分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zijie Yu
Furen Deng
Shijie Sun
Chenhui Niu
Jixia Li
Fengquan Wu
Wei-Yang Wang
Yougang Wang
Hui Feng
Lin Shu
Jie Hao
Reza Ansari
Albert Stebbins
Xuelei Chen
摘要： The Tianlai Dish Pathfinder array is a radio interferometer array consisting of 16 six meter dish antennas. The original digital backend integration time is at the seconds level, designed for HI intensity mapping experiment. A new digital backend with millisecond response is added to enable it to search for fast radio burst (FRB) during its observations. The design and calibration of this backend, and the real time search pipeline for it are described in this paper. It is capable of forming 16 digital beams for each linear polarisation, covering an area of 19.6 square degrees. The search pipeline is capable of searching for, recording and classifying FRBs automatically in real time. In commissioning, we succeeded in capturing the signal pulses from the pulsars PSR B0329+54 and B2021+51.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Knowing the conserved quantities that a galaxy's stellar orbits conform to is important in helping us understand the stellar distribution and structures within the galaxy. Isolating integrals of motion and resonances are particularly important, non-isolating integrals less so. We compare the behavior and results of two methods for counting the number of conserved quantities, one based on the correlation integral approach and the other a more recent method using machine learning. Both methods use stellar orbit trajectories in phase space as their only input, and we create such trajectories from theoretical spherical, axisymmetric and triaxial model galaxies. The orbits have known isolating integrals and resonances. We find that neither method is fully effective in recovering the numbers of these quantities, nor in determining the number of non-isolating integrals. From a computer performance perspective, we find the correlation integral approach to be the faster. Determining the algebraic formulae of (multiple) conserved quantities from the trajectories has not been possible due to the lack of an appropriate symbolic regression capability. Notwithstanding the shortcomings we have noted, it may be that the methods are usable as part of a trajectory analysis tool kit.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Wenkai Hu
Yichao Li
Yougang Wang
Fengquan Wu
Bo Zhang
Ming Zhu
Shifan Zuo
Guilaine Lagache
Yinzhe Ma
Mario G. Santos
Xuelei Chen
摘要： We investigate the 1/f noise of the Five-hundred-meter Aperture Spherical Telescope (FAST) receiver system using drift-scan data from an intensity mapping pilot survey. All the 19 beams have 1/f fluctuations with similar structures. Both the temporal and the 2D power spectrum densities are estimated. The correlations directly seen in the time series data at low frequency $f$ are associated with the sky signal, perhaps due to a coupling between the foreground and the system response. We use Singular Value Decomposition (SVD) to subtract the foreground. By removing the strongest components, the measured 1/f noise power can be reduced significantly. With 20 modes subtraction, the knee frequency of the 1/f noise in a 10 MHz band is reduced to $1.8 \times 10^{-3}\Hz$, well below the thermal noise over 500-seconds time scale. The 2D power spectra show that the 1/f-type variations are restricted to a small region in the time-frequency space and the correlations in frequency can be suppressed with SVD modes subtraction. The residual 1/f noise after the SVD mode subtraction is uncorrelated in frequency, and a simple noise diode frequency-independent calibration of the receiver gain at 8s interval does not affect the results. The 1/f noise can be important for HI intensity mapping, we estimate that the 1/f noise has a knee frequency $(f_{k}) \sim$ 6 $\times$ 10$^{-4}$Hz, and time and frequency correlation spectral indices $(\alpha) \sim 0.65$, $(\beta) \sim 0.8$ after the SVD subtraction of 30 modes. This can bias the HI power spectrum measurement by 10 percent.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We combine the kinematics of 159 globular clusters (GCs) provided by the Gaia
Early Data Release 3 (EDR3) with other observational data to classify the GCs,
and to estimate the mass of the Milky Way (MW). We use the age-metallicity
relation, integrals of motion, action space and the GC orbits to identify the
GCs as either formed in-situ (Bulge and Disk) or ex situ (via accretion). We
find that $45.3\%$ have formed in situ, $38.4\%$ may be related to known merger
events: Gaia-Sausage-Enceladus, the Sagittarius dwarf galaxy, the Helmi
streams, the Sequoia galaxy, and the Kraken galaxy. We also further identify
three new sub-structures associated with the Gaia-Sausage-Enceladus. The
remaining $16.3\%$ of GCs are unrelated to the known mergers and thought to be
from small accretion events. We select 46 GCs which have radii $8.0
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We combine the kinematics of 159 globular clusters (GCs) provided by the Gaia
Early Data Release 3 (EDR3) with other observational data to classify the GCs,
and to estimate the mass of the Milky Way (MW). We use the age-metallicity
