $ sim 3 sigma$ confidence level, with the parameter $ Omega_k=-0.245^{+0.075}_{-0.071}$. In extended power-law (EPL) model, the 95$ %$ confidence level upper limit of $ Omega_k$ is $<0.011$. As for the parameters of the lens models, constrains on the three models indicate that the mass profile of the lens galaxy could not be simply described by the standard SIS model.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：In our previous work, we investigated the occurrence rate of super-flares on various types of stars and their statistical properties, with a particular focus on G-type dwarfs, using entire Kepler data. The said study also considered how the statistics change with stellar rotation period, which in turn, had to be determined. Using such new data, as a by-product, we found 138 Kepler IDs of F and G types main sequence stars with rotation periods less than a day ($P_{ rm rot}<1$ d). On one hand, previous studies have revealed short activity cycles in F-type and G-type stars and the question investigated was whether or not short-term activity cycles are a common phenomenon in these stars. On the other hand, extensive studies exist which establish empirical connection between a star's activity cycle and rotation periods. In this study, we compile all available Kepler data with $P_{ rm rot}<1$ d, derive and use, established empirical relation between $P_{ rm cyc}$ and $P_{ rm rot}$ with the aim to provide predictions for very short $5.09 leq P_{ rm cyc} leq 38.46$ d cases in a tabular form. We propose an observation to measure $P_{ rm cyc}$ using monitoring program of stellar activity (e.g. activity-related chromospheric emission S-index) or similar means for the Kepler IDs found in this study in order put to test the derived here empirical relations between $P_{ rm cyc}$ and $P_{ rm rot}$. We also propose an alternative method for measuring very short $P_{ rm cyc}$, using flare-detection algorithms applied to future space mission data.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：The extremely low frequency (f<40MHz) is a very important frequency band for the modern radio astronomy observations. It is also a key frequency band for solar radio bursts, planetary radio bursts, fast radio bursts detected in the lunar space electromagnetic environment and the Earth's middle and upper atmosphere with low dispersion values. In this frequency band, the solar stellar activity, the early state of the universe, and the radiation characteristics of planetary magnetosphere and plasma layer can be explored. Since there are few observations with effective spatial resolution in extremely low frequency, it is highly possible to discover unknown astronomical phenomena on such band in the future. In conjunction with low frequency radio observation on the far side of the Moon, we initially set up a novel low-frequency radio array in the Qitai Observation Station of Xinjiang Astronomical Observatory deep in Tianshan Mountains, Xinjiang, China on August 23, 2021. The array covers an operating frequency range of 1~90MHz with a sensitivity of -78dBm/125kHz, a dynamic range of 72dB, and a typical gain value of 6dBi, which can realize unattended all-weather observations. The two antennas (1~62MHz) due south of the Qitai Low-Frequency Radio Array (Qitai LFRA) were put into trial observations on May 28, 2021, and the very quiet electromagnetic environment of the station has been confirmed. So far, many solar radio bursts and other foreign signals have been detected. The results show that this novel low frequency radio array has the advantages of good performance, strong direction and high antenna efficiency. It can play a unique role in Solar Cycle 25, and has potential value in prospective collaborative observation between the Earth and space for the extremely low frequency radio astronomy.