摘要:A multi-function digital baseband data acquisition system is designed for the sampling,distribution and recording of wide-band multi-channel astronomical signals.The system hires a SNAP2board as a digital baseband converter to digitize,channelize and packetize the received signal.It can be configured dynamically from a single channel to eight channels with a maximum bandwidth of 4096 MHz.Eight parallel HASHPIPE instances run on four servers,each carrying two NVMe SSD cards,achieving a total continuous write rate of 8 GB s^(-1).Data are recorded in the standard VDIF file format.The system is deployed on a 25-meter radio telescope to verify its functionality based on pulsar observations.Our results indicate that during the 30-minute observation period,the system achieved zero data loss at a data recording rate of 1 GB s^(-1) on a single server.The system will serve as a verification platform for testing the functions of the QTT(Qi Tai radio Telescope)digital backend system.In addition,it can be used as a baseband/VLBI(Very Long Baseline Interferometry)recorder or D-F-engine of correlator/beamformer as well.
摘要:We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR).Employment of external calibrator instead of internal calibrator commonly used in current EoR experiments allows removing instrumental effects such as beam pattern,receiver gain and instability of the system if the conventional three-position switch measurements are implemented in a short time interval.Furthermore,in the new design the antenna system is placed in an underground anechoic chamber with an open/closing ceiling to maximally reduce the environmental effect such as RFI and ground radiation/reflection.It appears that three of the four external calibrators proposed in this paper,including two indoor artificial transmitters and one outdoor celestial radiation(the Galactic polarization),fail to meet our purpose.Diurnal motion of the Galactic diffuse emission turns out to be the most probable source as an external calibrator,for which we have discussed the observational strategy and the algorithm of extracting the EoR signal.
摘要:The line-of-sight peculiar velocities are good indicators of the gravitational fluctuation of the density field.Techniques have been developed to extract cosmological information from the peculiar velocities in order to test cosmological models.These techniques include measuring cosmic flow,measuring two-point correlation and power spectrum of the peculiar velocity fields,and reconstructing the density field using peculiar velocities.However,some measurements from these techniques are biased due to the nonGaussianity of the estimated peculiar velocities.Therefore,we rely on the 2MTF survey to explore a power transform that can Gaussianize the estimated peculiar velocities.We find a tight linear relation between the transformation parameters and the measurement errors of log-distance ratio.To show an example for the implementation of Gaussianized peculiar velocities in cosmology,we develop a bulk flow estimator and estimate bulk flow from the Gaussianized peculiar velocities.We use 2MTF mocks to test the algorithm,and we find the algorithm yields unbiased measurements.We also find this technique gives smaller measurement errors compared to other techniques.In Galactic coordinates,at the depth of 30 h^(-1)Mpc,we measure a bulk flow of 332±27 km s^(-1) in the direction(l,b)=(293°±5°,13°±4°).The measurement is consistent with theΛCDM prediction.
摘要:The Spitzer Extended Deep Survey(SEDS)as a deep and wide mid-infrared(MIR)survey project provides a sample of 500000+sources spreading 1.46 square degree and a depth of 26 AB mag(3σ).Combining with the previous available data,we build a PSF-matched multi-wavelength photometry catalog from u band to 8μm.We fit the SEDS galaxies spectral energy distributions by the local galaxy templates.The results show that the SEDS galaxy can be fitted well,indicating the high redshift galaxy(z~1)shares the same templates with the local galaxies.This study would facilitate the further study of the galaxy luminosity and high redshift mass function.
摘要:LAMOST Data Release 5,covering 17000 deg^(2) from-10°to 80°in declination,contains 9 million co-added low-resolution spectra of celestial objects,each spectrum combined from repeat exposure of two to tens of times during Oct 2011 to Jun 2017.In this paper,we present the spectra of individual exposures for all the objects in LAMOST Data Release 5.For each spectrum,the equivalent width of 60lines from 11 different elements are calculated with a new method combining the actual line core and fitted line wings.For stars earlier than F type,the Balmer lines are fitted with both emission and absorption profiles once two components are detected.Radial velocity of each individual exposure is measured by minimizing χ^(2) between the spectrum and its best template.The database for equivalent widths of spectral lines and radial velocities of individual spectra are available online.Radial velocity uncertainties with different stellar type and signal-to-noise ratio are quantified by comparing different exposure of the same objects.We notice that the radial velocity uncertainty depends on the time lag between observations.For stars observed in the same day and with signal-to-noise ratio higher than 20,the radial velocity uncertainty is below 5 km s^(-1),and increases to 10 km s^(-1) for stars observed in different nights.
