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Otto F.,Chinese University of Hong Kong | Ji W.,Chinese University of Hong Kong | Ji W.,Xinjiang Astronomical Observatory | Li H.-B.,Chinese University of Hong Kong
Astrophysical Journal | Year: 2017

The complex interplay between turbulence, magnetic fields, and self-gravity leads to the formation of molecular clouds out of the diffuse interstellar medium (ISM). One avenue of studying this interplay is by analyzing statistical features derived from observations, where the interpretation of these features is greatly facilitated by comparisons with numerical simulations. Here we focus on the statistical anisotropy present in synthetic maps of velocity centroid data, which we derive from three-dimensional magnetohydrodynamic simulations of a turbulent, magnetized, self-gravitating patch of ISM. We study how the orientation and magnitude of the velocity anisotropy correlate with the magnetic field and with the structures generated by gravitational collapse. Motivated by recent observational constraints, our simulations focus on the supersonic (sonic Mach number M ≈ 2-17) but sub- to trans-alfvénic (alfvénic Mach number MA ≈ 0.2-1.2) turbulence regime, and we consider clouds that are barely to mildly magnetically supercritical (mass-to-flux ratio equal to once or twice the critical value). Additionally we explore the impact of the turbulence driving mechanism (solenoidal or compressive) on the velocity anisotropy. While we confirm previous findings that the velocity anisotropy generally aligns well with the plane-of-sky magnetic field, our inclusion of the effects of self-gravity reveals that in regions of higher column density, the velocity anisotropy may be destroyed or even reoriented to align with the gravitationally formed structures. We provide evidence that this effect is not necessarily due to the increase of MA inside the high-density regions. © 2017. The American Astronomical Society. All rights reserved.

Chen J.L.,Xinjiang Astronomical Observatory | Chen J.L.,University of Chinese Academy of Sciences | Wang H.G.,Guangzhou University | Wang H.G.,Max Planck Institute for Radio Astronomy | And 6 more authors.
Astrophysical Journal | Year: 2011

The mode-switching phenomenon of PSR B0329+54 is investigated based on the long-term monitoring from 2003 September to 2009 April made with the Urumqi 25 m radio telescope at 1540 MHz. At that frequency, the change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratios between the leading and trailing components are measured for the individual profiles averaged over a few minutes. It is found that the ratios follow normal distributions, where the abnormal mode has a greater typical width than the normal mode, indicating that the abnormal mode is less stable than the normal mode. Our data show that 84.9% of the time for PSR B0329+54 was in the normal mode and 15.1% was in the abnormal mode. From the two passages of eight-day quasi-continuous observations in 2004, supplemented by the daily data observed with the 15 m telescope at 610 MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales are constrained with the Bayesian inference method. It is found that the gamma distribution with the shape parameter slightly smaller than 1 is favored over the normal, log-normal, and Pareto distributions. The optimal scale parameters of the gamma distribution are 31.5 minutes for the abnormal mode and 154 minutes for the normal mode. The shape parameters have very similar values, i.e., 0.75+0.22 -0.17 for the normal mode and 0.84 +0.28 -0.22 for the abnormal mode, indicating that the physical mechanisms in both modes may be the same. No long-term modulation of the relative intensity ratios was found for either mode, suggesting that the mode switching was stable. The intrinsic timescale distributions, constrained for this pulsar for the first time, provide valuable information to understand the physics of mode switching. © 2011 The American Astronomical Society. All rights reserved.

Chen J.L.,Yuncheng University | Chen J.L.,Xinjiang Astronomical Observatory | Wang H.G.,Guangzhou University | Wang H.G.,CSIRO
Astrophysical Journal, Supplement Series | Year: 2014