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：This paper describes a fabrication process for the hyperboloidal concave mirror of a 46.5-nm telescope. The 180-mm aperture hyperboloidal concave mirror and 70-mm aperture compensator are machined directly from chemical mechanical polishing (CMP) of a spherical surface to a high-accuracy aspherical surface by ion beam figuring (IBF). The aspherical measurement method is the Dall null test. To minimize system errors in the measurement process, the rotational measurement method with six rotations is used in the null test. The results of the analysis for the ME (first solve the machined surface profile, then solve the system errors) and EM (first solve the system errors, then solve the machined surface profile) methods of calculation in the measurement are given. The ME method is a more accurate rotational test method, and the six rotations are appropriate for rotational measurements. After the figuring process, the hyperboloidal concave mirror surface profile reached 8.27 nm RMS and the compensator surface profile is approximately 4 nm RMS. The roughness of the hyperboloidal concave mirror is smooth to 0.160 nm RMS.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：The Swift/XRT detected the X-ray afterglow of long burst GRB 220117A, which began to rebrighten 300 seconds after triggering and followed a single power-law decay segment after thousands of seconds of orbital observation gap. This segment is different from the shallow decay segment (plateau) and flare, and may belong to a giant X-ray bump. We investigated this segment by the fall-back accretion model and found that the model can interpret this segment with reasonable parameter values. Within this physical model scenario, the fall-back accretion rate reaches a peak value $ sim 1.70 times10^{-5} M_{ odot} rm s^{-1}$ around 300s in the central engine frame, which is compatible with the late mass supply rate of some textbf{low-metallicity} massive progenitor stars. textbf{The initial black hole (BH) spin is} $a_{0}=0.64^{+0.24}_{-0.26}$ and imply this re-brightening signature requires a larger black hole(BH) spin. The total accretion mass during the fall-back process is $M_{ rm{acc}}=(3.09 pm0.02) times 10^{-2}M_{ odot}$. The jet energy from the fall-back accretion is $(9.77 pm0.65) times10^{52} rm {ergs}$, with a ratio of 0.066 to the isotropic-equivalent radiation energies of GRB prompt phase in the $1-10^{4}$ keV band. The fall-back radius $r_{p}$ corresponding to the peak time of fall-back $t_{p}$ is $(3.16 pm0.05) times10^{10} rm cm$, which is consistent with the typical radius of Wolf–Rayet stars. In summary, our results provide additional support for the origin of the long burst from the core collapse of Wolf-Rayet stars, and its late central engine activity is likely due to the fall-back accretion process.
分类： 天文学 >> 天文学 提交时间： 2023-09-08 合作期刊: 《Research in Astronomy and Astrophysics》
摘要：We study the colour and star formation rates of paired galaxies in filaments and sheets using the EAGLE simulations. We find that the major pairs with pair separation < 50 kpc are bluer and more star forming in filamentary environments compared to those hosted in sheet-like environments. This trend reverses beyond a pair separation of ∼ 50 kpc. The interacting pairs with larger separations (> 50 kpc) in filaments are on average redder and low-star forming when compared to those embedded in sheets. The galaxies in filaments and sheets may have different stellar mass and cold gas mass distributions. Using a Kolmogorov-Smirnov test, we find that for paired galaxies with pair separation < 50 kpc, there are no significant differences in these properties in sheets and filaments. The filaments transport gas towards the cluster of galaxies. Some earlier studies find preferential alignment of galaxy pairs with filament axis. Such alignment of galaxy pairs may lead to different gas accretion efficiency in galaxies residing in filaments and sheets. We propose that the enhancement of star formation rate at smaller pair separation in filaments is caused by the alignment of galaxy pairs. A recent study with the SDSS data (Das et al., 2023) reports the same findings. The confirmation of these results by the EAGLE simulations suggests that the hydrodynamical simulations are powerful theoretical tools for studying the galaxy formation and evolution in the cosmic web.
分类： 天文学 >> 天文学 提交时间： 2023-09-06
摘要：利用星系解构软件GALFIT通过面亮度轮廓拟合对近邻早型旋涡星系M81 (NGC 3031)进行形态学解 构, 旨在探究M81星系的结构组成并对其进行形态学量化. 通过6种解构模式, 对M81进行了不同复杂程度的结 构分解, 其中最复杂的解构模式包含核球、盘、外旋臂、内旋臂、星系核5个子结构. 研究结果显示, M81有 一个S´ersic指数约为5.0的经典核球, 其形态和光度在不同解构模式中均保持稳定; M81星系盘的S´ersic指数约 为1.2, 但它的形态参数和光度与是否分解内旋臂相关. 不同子结构的组合对作为混合体的星系整体的形态有不 可忽视的影响. 星系解构的结果提供了不同解构模式适用性的建议: 其中核球+盘+星系核的三成分解构适用 于大样本星系的核-盘研究; 而考虑旋臂的复杂解构则适合于对星系子结构的精确测量, 如小样本(或个源)研究. 基于Spitzer-The Infrared Array Camera (IRAC) 4.5 µm的单波段图像的形态学解构研究是后续一系列研究的 开始, 在此基础上未来将会对M81进行多波段解构, 同时研究不同子结构的光谱能量分布和星族性质, 并推断 M81各子结构的形成历史和演化过程.
