查看更多>>摘要：Leighton Chajnantor Telescope(LCT),i.e.,the former Caltech Submillimeter Observatory telescope,will be refurbished at the new site in Chajnantor Plateau,Chile in 2023.The environment of LCT will change significantly after its relocation,and the telescope will be exposed to large wind disturbances directly because its enclosure will be completely open during observation.The wind disturbance is expected to be a challenge for LCT's pointing control since the existing control method cannot reject this disturbance very well.Therefore,it is very necessary to develop a new pointing control method with good capability of disturbance rejection.In this research,a disturbance observer—based composite position controller(DOB-CPC)is designed,in which an H∞ feedback controller is employed to compress the disturbance,and a feedforward linear quadratic regulator is employed to compensate the disturbance precisely based on the estimated disturbance signal.Moreover,a controller switching policy is adopted,which applies the proportional controller to the transient process to achieve a quick response and applies the DOB-CPC to the steady state to achieve a small position error.Numerical experiments are conducted to verify the good performance of the proposed pointing controller(i.e.,DOB-CPC)for rejecting the disturbance acting on LCT.
查看更多>>摘要：During the long term evolution of globular clusters(GCs),some member stars are lost to the field.The recently found nitrogen-rich(N-rich)metal-poor field stars are promising candidates of these GC escapees,since N enhancement is the fingerprint of chemically enhanced populations in GCs.In this work,we discuss the possibility of identifying N-rich metal-poor field stars with the upcoming Chinese Space Station Telescope(CSST).We focus on the main survey camera with NUV,u,g,r,i,z,y filters and slitless spectrograph with a resolution about 200.The combination of UV sensitive equipment and prominent N-related molecular lines in the UV band bodes well for the identification:the color-color diagram of(u-g)versus(g-r)is capable of separating N-rich field stars from normal halo stars,if metallicity can be estimated without using the information on u-band photometry.Besides,the synthetic spectra show that a signal-to-noise ratio of 10 is sufficient to identify N-rich field stars.In the near future,a large sample of N-rich field stars found by CSST,combined with state-of-the-art N-body simulations will be crucial to deciphering GC-Galaxy co-evolution.
查看更多>>摘要：I analyze a new X-ray image of the youngest supernova remnant(SNR)in the Galaxy,which is the type Ⅰa SNR G1.9+0.3,and reveal a very clear point-symmetrical structure.Since explosion models of type Ⅰa supernovae(SNe Ia)do not form such morphologies,the point-symmetrical morphology must come from the circumstellar material(CSM)into which the ejecta expands.The large-scale point-symmetry that I identify and the known substantial deceleration of the ejecta of SNR Gl.9+0.3 suggest a relatively massive CSM of(≥)1 M⊙.I argue that the most likely explanation is the explosion of this SN Ⅰa into a planetary nebula.The scenario that predicts a large fraction of SN Ⅰa inside PNe(SNIPs)is the core degenerate scenario.Other SN Ⅰa scenarios might lead to only a very small fraction of SNIPs or none at all.
查看更多>>摘要：The Giant Radio Array for Neutrino Detection(GRAND)is a proposed large-scale observatory designed to detect cosmic rays,gamma-rays,and neutrinos with energies exceeding 100 PeV.The GRANDProto300 experiment is proposed as the early stage of the GRAND project,consisting of a hybrid array of radio antennas and scintillator detectors.The latter,as a mature and traditional detector,is used to cross-check the nature of the candidate events selected from radio observations.In this study,we developed a simulation software called G4GRANDProto300,based on the Geant4 software package,to optimize the spacing of the scintillator detector array and to investigate its effective area.The analysis was conducted at various zenith angles under different detector spacings,including 300,500,600,700,and 900 m.Our results indicate that,for large zenith angles used to search for cosmic-ray in the GRAND project,the optimized effective area is with a detector spacing of 500 m.The G4GRANDProto300 software that we developed could be used to further optimize the layout of the particle detector array in future work.
查看更多>>摘要：The thermal gradient is an important factor that causes degradation to the image quality of telescopes.In order to ensure the accurate alignment of the primary focus unit and the primary mirror,the hexapod platform(as a corrector)is investigated in this paper.First,a ground-based telescope with 2.5 m aperture and 3.5 deg field of view is described.The telescope is under construction,and it is expected to be finished in 2023.Second,the hexapod platform with flexure hinges utilized to adjust the primary focus unit is proposed,which is applied as a corrector.Then,the inverse kinematics of the platform is established and an open-loop control system is built based on it.Finally,the cryogenic performance test for the hexapod platform is performed.The experimental results show that the resolution and repeatability of the translation for the hexapod platform can be achieved at the micrometer level.The resolution and repeatability of the rotation can be achieved at the arc-second level.Therefore,the cryogenic performance of the hexapod platform can meet the optical imaging requirements of the wide-field ground-based telescope.The kinematic analysis and cryogenic performance tests in the paper provide a technical reference for the precise alignment of the primary focus unit and the primary mirror,which can improve the imaging quality of the telescope.
