Keywords

The scientific research mission of the Survey Space Telescope (also known as the China Space Station Telescope, CSST) imposes rigorous requirements on the observation precision. However, microvibrations generated by the shutter mechanism can seriously affect the performance of the highly precise and sensitive instruments onboard. This study presents a dynamic model and verifies its performance using experimental results. Indeed, a five-degree-of-freedom nonlinear dynamic model that considers all interference sources and bearing nonlinearity is established. The validity of the proposed model is verified using experimental measurements of the microvibration characteristics. The obtained results proved that the proposed dynamic model can accurately predict the characteristics of the microvibrations caused by the shutter mechanism in the design stage and provide a theoretical basis for the development of CSST.

The surface accuracy of the large-aperture reflector antenna has a significant influence on the observation efficiency. Recent researchers have focused on using the finite element (FE) simulation to study the effect of gravity and heat on the deformation distribution of the main reflector. However, the temperature distribution of the antenna is challenging to obtain, and it takes a long time for the FE simulation to carry out FE modeling and post-processing. To address these limitations, this study presents a surrogate model based on Extreme Gradient Boosting (XGBoost) and deep Convolutional Neural Network (CNN) to get the deformation distribution of the main reflector quickly. In the design of the surrogate model, using the XGBoost algorithm and sparse sampling to solve the difficulty of obtaining the entire temperature distribution is first proposed, and then a deep CNN is developed for estimating deformation. Based on the effect of dynamic loads on the antenna structure, a diverse data set is generated to train and test the surrogate model. The results show that the surrogate model reduces the calculating time dramatically and can obtain the indistinguishable deformation compared to the FE simulation. This technique provides a valuable tool for temperature and deformation calculation of large-aperture antennas.

We study the subhalo and satellite populations in haloes similar to the Milky Way (MW)-Andromeda paired configuration in the Millennium II and P-Millennium simulations. We find subhaloes are 5%–15% more abundant in paired haloes than their isolated counterparts that have the same halo mass and large-scale environmental density. Paired haloes tend to reside in a more isotropic environment than isolated haloes, the shear tensor of their large-scale tidal field is possibly responsible for this difference. We also study the thickness of the spatial distribution of the top 11 most massive satellite galaxies obtained in the semi-analytic galaxy sample constructed from the Millennium II simulation. Moreover, satellites that have lost their host subhaloes due to the resolution limit of the simulation have been taken into account. As a result, we find that the difference in the distribution of the satellite thickness between isolated and paired haloes is indistinguishable, which suggests that the paired configuration is not responsible for the observed plane of satellites in the MW. The results in this study indicate the paired configuration could bring some nonnegligible effect on the subhalo abundance in the investigation of the MW's satellite problems.

This paper is concerned with the characteristics of a tracking differentiator (TD) in arranging transition process for the physical system. In order to overcome disadvantages of linear TD (LTD) in practice, a novel variable parameter linear tracking differentiator (VLTD) is proposed. By designing the speediness factor as a function of the tracking error, the VLTD can track a large range of set values with reasonable speed and acceleration. Analysis shows that VLTD can converge to its set value under certain conditions. Meanwhile, the speed and acceleration bounds are added to the VLTD, which guarantees that the proposed transition signal really plays a transitional role. The numerical simulation results emphasize necessity for adding speed and acceleration bounds to the VLTD. By comparing VLTD with the nonlinear TD (NLTD) in the simulations and experiments, VLTD can achieve almost the same performance as the NLTD while it is easier to be implemented.

We analyzed the optical monitoring data in the R band of TeV blazar 1ES 1959+650 from 2002 to 2018, and provided evidence of a quasi-periodic oscillation in this object. The light curve shows a stable and persistent periodicity at ∼540 days, detected by the Lomb–Scargle periodogram, Jurkevich and weighted wavelet z-transform techniques. The red noise power spectrum was estimated using the PSRESP method, and this period was found to be at >3σ significance level. There are also two possible periodicities at ∼268 and ∼1100 days detected by all three methods. However, their significance levels are relatively low, and thus these two periods cannot be verified by the present data. We discuss several possible physical models that could explain the periodic variability in this object.