分类： 天文学 >> 天体物理学 分类： 天文学 >> 天文学 提交时间： 2023-08-31
摘要：The radiation mechanism of fast radio bursts (FRBs) has been extensively studied but still remains elusive. Coherent radiation is identified as a crucial component in the FRB mechanism, with charged bunches also playing a significant role under specific circumstances. In the present research, we propose a phenomenological model that draws upon the coherent curvature radiation framework and the magnetized neutron star, taking into account the kinetic energy losses of outflow particles due to inverse Compton scattering (ICS) induced by soft photons within the magnetosphere. By integrating the ICS deceleration mechanism for particles, we hypothesize a potential compression effect on the particle number density within a magnetic tube/family, which could facilitate achieving the necessary size for coherent radiation in the radial direction. This mechanism might potentially enable the dynamic formation of bunches capable of emitting coherent curvature radiation along the curved magnetic field. Moreover, we examine the formation of bunches from an energy perspective. Our discussion suggests that within the given parameter space the formation of bunches is feasible. Finally, we apply this model to FRB 20190520B, one of the most active repeating FRBs discovered and monitored by FAST. Several observed phenomena are explained, including basic characteristics, frequency downward drifting, and bright spots within certain dynamic spectral ranges.
分类： 天文学 >> 天文学 提交时间： 2023-08-04
摘要：The inner boundary of a black hole accretion disk is often set to the marginally stable circular orbit (or the innermost stable circular orbit, ISCO) around the black hole. It is important for the theories of black hole accretion disks and their applications to astrophysical black hole systems. Traditionally, the marginally stable circular orbit is obtained by considering the equatorial motion of a test particle around a black hole. However, in reality the accretion flow around black holes consists of fluid, in which the pressure often plays an important role. Here we consider the influence of fluid pressure on the location of marginally stable circular orbit around black holes. It is found that when the temperature of the fluid is so low that the thermal energy of a particle is much smaller than its rest energy, the location of marginally stable circular orbit is almost the same as that in the test particle case. However, we demonstrate that in some special cases the marginally stable circular orbit can be different when the fluid pressure is large and the thermal energy becomes non-negligible comparing with the rest energy. We present our results for both the cases of non-spinning and spinning black holes. The influences of our results on the black hole spin parameter measurement in X-ray binaries and the energy release efficiency of accretion flows around black holes are discussed.
分类： 天文学 >> 天文学 提交时间： 2023-07-28
摘要：The power law and reflection emission have been observed in the X-ray spectra of both black hole X-ray binaries
(BHXRBs) and active galactic nuclei (AGNs), indicating a common physical origin of the X-ray emission from
these two types of sources. The relevant parameters describing the shape of both components and the potential
correlation between these parameters can provide important clues on the geometric and physical properties of the
disk and the corona in these sources. In this work, we present a positive correlation between the photon index Γ
and the reflection strength R for the low-mass BHXRBs in the hard state by modeling NuSTAR data, which is
qualitatively consistent with the previous studies. We compare our results with the predictions from different
theoretical disk-corona models. We show that the R − Γ correlation found in this work seems to favor the moving
corona model proposed by Beloborodov. Our results indicate that the coronal geometry varies significantly among
BHXRBs. We further compare our results with that of AGNs. We find that the reflection strength R is smaller than
unity in the hard state of BHXRBs, while it can be as large as ∼5 in AGNs, which implies that the variations of the
disk-coronal geometry of AGNs are more vigorous than that of the BHXRBs in the hard state.