查看更多>>摘要：Gamma-ray bursts(GRBs)are among the brightest objects in the Universe and,hence,can be observed up to a very high redshift.Properly calibrated empirical correlations between intensity and spectral correlations of GRBs can be used to estimate the cosmological parameters.However,the possibility of the evolution of GRBs with redshift is a long-standing puzzle.In this work,we used 162 long-duration GRBs to determine whether GRBs below and above a certain redshift have different properties.The GRBs are split into two groups,and we fit the Amati relation for each group separately.Our findings demonstrate that estimations of the Amati parameters for the two groups are substantially dissimilar.We perform simulations to investigate whether the selection effects could cause the difference.Our analysis shows that the differences may be either intrinsic or due to systematic errors in the data,and the selection effects are not their true origin.However,in-depth analysis with a new data set comprised of 119 long GRBs shows that intrinsic scatter may partly be responsible for such effects.
查看更多>>摘要：Fast radio bursts(FRBs)are short pulses observed in radio frequencies usually originating from cosmological distances.The discovery of FRB 200428 and its X-ray counterpart from the Galactic magnetar SGR J1935+2154 suggests that at least some FRBs can be generated by magnetars.However,the majority of X-ray bursts from magnetars are not associated with radio emission.The fact that only in rare cases can an FRB be generated raises the question regarding the special triggering mechanism of FRBs.Here we report long time spin evolution of SGR J1935+2154 until the end of 2022.According to v and v,the spin evolution of SGR J1935+2154 could be divided into two stages.The first stage evolves relatively steady evolution until 2020 April 27.After the burst activity in 2020,the spin of SGR J1935+2154 shows strong variations,especially for v.After the burst activity in 2022 October,a new spin-down glitch with Δv/v=(-7.2±0.6)×10-6 is detected around MJD 59876,which is the second event in SGR J1935+2154.At the end,spin frequency and pulse profile do not show variations around the time of FRB 200428 and radio bursts 221014 and 221021,which supply strong clues to constrain the trigger mechanism of FRBs or radio bursts.
查看更多>>摘要：With the development of cutting-edge multi-object spectrographs,fiber positioners located in the focal plane are being scaled down in size,and miniature hollow-cup Permanent Magnet motors are now being considered as a suitable replacement for Faulhaber Precistep stepper motors.However,the small electrical time constant of such coreless motors poses a challenge,as the problem of severe commutation torque ripple in a fiber positioner running a position loop has been tricky.To overcome this challenge,it is advised to increase the Pulse Width Modulation(PWM)frequency as much as possible to mitigate the effects of the current fluctuation.This must be done while ensuring adequate resolution of the PWM generator.By employing a voltage open-loop field-oriented control based on a modulation frequency of 1 MHz,the drive current only costs 25 mA under a 3.3 V power supply.The sine degree of phase current is immaculate,and the repeat positioning accuracy can reach 2 μm.Moreover,it is possible to further shrink the bill of devices and the layout area of the Printed Circuit Board,especially in size-sensitive applications.This device has been developed under the new generation of The Large Sky Area Multi-Object Fiber Spectroscopic Telescope.
查看更多>>摘要：We present a study of low surface brightness galaxies(LSBGs)selected by fitting the images for all the galaxies in α.40 SDSS DR7 sample with two kinds of single-component models and two kinds of two-component models(disk+bulge):single exponential,single sérsic,exponential+deVaucular(exp+deV),and exponential+sérsic(exp+ser).Under the criteria of the B band disk central surface brightness μ0,disk(B)≥ 22.5 mag arcsec-2 and the axis ratio b/a＞0.3,we selected four none-edge-on LSBG samples from each of the models which contain 1105,1038,207,and 75 galaxies,respectively.There are 756 galaxies in common between LSBGs selected by exponential and sérsic models,corresponding to 68.42％of LSBGs selected by the exponential model and 72.83％of LSBGs selected by the sérsic model,the rest of the discrepancy is due to the difference in obtaining μ0 between the exponential and sérsic models.Based on the fitting,in the range of 0.5 ≤ n≤1.5,the relation of μ0 from two models can be written as μ0,sérsic-μ0,exp=-1.34(n-1).The LSBGs selected by disk+bulge models(LSBG_2comps)are more massive than LSBGs selected by single-component models(LSBG_lcomp),and also show a larger disk component.Though the bulges in the majority of our LSBG_2comps are not prominent,more than 60％of our LSBG_2comps will not be selected if we adopt a single-component model only.We also identified 31 giant low surface brightness galaxies(gLSBGs)from LSBG_2comps.They are located at the same region in the color-magnitude diagram as other gLSBGs.After we compared different criteria of gLSBGs selection,we find that for gas-rich LSBGs,M*＞1010M⊙ is the best to distinguish between gLSBGs and normal LSBGs with bulge.
查看更多>>摘要：The simulation of radio frequency interference(RFI)cancellation by applying a spatial filtering technique for phased array feed(PAF)is presented.In order to better reflect the characteristics of PAF,a new signal model is to add the coupling coefficient among elements of PAF to the conventional array signal model.Then the subspace projection(SP)algorithm is used to cancel RFI from the correlation matrix of the signal,and finally,the 2D power image is drawn.The power variation of signal-of-interest direction and RFI direction before and after using the SP algorithm is analyzed.The new signal model and simulation strategy can be used to test and verify the beamformer.
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