分类： 天文学 >> 天体物理学 提交时间： 2023-09-15
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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： 物理学 >> 核物理学 分类： 天文学 >> 天文仪器与技术 提交时间： 2023-08-29
Wan, Qiang
Guo, Jianhua
Xu, Xing
Wang, Shen
Zhang, Yongqiang
Hu, Yiming
Zhang, Yan
Pan, Xu
Li, Xiang
Yue, Chuan
Jiang, Wei
Cui, Yuxin
Chen, Dengyi
摘要：In the future, the Very Large Area gamma-ray Space Telescope (VLAST) is expected to observe high-energy electrons and gamma rays in the MeV to TeV range with unprecedented acceptance. As part of the detector suite, a high-energy imaging calorimeter (HEIC) is currently being developed as a homogeneous calorimeter that utilizes long bismuth germanate (BGO) scintillation crystals as both absorbers and detectors. To accurately measure the energy deposition in the BGO bar of HEIC, a high-dynamic-range readout method using a silicon photoMultiplier (SiPM) and multiphotodiode (PD) with different active areas has been proposed. A prototype readout system that adopts multichannel charge measurement ASICs was also developed to read out the combined system of SiPMs and PDs. Preliminary tests confirmed the feasibility of the readout scheme, which is expected to have a dynamic range close to 106.
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： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：观测表明, 黑洞双星的B型准周期振荡(Quasi-Periodic Oscillation, QPO)频率与幂律通量之间存在正相关性. 试图基于阿尔文波振荡模型定量解释该相关性. 标准薄吸积盘辐射通量极大值处的阿尔文波振荡产生QPO. 标准薄盘上的软光子与冕或喷流基部的热电子介质发生逆康普顿散射产生幂律通量. 通过吸积率的连续变化, 得到QPO频率与幂律通量关系的分析解和数值解. 模拟得到的相关性在合理的参数范围内与观测值相吻合. QPO频率与幂律通量的正相关性可以理解为, 较强的磁场导致较高的阿尔文波频率和较高的电子温度从而得到较高的幂律通量. 结果表明B型QPO可能与吸积盘或喷流中的环向磁场的活动有关.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：对于在轨运行的BDS (BeiDou Navigation Satellite System)卫星, 太阳光压是作用在卫星上主要的非引力摄动. 受多种因素的影响, 太阳光压摄动力难以精确建模, 是BDS卫星精密定轨和轨道预报过程中重要的误差来源. 由于ECOMC (Empirical CODE Orbit Model 1 and 2 Combined)模型兼顾了ECOM1 (Empirical CODE Orbit Model 1)和ECOM2 (Empirical CODE Orbit Model 2)模型的特点, 在模型中引入了较多的待估参数, 使得参数之间存在强相关性. 针对ECOMC模型的这一缺陷, 文中收集了2019年1月至2022年4月武汉大学分析中心提供的BDS-3卫星精密星历, 采用动力学轨道拟合方法得到了ECOMC模型的13个光压参数. 通过对该模型的光压参数进行时间序列分析, 分别给出了BDS-3 IGSO (Inclined Geosynchronous Orbit)和MEO (Medium Earth Orbit)卫星光压模型的参数选择策略. 并利用轨道拟合和轨道预报试验, 验证了光压模型参数选择策略的合理性. 