摘要:Measuring weak lensing cosmic magnification signal is very challenging due to the overwhelming intrinsic clustering in the observed galaxy distribution.In this paper,we modify the Internal Linear Combination(ILC)method to reconstruct the lensing signal with an extra constraint to suppress the intrinsic clustering.To quantify the performance,we construct a realistic galaxy catalogue for the LSST-like photometric survey,covering 20000 deg^(2) with mean source redshift at z_(s)~1.We find that the reconstruction performance depends on the width of the photo-z bin we choose.Due to the correlation between the lensing signal and the source galaxy distribution,the derived signal has smaller systematic bias but larger statistical uncertainty for a narrower photo-z bin.We conclude that the lensing signal reconstruction with the Modified ILC method is unbiased with a statistical uncertainty <5% for bin width Δz^(P)=0.2.
摘要:The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST)started a medianresolution spectroscopic(MRS,R~7500)survey since October 2018.The main scientific goals of MRS,including binary stars,pulsators and other variable stars,were launched with a time-domain spectroscopic survey.However,the systematic errors,including the bias induced from wavelength calibration and the systematic difference between different spectrographs,have to be carefully considered during radial velocity measurement.In this work,we provide a technique to correct the systematics in the wavelength calibration based on the relative radial velocity measurements from LAMOST MRS spectra.We show that,for the stars with multi-epoch spectra,the systematic bias which is induced from the exposures on different nights can be corrected well for LAMOST MRS in each spectrograph.In addition,the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km s^(-1).As a by-product,we also give the constant star candidates^(**),which can be the secondary radial-velocity standard star candidates of LAMOST MRS time-domain surveys.
摘要:Classification of edge-on galaxies is important to astronomical studies due to our Milky Way galaxy being an edge-on galaxy.Edge-on galaxies pose a problem to classification due to their less overall brightness levels and smaller numbers of pixels.In the current work,a novel technique for the classification of edge-on galaxies has been developed.This technique is based on the mathematical treatment of galaxy brightness data from their images.A special treatment for galaxies’brightness data is developed to enhance faint galaxies and eliminate adverse effects of high brightness backgrounds as well as adverse effects of background bright stars.A novel slimness weighting factor is developed to classify edge-on galaxies based on their slimness.The technique has the capacity to be optimized for different catalogs with different brightness levels.In the current work,the developed technique is optimized for the EFIGI catalog and is trained using a set of 1800 galaxies from this catalog.Upon classification of the full set of 4458 galaxies from the EFIGI catalog,an accuracy of 97.5% has been achieved,with an average processing time of about 0.26 seconds per galaxy on an average laptop.
摘要:To evaluate performances of a back-illuminated scientific CMOS(sCMOS)camera for astronomical observations,comparison tests between Andor Marana s CMOS and Andor iKon-L 936 CCD cameras were conducted in a laboratory and on a telescope.The laboratory tests showed that the readout noise of the sCMOS camera is about half lower,the dark current is about 17 times higher,the dynamic range is lower in the 12-bit setting and higher in the 16-bit setting,and the linearity and bias stability are comparable relative to those of the CCD camera.In field tests,we observed the open cluster M67 with the sCMOS and CCD cameras on a 60 cm telescope.Unlike the CCD camera,the sCMOS camera has a dual-amplifier architecture.Since a 16-bit image of the sCMOS camera is composed of two 12-bit images sampled with 12-bit high gain and low gain amplifiers simultaneously,it is not real 16-bit output data.The evaluation tests indicated that the dual-amplifier architecture of the s CMOS camera leads to a decline of photometric stability by about six times around specific pixel counts.For photometry of bright objects with similar magnitudes that require high frame rates,the s CMOS camera under 12-bit setting is a good choice.Therefore,the sCMOS camera is fitted with survey observations of variable objects requiring short exposure times,mostly less than 1 s,and high frame rates.It also satisfies the requirements for an offset guiding instrument owing to its high sensitivity,high temporal resolution and high stability.
