Application of Regularization Methods in the Sky Map Reconstruction of the Tianlai Cylinder Pathfinder Array

The Tianlai cylinder pathfinder is a radio interferometer array to test 21 cm intensity mapping techniques in the post-reionization era.It works in passive drift scan mode to survey the sky visible in the northern hemisphere.To deal with the large instantaneous field of view and the spherical sky,we decompose the drift scan data into m-modes,which are linearly related to the sky intensity.The sky map is reconstructed by solving the linear interferometer equations.Due to incomplete uv coverage of the interferometer baselines,this inverse problem is usually ill-posed,and regularization method is needed for its solution.In this paper,we use simulation to investigate two frequently used regularization methods,the Truncated Singular Value Decomposition(TSVD),and the Tikhonov regularization techniques.Choosing the regularization parameter is very important for its application.We employ the generalized cross validation method and the L-curve method to determine the optimal value.We compare the resulting maps obtained with the different regularization methods,and for the different parameters derived using the different criteria.While both methods can yield good maps for a range of regularization parameters,in the Tikhonov method the suppression of noisy modes are more gradually applied,produce more smooth maps which avoids some visual artefacts in the maps generated with the TSVD method.

Detecting HI Galaxies with Deep Neural Networks in the Presence of Radio Frequency Interference

In the neutral hydrogen(H I)galaxy survey,a significant challenge is to identify and extract the H I galaxy signal from the observational data contaminated by radio frequency interference(RFI).For a drift-scan survey,or more generally a survey of a spatially continuous region,in the time-ordered spectral data,the H I galaxies and RFI all appear as regions that extend an area in the time-frequency waterfall plot,so the extraction of the H I galaxies and RFI from such data can be regarded as an image segmentation problem,and machine-learning methods can be applied to solve such problems.In this study,we develop a method to effectively detect and extract signals of H I galaxies based on a Mask R-CNN network combined with the PointRend method.By simulating FAST-observed galaxy signals and potential RFI impact,we created a realistic data set for the training and testing of our neural network.We compared five different architectures and selected the best-performing one.This architecture successfully performs instance segmentation of H I galaxy signals in the RFI-contaminated time-ordered data,achieving a precision of 98.64%and a recall of 93.59%.

Classifying Globular Clusters and Applying them to Estimate the mass of the Milky Way

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 agemetallicity 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,while 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 <r<37.3 kpc and obtain the anisotropy parameter β=0.315_(-0.049)^(+0.055),which is lower than the recent result using the sample of GCs in Gaia Data Release 2,but still in agreement with it by considering the error bar.By using the same sample,we obtain the MW mass inside the outermost GC as M(<37.3 kpc)=0.423_(-0.02)^(+0.02)×10^(12)M_(⊙),and the corresponding M_(200)=1.11_(-0.18)^(+0.25)×10^(12)M_(⊙).The estimated mass is consistent with the results in many recent studies.We also find that the estimated β and mass depend on the selected sample of GCs.However,it is difficult to determine whether a GC fully traces the potential of the MW.

A Simulation of Calibration and Map-making Errors of the Tianlai Cylinder Pathfinder Array

The Tianlai cylinder array is a pathfinder for developing and testing 21 cm intensity mapping techniques.In this paper,we use numerical simulation to assess how its measurement is affected by thermal noise and the errors in calibration and map-making process,and the error in the sky map reconstructed from a drift scan survey.Here we consider only the single frequency,unpolarized case.The beam is modeled by fitting to the electromagnetic simulation of the antenna,and the variations of the complex gains of the array elements are modeled by Gaussian processes.Mock visibility data are generated and run through our data processing pipeline.We find that the accuracy of the current calibration is limited primarily by the absolute calibration,where the error comes mainly from the approximation of a single dominating point source.We then studied the m-mode map-making with the help of Moore-Penrose inverse.We find that discarding modes with singular values smaller than a threshold could generate visible artifacts in the map.The impacts of the residue variation of the complex gain and thermal noise are also investigated.The thermal noise in the map varies with latitude,being minimum at the latitude passing through the zenith of the telescope.The angular power spectrum of the reconstructed map show that the current Tianlai cylinder pathfinder,which has a shorter maximum baseline length in the North-South direction,can measure modes up to l■2πb_(NS)/λ~200 very well,but would lose a significant fraction of higher angular modes when noise is present.These results help us to identify the main limiting factors in our current array configuration and data analysis procedure,and suggest that the performance can be improved by reconfiguration of the array feed positions.

