Subjects: Astronomy submitted time 2024-07-03 Cooperative journals: 《天文学进展》
Abstract： In 2019, the Event Horizon Telescope (EHT) used very long baseline interfer#2;ometry to capture the first-ever image of a black hole. To meet the demand for dynamic imaging of black holes, the next generation of the EHT array will incorporate more stations located in the eastern hemisphere. The Tibet Plateau in China is the highest region on average elevation in the world, and its unique meteorological conditions make it a potential candidate for site selection. In this paper, we conducted a preliminary evaluation of the high-frequency radio observation conditions in the Tibet Plateau region using MERRA-2 data from the past three years (2019―2021), consisting of 759 data grid points. Based on the opacity performance of these grid points in different seasons, we selected the optimal grid points for each of the four seasons. In addition to the meteorological conditions of the already established sites, we evaluated these four grid points based on three meteorological factors: precipitable water vapor (PWV), liquid water path (LWP), and wind speed. Furthermore, using the meteorological conditions of the Atacama Large Millimeter/submillimeter Array (ALMA) as a standard, we defined the preliminary range for the site selection of the new station.
Subjects: Astronomy submitted time 2024-07-03 Cooperative journals: 《天文学进展》
FANG Wen-feng
YAO Song
YU Yong
WANG Lei
SHAN Xing-mei
SHEN Lu-run
WANG Qi
CHEN Zhen-dong
LIN Qing
DU Zhi-mao
YAO Jia-wen
Abstract： The Double-focus One-meter Telescope, located at the Wangshu Observatory of the Shanghai Astronomy Museum, is currently the largest aperture telescope dedicated to popular science in China. The telescope adopts a dual-focus design scheme that allows manual switching between the prime and Nasmyth focus. The Nasmyth focus primarily serves visual observations for popular science, while the prime focus is equipped with a scientific-grade CMOS sensor, offering a field of view of 1.5 ◦ × 1.1 ◦ . The larger field of view is suitable not only for live broadcast events but also for extensive research on various topics such as surveys of novae and supernovae, investigations of small celestial bodies in the solar system, and monitoring of artificial satellites. The astrometric precison at the prime focus of the telescope was analyzed and evaluated based on actual observation data. The results indicate that for star images with a signal-to-noise ratio greater than 5, the repeatability of measured coordinates is better than 0.1 pixel. When the signal-to-noise ratio is 30, the repeatability improves to better than 0.05 pixel. Using the high-precision Gaia DR3 catalog as the reference, the analysis reveals the presence of significant non-linear characteristics in the observation images obtained at the prime focus of the telescope. Consequently, a 3-order model (20-parameter) is necessary for data reduction. For the test observation data, the observational precision for stars brighter than 15th magnitude is approximately 0.05′′ , with the precision decreasing gradually as the magnitude becomes fainter. The observational precision for 17.5th magnitude is around 0.1′′ .
Subjects: Astronomy submitted time 2024-07-03 Cooperative journals: 《天文学进展》
Abstract： The annual signals in the Global Navigation Satellite System (GNSS) surface deformation time series include not only load deformation but also thermal expansion caused by the change of Earth’s surficial temperature. Using a global three-dimensional thermoe#2;lastic deformation model, the thermal expansion effects of 39 crustal movement observation network of China (CMONOC) stations in the U, E, and N directions in southwestern China are estimated. Then the consistency between GNSS time series (before and after correcting thermal expansion effects) and load deformation derived from satellite gravity observations is analyzed, by comparing GNSS results with the calculations from Gravity Recovery and Climate Experience (GRACE) and GRACE Follow-On (GRACE-FO). After correcting for thermal expansion effects, the results indicate that the correspondence between the vertical component (U direction) of GNSS and the GRACE/GRACE-FO results is slightly weakened. On the horizontal component, the consistence in the E direction improves to some extent, but there exists a significant decrease in the N direction. The results presented in this paper in the southwest region are inconsistent with existing global or large-scale regional studies, indicating that the accuracy of estimating thermal expansion effects in different regions by using global thermoelastic deformation models needs to be further evaluated, and its impact on three-dimensional surface deformation is also worth further in-depth research.
Subjects: Astronomy submitted time 2024-07-03 Cooperative journals: 《天文学进展》
Abstract： We have collected and analyzed the limited the Very Large Array (VLA) archival data of J1458+4121, which is a possible candidate of neutrino event IC-220624A, based on which we observed the source first time with the Very Long Baseline Array (VLBA) at L and C bands. Through data processing and analysis, we obtained the VLBA images of J1458+4121. In addition, its more precise coordinate (α=14:58:20.772, δ=41:21:01.911) was obtained by phase reference technique. The total flux density observed by VLA decreases from 1.4 to 8.4 GHz, so its radio spectrum may be power-law. However, the total flux density observed by VLBA shows an increasing trend from 1.5 to 5 GHz, which means an inversted spectrum in the GHz band. Therefore, it is highly likely that J1458+4121 is a young radio source, and its neutrino production may be caused by the emergence of a new jet component. The results of this paper are beneficial to the follow-up study of J1458+4121, and extend the focus on neutrino origin from bright blazars to other types of active galactic nuclei.
Subjects: Astronomy submitted time 2024-07-03 Cooperative journals: 《天文学进展》
Abstract： Open clusters are ideal laboratories for studying star formation and evolution. However the pattern of the formation of star clusters remains a fundamental and unresolved question. To explore the formation of star clusters, it is essential to search physically con#2;nected pairs or multiple systems of open clusters that could constrain the theories of star cluster formation. Revealing the properties of binary clusters and cluster groups can also yield important insights into the hierarchical formation of stars. Additionally, binary star clusters and cluster groups provide exceptional opportunities to investigate the formation and evolution patterns of star clusters, which are born in the same molecular cloud and to reveal the connection between the cloud and star clusters in morphological and kinematic properties. This article begins by discussing the selection of samples and the patterns of formation for binary star clusters and cluster groups, providing a concise review of research conducted within the Milky Way, and focusing on recent advancements in this field during the Gaia era.
Subjects: Astronomy submitted time 2024-03-22 Cooperative journals: 《天文学进展》
Abstract： Electromagnetic waves will be absorbed when they pass through the atmo#2;sphere. Electromagnetic waves with some frequencies are absorbed more, while electromag#2;netic waves with other frequencies are absorbed less. The frequency range within which the electromagnetic waves are less absorbed is called the atmospheric window of electro#2;magnetic waves. Similarly, the propagation of electromagnetic waves in the universe will be absorbed by the galactic medium. This paper investigates the problem of the absorption of electromagnetic wave when it propagates in the Milky Way. Our analysis shows that there is a lower cut-off frequency for the electromagnetic waves which can reach the earth passing through the Milky Way. Interestingly, the lower cut-off frequency is different respect to different direction on the celestial sphere, which gives a distribution. This distribution is closely related to the distribution of free electron gas in the Milky Way. We propose to determine the distribution of free electron gas in the Milky Way by measuring the cut-off frequency distribution.
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