分类： 天文学 >> 天文学 提交时间： 2023-07-28
摘要：We create mock X-ray observations of hot gas in galaxy clusters with a new extension of the L-Galaxies semianalytic
model of galaxy formation, which includes the radial distribution of hot gas in each halo. Based on the
model outputs, we first build some mock light cones, then generate mock spectra with the SOXS package and
derive the mock images in the light cones. Using the mock data, we simulate mock X-ray spectra for the ROSAT
all-sky survey, and compare the mock spectra with the observational results. Then, we consider the design
parameters of the HUBS mission and simulate the observation of the halo hot gas for HUBS as an important
application of our mock work. We find: (1) our mock data match the observations by current X-ray telescopes. (2)
The survey of hot baryons in resolved clusters by HUBS is effective below redshift 0.5, and the observations of the
emission lines in point-like sources at z > 0.5 by HUBS help us understand the hot baryons in the early universe.
(3) By taking advantage of the large simulation box and flexibility in semi-analytic models, our mock X-ray
observations provide the opportunity to select targets and observation strategies for forthcoming X-ray facilities.
分类： 天文学 >> 天文学 提交时间： 2023-07-28
摘要：Cross-correlating the data on neutral hydrogen (H I) 21 cm intensity mapping with galaxy surveys is an effective
method to extract astrophysical and cosmological information. In this work, we investigate the cross-correlation of
MeerKAT single-dish mode H I intensity mapping and China Space Station Telescope (CSST) spectroscopic
galaxy surveys. We simulate a survey area of ∼300 deg2 of MeerKAT and CSST surveys at z = 0.5 using Multi-
Dark N-body simulation. The PCA algorithm is applied to remove the foregrounds of H I intensity mapping, and
signal compensation is considered to solve the signal loss problem in H I-galaxy cross power spectrum caused by
the foreground removal process. We find that from CSST galaxy auto and MeerKAT-CSST cross power spectra,
the constraint accuracy of the parameter product ΩH IbH IrH I,g can reach ∼1%, which is about one order of
magnitude higher than the current results. After performing the full MeerKAT H I intensity mapping survey with
5000 deg2 survey area, the accuracy can be enhanced to <0.3%. This implies that the MeerKAT-CSST crosscorrelation
can be a powerful tool to probe the cosmic H I property and the evolution of galaxies and the Universe.
分类： 天文学 >> 天文学 提交时间： 2023-07-28
摘要：In this work, the pulsation analysis is performed on 83 high-amplitude δ Scuti stars (HADS), which have been
observed by the Transiting Exoplanet Survey Satellite. The results show that 49 of these HADS show single-mode
pulsation, 27 of them show radial double-modes pulsation (in which 22 of them pulsate with the fundamental and
first overtone modes and five of them pulsate with the first and second overtone modes), and seven of them show
radial triple-modes pulsation (three of which are newly confirmed triple-mode HADS). The histogram of the
fundamental periods and the ratios between the fundamental and first overtone periods show bimodal structures,
which might be caused by the stellar evolution in this specific phase. Most of the radial triple-mode HADS have a
fundamental amplitude of 41–54 mmag, and 50% of them have similar amplitudes of the fundamental and first
overtone pulsation modes. All these hints require further confirmation not only in observations with more HADS
samples, but also in theoretical models with suitable treatments of stellar evolution and pulsation.
分类： 天文学 >> 天文学 提交时间： 2023-07-27
摘要：We have started a systematic survey of molecular clumps with infall motions to study the very early phase of star
formation. Our first step is to utilize the data products by MWISP to make an unbiased survey for blue asymmetric
line profiles of CO isotopical molecules. Within a total area of ∼2400 square degrees nearby the Galactic plane, we
have found 3533 candidates showing blue-profiles, in which 3329 are selected from the 12CO&13CO pair and
204 are from the 13CO&C18O pair. Exploration of the parametric spaces suggests our samples are in the cold phase
with relatively high column densities ready for star formation. Analysis of the spatial distribution of our samples
suggests that they exist virtually in all major components of the galaxy. The vertical distribution suggest that the
sources are located mainly in the thick disk of ∼85 pc, but still a small part are located far beyond Galactic
midplane. Our follow-up observation indicates that these candidates are a good sample to start a search for infall
motions, and to study the condition of very early phase of star formation.