Stellar Dynamical Modeling—Counting Conserved Quantities

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.

Hopf Bifurcation of a Nonresident Computer Virus Model with Delay

In this paper, a delayed nonresident computer virus model with graded infection rate is considered in which the following assumption is imposed: latent computers have lower infection ability than infectious computers. With the aid of the bifurcation theory, sufficient conditions for stability of the infected equilibrium of the model and existence of the Hopf bifurcation are established. In particular, explicit formulae which determine direction and stability of the Hopf bifurcation are derived by means of the normal form theory and the center manifold reduction for functional differential equations. Finally, a numerical example is given in order to show the feasibility of the obtained theoretical findings.

The Electromagnetic Characteristics of the Tianlai Cylindrical Pathfinder Array

A great challenge for 21 cm intensity mapping experiments is the strong foreground radiation which is orders of magnitude brighter than the 21 cm signal.Removal of the foreground takes advantage of the fact that its frequency spectrum is smooth while the redshifted 21 cm signal spectrum is stochastic.However,a complication is the nonsmoothness 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 a 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,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 reproduced well 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 Sparameters,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,and the results show very good agreement with the data in both magnitude and frequency structures.These results provide useful insights on the problem of 21 cm signal extraction for real instruments.

A Fast Radio Burst Backend for the Tianlai Dish Pathfinder Array

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 H I intensity mapping experiment.A new digital backend with millisecond response is added to enable it to search for fast radio burst 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 polarization,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.

Constraining Brans–Dicke Cosmology with the CSST Galaxy Clustering Spectroscopic Survey

The Brans-Dicke(BD)theory is the simplest Scalar-Tensor theory of gravity,which can be considered as a candidate of modified Einstein’s theory of general relativity.In this work,we forecast the constraints on BD theory in the CSST galaxy clustering spectroscopic survey with a magnitude limit~23 AB mag for point-source 5σdetection.We generate mock data based on the zCOSMOS catalog and consider the observational and instrumental effects of the CSST spectroscopic survey.We predict galaxy power spectra in the BD theory from z=0 to 1.5,and the galaxy bias and other systematical parameters are also included.The Markov Chain Monte Carlo technique is employed to find the best-fits and probability distributions of the cosmological and systematical parameters.A BD parameterζis introduced,which satisfiesζ=In(1+(1/ω)).We find that the CSST spectroscopic galaxy clustering survey can give|ξ|<10^(-2),or equivalently|ω|>O(10^(2))and|■/G|<10^(-13),under the assumptionζ=0.These constraints are almost at the same order of magnitude compared to the joint constraints using the current cosmic microwave background,baryon acoustic oscillations and TypeⅠa supernova data,indicating that the CSST galaxy clustering spectroscopic survey would be powerful for constraining the BD theory and other modified gravity theories.

The Tianlai Cylinder Pathfinder array: System functions and basic performance analysis

The Tianlai Cylinder Pathfinder is a radio interferometer array designed to test techniques for 21 cm intensity mapping in the post-reionization Universe,with the ultimate aim of mapping the large scale structure and measuring cosmological parameters such as the dark energy equation of state.Each of its three parallel cylinder reflectors is oriented in the north-south direction,and the array has a large field of view.As the Earth rotates,the northern sky is observed by drift scanning.The array is located in Hongliuxia,a radio-quiet site in Xinjiang,and saw its first light in September 2016.In this first data analysis paper for the Tianlai cylinder array,we discuss the sub-system qualification tests,and present basic system performance obtained from preliminary analysis of the commissioning observations during 2016-2018.We show typical interferometric visibility data,from which we derive the actual beam profile in the east-west direction and the frequency band-pass response.We describe also the calibration process to determine the complex gains for the array elements,either using bright astronomical point sources,or an artificial on site calibrator source,and discuss the instrument response stability,crucial for transit interferometry.Based on this analysis,we find a system temperature of about 90 K,and we also estimate the sensitivity of the array.