The frequency dependence of the pulse width is studied for 150 normal pulsars, mostly selected from the European Pulsar Network, for which the 10% multifrequency pulse widths can be well fit with the Thorsett relationship W10 = Aνμ + W10, min. The relative fraction of pulse width change between 0.4 GHz and 4.85 GHz, η = (W4.85-W0.4)/W0.4, is calculated in terms of the best-fit relationship for each pulsar. It is found that 81 pulsars (54%) have η < -10% (group A), showing considerable profile narrowing at high frequencies, 40 pulsars (27%) have -10% ≤η ≤ 10% (group B), meaning a marginal change in pulse width, and 29 pulsars (19%) have η > 10% (group C), showing a remarkable profile broadening at high frequencies. The fractions of the group-A and group-C pulsars suggest that the profile narrowing phenomenon at high frequencies is more common than the profile broadening phenomenon, but a large fraction of the group-B and group-C pulsars (a total of 46%) is also revealed. The group-C pulsars, together with a portion of group-B pulsars with slight pulse broadening, can hardly be explained using the conventional radius-to-frequency mapping, which only applies to the profile narrowing phenomenon. Based on a recent version of the fan beam model, a type of broadband emission model, we propose that the diverse frequency dependence of pulse width is a consequence of different types of distribution of emission spectra across the emission region. The geometrical effect predicting a link between the emission beam shrinkage and spectrum steepening is tested but disfavored. © 2014. The American Astronomical Society. All rights reserved..

Chen J.L.,Xinjiang Astronomical Observatory | Chen J.L.,University of Chinese Academy of Sciences | Wang H.G.,Guangzhou University | Wang N.,Xinjiang Astronomical Observatory
Journal of Astrophysics and Astronomy | Year: 2011

Using the published data of multi-frequency time-aligned pulse profiles from Kuzmin et al. (1998), we calculate the phase-resolved spectra of PSRs B0525+21 and B2020+28. The results reveal that conal-double pulsars have common 'M'-shaped phase-resolved spectra. © 2011 Indian Academy of Sciences.

Chen D.,CAS Center for Space Science and Applied Research | Zhu X.-Z.,Chinese Academy of Sciences | Zhu X.-Z.,University of Chinese Academy of Sciences | Wang N.,Xinjiang Astronomical Observatory
Chinese Astronomy and Astrophysics | Year: 2012

The applications of pulsar timing to several important aspects are introduced and a skeletal definition of the pulsar time scale is given. By taking advantage of the observational timing data of millisecond pulsars obtained at the Parkes Astronomical Observatory, Australia, the ensemble pulsar time based on 4 millisecond pulsars is built up, the comparison of its stability σ z, with that of the atomic time is made, and finally a few of important factors which affect the ensemble pulsar time, as well as their influences on the pulsar timing accuracy and application are analyzed. © 2012.

Hu K.,Xinjiang Astronomical Observatory | Yusup A.,Xinjiang Astronomical Observatory | Chang W.,Xinjiang Astronomical Observatory
ICEIEC 2013 - Proceedings of 2013 IEEE 4th International Conference on Electronics Information and Emergency Communication | Year: 2013

This paper is an article of the control algorithm, the main object of study is linear quadratic Gaussian optimal controller, we call it LQG controller. There is a construction of 110m diameter radio telescope at Xinjiang Astronomical Observatory, CAS, large-diameter telescope has higher requirements of the control algorithm,this is the background of this paper: Based on the existing servo system of 25m telescope in Xinjiang, LQG controller can be studied. Firstly this paper describes the disadvantage of PID controller compared to the LQG controller, then introduces details of the LQG controller derivation of the theoretical model, including LQ controller model, Kalman filter model and the combination of LQG controller on this basis, finally introduces the process of simulation. We simulate LQG controller and PID controller using Simulink. By setting the controller parameters combined with 25 m telescope existing motor parameters the step response of positon loop of antenna control system and bandwidth of the closed loop system can be obtained. Finally we add noise to the controller and get out the filtered and get out the filtered and unfiltered response curve. Then satisfactory results can be obtained by comparing to two curves and the advantages of LQG controller is verified in the antenna control system. © 2013 IEEE.