分类： 天文学 >> 天文学 提交时间： 2023-07-15
摘要：In observational astronomy, we essentially measure the location, flux density (at certain frequency and certain time), distance and angular size of the sources. A parameter space of observational astronomy can be constructed with the parameters such as the sample size of sources, frequency (bandwidth and frequency resolution), time (observing length and time resolution), sensitivity, and angular resolution of the telescope. Based on the previous experience, we would always obtain new knowledge of the universe with instruments that fill in the blanks in the parameter space, e.g. telescopes used for better surveys (which enlarge the sample sizes of sources), telescopes with higher sensitivity, angular resolution, frequency resolution or time resolution.
分类： 天文学 >> 天文学 提交时间： 2023-06-07 合作期刊: 《天文学进展》
摘要：太阳系外行星作为研究恒星演化重要的天体和探索生命起源的基础，多年来一直是天文学前沿研究的热点之一，目前已发展出近 10 种系外行星的探测方法。随着天文观测设备探测精度越来越高，自 1992 年至今，已发现 4 000 余颗系外行星。近几年天基天文观测手段日趋成熟，探测系外行星的精度与效率得到了极大提高，并带动着更多地基系外行星探测项目开展。继目前最成功的太阳系外行星探测卫星开普勒 (Kepler) 之后，2018 年 4 月 TESS 卫星成功发射，全球各地大量天文学家和地基光学望远镜设备纷纷加入太阳系外行星后随测光和光谱观测中，系外行星探测研究迎来了“黄金时代”。介绍了目前系外行星的探测方法和代表性探测项目，综述了类地行星、气态巨行星的探测研究现状和演化理论，并对未来 5 ∼ 10 年内系外行星探测研究的发展趋势进行了展望。
分类： 天文学 >> 天文学 提交时间： 2023-06-07 合作期刊: 《天文学进展》
摘要：GNSS 信号在卫星发射以及接收机接收的传输过程中存在设备时延。设备时延与钟差、模糊度等密切相关，很难将其与它们完全分开。通常，根据实际需要将设备时延与其相关参数组合形成不同的相对设备时延。如果能更好地将这些相对设备时延进行分类和区分，对高精度的GNSS 应用具有重要的参考意义。分别介绍了目前基于设备时延提出的几个常用偏差：码间偏差、未校准的相位时延、频间钟差偏差和系统间偏差，详细说明了它们的概念、产生原因、解算策略及相关研究进展。
分类： 天文学 >> 天文学 提交时间： 2023-06-07 合作期刊: 《天文学进展》
摘要：从 RNSS (radio navigation satellite system) 米级服务角度给出了 BDCS (Beidou coordinate system) 实现精度估算方法，包括广播星历定位法、精密星历与广播星历比对法和卫星激光测距 (satellite laser ranging, SLR) 坐标估计法。广播星历定位法适用于用户对参考框架实现精度的实时监测，但监测精度受限于广播星历精度。轨道比对法引入了事后精密产品，监测精度较高，但其时效性相对较差。SLR 坐标估计法不受电离层延迟和卫星钟差影响，在三种方法中精度最高，但由于激光测距数据量较少，无法做短期监测。结果表明，三种方法估算 BDCS 与ITRF 对齐精度均为厘米级。最后，在中俄卫星导航系统协作背景下，估算了 BDCS 和 PZ-90对齐精度，结果表明，两者在厘米量级对齐，能够满足米级定位用户应用需求。随着北斗三号广 播星历精度的提升和 SLR 的发展，上述三种方法的估算精度将会进一步提高。
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