摘要:The discovery of pulsars is of great significance in the field of physics and astronomy.As the astronomical equipment produces a large number of pulsar data,an algorithm for automatically identifying pulsars becomes urgent.We propose a deep learning framework for pulsar recognition.In response to the extreme imbalance between positive and negative examples and the hard negative sample issue presented in the High Time Resolution Universe Medlat Training Data,there are two coping strategies in our framework:the smart under-sampling and the improved loss function.We also apply the early-fusion strategy to integrate features obtained from different attributes before classification to improve the performance.To our best knowledge,this is the first study that integrates these strategies and techniques in pulsar recognition.The experiment results show that our framework outperforms previous works with respect to either the training time or F 1 score.We can not only speed up the training time by 10×compared with the state-ofthe-art work,but also get a competitive result in terms of F1 score.
摘要:The largest and most detailed map of the distribution of dark matter in the Universe has been recently created by the Dark Energy Survey(DES)team.The distribution was found to be slightly(by a few percent)smoother and less clumpy than predicted by general relativity.This result was considered as a hint of some new physical laws.In the present paper we offer a relatively simple model that could explain the above result without resorting to any new physical laws.The model deals with the dynamics of a system consisting of a large number of gravitating neutral particles,whose mass is equal to the mass of hydrogen atoms.The central point of the model is a partial inhibition of the gravitation for a relatively small subsystem of the entire system.It would be sufficient for this subsystem to constitute just about a few percent of the total ensemble of particles for explaining the few percent more smooth distribution of dark matter(observed by the DES team)compared to the prediction of general relativity.The most viable candidate for the dark matter particles in this model is the second flavor of hydrogen atoms(SFHA)that has only S-states and therefore does not couple to the electric dipole radiation or even to higher multipole radiation,so that the SFHA is practically dark.The SFHA has experimental confirmation from atomic experiments,it does not go beyond the Standard Model,it is based on standard quantum mechanics and it explains puzzling astrophysical observations of the redshifted line 21 cm from the early Universe.Thus,our model explaining the DES result of a little too smooth distribution of dark matter without resorting to any new physical laws seems to be self-consistent.
摘要:In the paper“In Search for Infall Motion in molecular clumps II:HCO+(1-0)and HCN(1-0)Observations toward a Sub-sample of Infall Candidates”by Yang etal.(RAA 2020 Vol.20 No.8,115(14pp),doi:10.1088/1674C4527/20/8/115),the formula(4)is incorrectly reproduced from Myers et al.(1996).
摘要:The Fourier transform spectrometer(FTS)is a core instrument for solar observation with high spectral resolution,especially in the infrared.The Infrared System for the Accurate Measurement of Solar Magnetic Field(AIMS),working at 10-13μm,will use an FTS to observe the solar spectrum.The Bruker IFS-125 HR,which meets the spectral resolution requirement of AIMS but simply equips with a point source detector,is employed to carry out preliminary experiment for AIMS.A sun-light feeding experimental system is further developed.Several experiments are taken with them during 2018 and 2019 to observe the solar spectrum in the visible and near infrared wavelength,respectively.We also proposed an inversion method to retrieve the solar spectrum from the observed interferogram and compared it with the standard solar spectrum atlas.Although there is a wavelength limitation due to the present sun-light feeding system,the results in the wavelength band from 0.45-1.0μm and 1.0-2.2μm show a good consistency with the solar spectrum atlas,indicating the validity of our observing configuration,the data analysis method and the potential to work in longer wavelength.The work provided valuable experience for the AIMS not only for the operation of an FTS but also for the development of its scientific data processing software.