relation, integrals of motion, action space and the GC orbits to identify the
GCs as either formed in-situ (Bulge and Disk) or ex situ (via accretion). We
find that $45.3\%$ have formed in situ, $38.4\%$ may be related to known merger
events: Gaia-Sausage-Enceladus, the Sagittarius dwarf galaxy, the Helmi
streams, the Sequoia galaxy, and the Kraken galaxy. We also further identify
three new sub-structures associated with the Gaia-Sausage-Enceladus. The
remaining $16.3\%$ of GCs are unrelated to the known mergers and thought to be
from small accretion events. We select 46 GCs which have radii $8.0
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shijie Sun
Jixia Li
Fengquan Wu
Peter Timbie
Reza Ansari
Jingchao Geng
Huli Shi
Albert Stebbins
Yougang Wang
Juyong Zhang
Xuelei Chen
摘要： A great challenge for 21 cm intensity mapping experiments is the strong foreground radiation which is orders of magnitude brighter than the 21cm signal. Removal of the foreground takes advantage of the fact that its frequency spectrum is smooth while the redshifted 21cm signal spectrum is stochastic. However, a complication is the non-smoothness of the instrument response. This paper describes the electromagnetic simulation of the Tianlai cylinder array, a pathfinder for 21 cm intensity mapping experiments. Due to the vast scales involved, a direct simulation requires large amount of computing resources. We have made the simulation practical by using a combination of methods: first simulate a single feed, then an array of feed units, finally with the feed array and a cylindrical reflector together, to obtain the response for a single cylinder. We studied its radiation pattern, bandpass response and the effects of mutual coupling between feed units, and compared the results with observation. Many features seen in the measurement result are well reproduced in the simulation, especially the oscillatory features which are associated with the standing waves on the reflector. The mutual coupling between feed units is quantified with S-parameters, which decrease as the distance between the two feeds increases. Based on the simulated S-parameters, we estimate the correlated noise which has been seen in the visibility data, the results show very good agreement with the data in both magnitude and frequency structures. These results provide useful insights on the problem of 21cm signal extraction for real instruments.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Fengquan Wu
Jixia Li
Shifan Zuo
Xuelei Chen
Santanu Das
John P. Marriner
Trevor M. Oxholm
Anh Phan
Albert Stebbins
Peter T. Timbie
Reza Ansari
Jean-Eric Campagne
Zhiping Chen
Yanping Cong
Qizhi Huang
Yichao Li
Tao Liu
Yingfeng Liu
Chenhui Niu
Calvin Osinga
摘要： The Tianlai Dish Pathfinder Array is a radio interferometer designed to test techniques for 21~cm intensity mapping in the post-reionization universe as a means for measuring large-scale cosmic structure. It performs drift scans of the sky at constant declination. We describe the design, calibration, noise level, and stability of this instrument based on the analysis of about $\sim 5 \%$ of 6,200 hours of on-sky observations through October, 2019. Beam pattern determinations using drones and the transit of bright sources are in good agreement, and compatible with electromagnetic simulations. Combining all the baselines, we make maps around bright sources and show that the array behaves as expected. A few hundred hours of observations at different declinations have been used to study the array geometry and pointing imperfections, as well as the instrument noise behaviour. We show that the system temperature is below 80~K for most feed antennas, and that noise fluctuations decrease as expected with integration time, at least up to a few hundred seconds. Analysis of long integrations, from 10 nights of observations of the North Celestial Pole, yielded visibilities with amplitudes of 20-30~mK, consistent with the expected signal from the NCP radio sky with $<10\,$mK precision for $1 ~\mathrm{MHz} \times 1~ \mathrm{min}$ binning. Hi-pass filtering the spectra to remove smooth spectrum signal yields a residual consistent with zero signal at the $0.5\,$mK level.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zijie Yu
Furen Deng
Shijie Sun
Chenhui Niu
Jixia Li
Fengquan Wu
Wei-Yang Wang
Yougang Wang
Hui Feng
Lin Shu
Jie Hao
Reza Ansari
Albert Stebbins
Xuelei Chen
摘要： The Tianlai Dish Pathfinder array is a radio interferometer array consisting of 16 six meter dish antennas. The original digital backend integration time is at the seconds level, designed for HI intensity mapping experiment. A new digital backend with millisecond response is added to enable it to search for fast radio burst (FRB) during its observations. The design and calibration of this backend, and the real time search pipeline for it are described in this paper. It is capable of forming 16 digital beams for each linear polarisation, covering an area of 19.6 square degrees. The search pipeline is capable of searching for, recording and classifying FRBs automatically in real time. In commissioning, we succeeded in capturing the signal pulses from the pulsars PSR B0329+54 and B2021+51.
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