Hu K.,Xinjiang Astronomical Observatory | Yusup A.,Xinjiang Astronomical Observatory | Chang W.,Xinjiang Astronomical Observatory
ICEIEC 2013 - Proceedings of 2013 IEEE 4th International Conference on Electronics Information and Emergency Communication | Year: 2013

This paper is the article which researches the control algorithm applied to servo system of antenna sub-reflector, the main control algorithm is fuzzy sliding mode control algorithm. This paper first introduces the idea of fuzzy-sliding mode algorithm, then describes the background knowledge about applying to the antenna sub-reflector. The main content of this paper is simulation and comparative analysis, firstly introduces the details of the theoretical model of fuzzy sliding mode controller including sliding mode controller and fuzzy controller, secondly combines these two controller by using a simply way, new controller is fuzzy-sliding model controller. Then this paper gets out some figures of response curves by Simulink. According to the comparison with the response curves of PID algorithm, conclusion can be obtained that: Compared with the PID controller, fuzzy-sliding mode controller has better superiority. It can satisfy the higher requirements of sub-reflector to the speed of response and control accuracy. © 2013 IEEE.

Chen J.,University of Chinese Academy of Sciences | Chen M.,Xinjiang Astronomical Observatory
2013 IEEE 3rd International Conference on Information Science and Technology, ICIST 2013 | Year: 2013

The application of an LQG controller in antenna servo system is presented in this work, using Nanshan 25-m antenna of Xinjiang Astronomical Observatory. A linear quadratic optimal controller is designed, based on establishing controlled object model and according to the quadratic performance index. Meanwhile, taking into account the actual situation of the antenna work, the Gaussian noise signal is added to simulate the actual interference signal. Then a Kalman filter is designed for the noise. A traditional PI (Proportional and Integral) controller is joined in order to further improve control performance. The good simulation results are obtained by repeatedly optimizing the controller. The results indicate that the designed LQG controller achieves desired results in control performance (such as settling time and overshoot) and has good disturbance performance. Besides, the system is relatively stable. © 2013 IEEE.

Hu K.,Xinjiang Astronomical Observatory | Yusup A.,Xinjiang Astronomical Observatory
ICEIEC 2013 - Proceedings of 2013 IEEE 4th International Conference on Electronics Information and Emergency Communication | Year: 2013

The pointing of an antenna always exists deviation due to track deformation, gravity and thermal influence. Through long time observation, we find that sunlight also has some influence on antenna structure. In order to get the antenna pointing deviation in the effects of the sunlight of 25m antenna in Urumqi, we use the frame-typed level and thermometer to make a week's measurement about the temperature around the antenna and the antenna tilt. The measured data was analyzed and combined with the track deformation data, and the pointing error fixed model as well as the thermal load deformation model were built. The measured data was fitted by the two models, and the result shows that the model can well reflect the pointing error and thermal load error, thus can give a better understanding of the antenna deformation, and also can give some good reference to antenna pointing calibration and the antenna's reconstruction. © 2013 IEEE.

Wen Z.G.,Xinjiang Astronomical Observatory | Wen Z.G.,University of Chinese Academy of Sciences | Wang N.,Xinjiang Astronomical Observatory | Wang N.,Chinese Academy of Sciences | And 9 more authors.
Astronomy and Astrophysics | Year: 2016

Aims. We intend to study of the nulling and subpulse drifting in PSR J1727-2739 in detail to investigate its radiation properties. Methods. The observations were carried out on 20 March, 2004 using the Parkes 64-m radio telescope with a central frequency of 1518 MHz. A total of 5568 single pulses were analysed. Results. This pulsar shows well-defined nulls with lengths lasting from 6 to 281 pulses and separated by burst phases ranging from 2 to 133 pulses. We estimate a nulling fraction of around 68%. No emission in the average pulse profile integrated over all null pulses is detected with significance above 3σ. Most transitions from nulls to bursts are within a few pulses, whereas the transitions from bursts to nulls exhibit two patterns of decay; these transitions either decrease gradually or rapidly. In the burst phase, we find that there are two distinct subpulse drift modes with vertical spacing between the drift bands of 9.7 ± 1.6 and 5.2 ± 0.9 pulse periods, while sometimes there is a third mode with no subpulse drifting. Some mode transitions occur within a single burst, while others are separated by nulls. Different modes have different average pulse profiles. Possible physical mechanisms are discussed. © ESO, 2016.

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