We study the origin of the UV-excess in star clusters by performing N-body simulations of six clusters with N = 10 k and N = 100 k (single stars & binary systems) and metallicities of Z = 0.01, 0.001 and 0.0001, using petar. All models initially have a 50% primordial binary fraction. Using GalevNB we convert the simulated data into synthetic spectra and photometry for the China Space Station Telescope (CSST) and Hubble Space Telescope (HST). From the spectral energy distributions we identify three stellar populations that contribute to the UV-excess: (1) second asymptotic giant branch stars, which contribute to the UV flux at early times; (2) naked helium stars and (3) white dwarfs, which are long-term contributors to the FUV spectra. Binary stars consisting of a white dwarf and a main sequence star are cataclysmic variable (CV) candidates. The magnitude distribution of CV candidates is bimodal up to 2 Gyr. The bright CV population is particularly bright in FUV − NUV. The FUV − NUV color of our model clusters is 1–2 mag redder than the UV-excess globular clusters in M87 and in the Milky Way. This discrepancy may be induced by helium enrichment in observed clusters. Our simulations are based on simple stellar evolution; we do not include the effects of variations in helium and light elements or multiple stellar populations. A positive radial color gradient is present in CSST NUV − y for main sequence stars in all models with a color difference of 0.2–0.5 mag, up to 4 half-mass radii. The CSST NUV − g color correlates strongly with HST FUV − NUV for NUV − g > 1 mag, with the linear relation FUV − NUV =(1.09 ± 0.12) × (NUV − g) + (−1.01 ± 0.22). This allows for conversion of future CSST NUV − g colors into HST FUV − NUV colors, which are sensitive to UV-excess features. We find that CSST will be able to detect UV-excess in Galactic/extragalactic star clusters with ages >200 Myr.

We present the broadband numerical modeling of afterglows for two remarkably bright long gamma-ray bursts (GRBs), GRB 050820A and GRB 070125, with a wide range of observations from the radio band to the X-ray band. In our work, we fit light curves and constrain physical parameters using a standard forward shock model from the afterglowpy Python package, considering different jet structures and the jet lateral expansion. For GRB 050820A, the constrained jet is close to a top-hat jet with an extremely small half opening angle of about 0.015 rad, and the circumburst matter density is as small as 10⁻⁷ cm⁻³, which suggests that this peculiar long GRB might originate from metal-poor stars with low mass-loss rates. To explain the late time optical light curves of GRB 070125, the effects of the lateral expansion and the participation factor of electrons that are accelerated by the shock have to be taken into account. The constrained results for GRB 070125 show that the jet is also close to a top-hat jet with a half opening angle of about 0.1 rad, the viewing angle is about 0.05 rad, the circumburst density is about 10 cm⁻³, and the participation factor is about 0.1. The jet energy of the two bursts is required to be ∼10⁵¹–10⁵² erg, which can be produced by a millisecond magnetar or a hyper-accreting black hole.

Recent studies have suggested that type Iax supernovae (SNe Iax) are likely to result from a weak deflagration explosion of a Chandrasekhar-mass white dwarf in a binary system with a helium (He)-star companion. Assuming that most SNe Iax are produced from this scenario, in this work we extend our previous work on the three-dimensional hydrodynamical simulation of ejecta-companion interaction by taking the orbital and spin velocities of the progenitor system into account. We then follow the post-impact evolution of a surviving He-star companion by using the one-dimensional stellar evolution code MESA. We aim to investigate the post-explosion rotation properties of a He-star companion in SNe Iax. We find that the He-star companion spins down after the impact due to the angular-momentum loss and expansion caused by the mass-stripping and shock heating during the interaction. This leads to the situation where the surface rotational speed of the surviving companion can drop to one-third of its pre-explosion value when it expands to a maximum radius a few years after the impact. Subsequently, the star shrinks and spins up again once the deposited energy is released. This spin-switching feature of the surviving He-star companions of SNe Iax may be useful for the identification of such objects in future observations.

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 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 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 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, 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.

The telescopes and the infrastructures may alter the local wind environment around the observatory and further affect the observing environment. After the completion of site testing, it is necessary to analyze the wind environment of the entire site and plan the telescope layout to make use of the excellent conditions scientifically and rationally. Taking a typical observatory as an example, the effect of topographical features on the wind environment and the mutual interference between telescope enclosures are analyzed by using the Computational Fluid Dynamics (CFD) method. The CFD simulations are compared with the seeing data from the Differential Image Motion Monitor, and the results are in good agreement, which verifies the effectiveness of the CFD method. The results of wind environment analysis can provide reasonable suggestions for site layout and construction, reducing the interference effects and improving the observing environment.