结果表明, 采用改进型ECOMC模型进行BDS-3 IGSO和MEO卫星轨道拟合的效果最佳, 同时, 也能够提升BDS-3 IGSO和MEO卫星中长期轨道预报的精度.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：偏心率是描述天体运动轨道的重要参数之一, 能够为揭示天体的动力学演化提供重要线索, 进而帮助理解天体形成与演化的过程及背后的物理机制. 随着天文观测技术的不断发展, 人们对于天体运动轨道的研究已经走出太阳系, 包含的系统也从大质量端的恒星系统延伸到了低质量端的行星系统. 聚焦天体轨道偏心率研究, 回顾了目前在恒星系统(包括主序恒星、褐矮星以及致密星)和行星系统(包括太阳系外巨行星以及``超级地球''、``亚海王星''等小质量系外行星)方面取得的进展, 总结了不同尺度结构下偏心率研究的一些共同之处和待解决的问题. 并结合当下和未来的相关天文观测设备和项目, 对未来天体轨道偏心率方面的研究工作进行了展望.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：中国巡天空间望远镜(Chinese Survey Space Telescope, CSST)是中国的首个大型空间光学望远镜, 将对包括系外行星探测在内的诸多科学目标开展研究, 有望取得前沿科学进展. 时序测光精度是CSST重要的性能指标, 受到物理噪声和仪器噪声的影响, 需要通过数值模拟对其分析和评估. 模拟基于目前公布的CSST主要技术参数, 建立了时序的恒星信号和噪声模型, 以CSST的i波段为例, 分析凝视观测模式下的测光精度. 通过数值仿真, 展示了孔径测光中各项噪声的贡献, 特别是由指向抖动和像素响应不均匀性导致的抖动噪声. 模拟结果还给出了测光孔径的推荐范围. 为了获得更高的信噪比, 可以减小仪器抖动振幅和像素不均匀性, 或者采用参考星较差测光的方式. 结果为CSST后续的时序测光精度与不同指标参数的相关性分析、系外行星探测能力评估以及测光数据处理提供了模拟数据的支撑.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：经典的初轨确定方法包括Laplace方法和Gauss方法以及它们的各种变化形式. 除这些经典方法之外, 基于当今光学观测数据的特点, 学者们也陆续提出了一些其他的初轨确定方法, 包括双$r$ (目标距离观测者的距离)方法和可行域方法. 双$r$方法的一种实现方式是通过猜测某两个时刻(通常是定轨弧段的首、末时刻)目标离观测者的距离, 结合观测者在空间中的位置矢量, 即可求解相应的Lambert弧段作为目标轨道的初始猜测. 进一步, 以其他观测时刻的RMS (Root Mean Square)为优化变量可以改进初始猜测从而确定初轨. 可行域方法则是针对一组初始观测参数(包括赤经、赤纬及其变率), 根据一些初始假设将目标(离观测者的)距离及其变率约束在可行域内, 并通过三角划分逐步逼近的方式寻找到使观测RMS最小的猜测解. 针对一系列模拟观测数据以及实测数据, 将智能优化算法(粒子群算法)应用于这两种初轨方法, 并将结果与改进的Laplace算法的结果进行比较. 由于双$r$方法不仅可以用于短弧定轨还可用于长弧关联, 所以进一步给出了针对长弧段数据的关联结果.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：高灵敏度太赫兹探测模块(High Sensitivity Terahertz Detection Module, HSTDM)是中国空间巡天望远镜---巡天光学设施的后端模块之一, 其核心探测器采用可工作于10\;K温区的氮化铌超导SIS (Superconductor Insulator Superconductor)混频器. 超导SIS混频前端的锁定机构在力学和热学方面需相应的特殊设计, 以应对发射阶段的力学振动以及工作运行阶段的隔热要求. 为了确认在80\;K温区锁定机构与混频器前端有效分离, 针对超导SIS混频前端发射锁定机构所用特氟龙材料热胀特性, 开展基于低温LVDT (Linear Variable Differential Transformer)测量和标记划痕法测量以及两种测量方法交叉验证. LVDT实验测量结果表明特氟龙材料收缩率随温度变化与理论模型基本一致, 在80\;K测得材料收缩率为1.86\%. 据此分析, 超导SIS混频前端锁定机构在\lk 80\;K温度下可与10\;K冷级的超导SIS混频前端实现有效分离.
分类： 提交时间： 2023-08-02 合作期刊: 《天文学报》
摘要：在不同的轨道预报场景中, 使用的动力学模型也不同. 例如, 在低轨空间碎片的预报中大气阻力是十分重要的摄动力, 而到了中高轨, 大气阻力就可以忽略不计. 如何为不同轨道类型的空间碎片选择最优(满足精度要求下的最简)动力学模型还没有系统、详尽的研究. 将对不同精度需求、不同轨道类型下的大批量轨道进行预报, 通过比较不同动力学模型下的预报结果, 给出各种预报场景的最优动力学模型建议. 可以为不同轨道类型的空间碎片在轨道预报时选择基准动力学模型提供参考或标准.
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