摘要:In this paper,we carried out multiwavelength observations of three recurring jets on 2014November 7.The jets originated from the same region at the edge of AR 12205 and propagated along the same coronal loop.The eruptions were generated by magnetic reconnection,which is evidenced by continuous magnetic cancellation at the jet base.The projected initial velocity of jet2 is~402 km s^(-1).The accelerations in the ascending and descending phases of jet2 are not consistent,the former is considerably larger than the value of g_(⊙) at the solar surface,while the latter is lower than g_(⊙).There are two possible candidates of extra forces acting on jet2 during its propagation.One is the downward gas pressure from jet1when it falls back and meets with jet2.The other is the viscous drag from the surrounding plasma during the fast propagation of jet2.As a contrast,the accelerations of jet3 in the rising and falling phases are constant,implying that the propagation of jet3 is not significantly influenced by extra forces.
摘要:In the Sun and Sun-like stars,it is believed that cycles of the large-scale magnetic field are produced due to the existence of differential rotation and helicity in the plasma flows in their convection zones(CZs).Hence,it is expected that for each star,there is a critical dynamo number for the operation of a large-scale dynamo.As a star slows down,it is expected that the large-scale dynamo ceases to operate above a critical rotation period.In our study,we explore the possibility of the operation of the dynamo in the subcritical region using the Babcock–Leighton type kinematic dynamo model.In some parameter regimes,we find that the dynamo shows hysteresis behavior,i.e.,two dynamo solutions are possible depending on the initial parameters—decaying solution if starting with weak field and strong oscillatory solution(subcritical dynamo)when starting with a strong field.However,under large fluctuations in the dynamo parameter,the subcritical dynamo mode is unstable in some parameter regimes.Therefore,our study supports the possible existence of subcritical dynamo in some stars which was previously demonstrated in a mean-field dynamo model with distributedαand MHD turbulent dynamo simulations.
摘要:Data archiving is one of the most critical issues for modern astronomical observations.With the development of a new generation of radio telescopes,the transfer and archiving of massive remote data have become urgent problems to be solved.Herein,we present a practical and robust file-level flow-control approach,called the Unlimited Sliding-Window(USW),by referring to the classic flow-control method in the TCP protocol.Based on the USW and the Next Generation Archive System(NGAS)developed for the Murchison Widefield Array telescope,we further implemented an enhanced archive system(ENGAS)using ZeroMQ middleware.The ENGAS substantially improves the transfer performance and ensures the integrity of transferred files.In the tests,the ENGAS is approximately three to twelve times faster than the NGAS and can fully utilize the bandwidth of network links.Thus,for archiving radio observation data,the ENGAS reduces the communication time,improves the bandwidth utilization,and solves the remote synchronous archiving of data from observatories such as Mingantu spectral radioheliograph.It also provides a better reference for the future construction of the Square Kilometer Array(SKA)Science Regional Center.
摘要:We monitored BL Lacertae in the B,V,R and I bands for 14 nights during the period of 2016-2018.The source showed significant intra-day variability on 12 nights.We performed color-magnitude analysis and found that the source exhibited bluer-when-brighter chromatism.This bluer-when-brighter behavior is at least partly caused by the larger variation amplitude at shorter wavelengths.The variations at different wavelengths are correlated well and show no inter-band time lag.
摘要:As the advent of precision cosmology,the Hubble constant(H0)inferred from the Lambda Cold Dark Matter fit to the Cosmic Microwave Background data is increasingly in tension with the measurements from the local distance ladder.To approach its real value,we need more independent methods to measure,or to make constraint of,the Hubble constant.In this paper,we apply a plain method,which is merely based on the Friedman-Lema??tre-Robertson-Walker cosmology together with geometrical relations,to constrain the Hubble constant by proper motions of radio components observed in AGN twin-jets.Under the assumption that the ultimate ejection strengths in both sides of the twin-jet concerned are intrinsically the same,we obtain a lower limit of H0,min=51.5±2.3 km s^(-1)Mpc^(-1) from the measured maximum proper motions of the radio components observed in the twin-jet of NGC 1052.
摘要:In this paper,co-phasing errors of a segmented primary mirror tiled by hexagonal segments are successfully calculated for the 12-meter Large aperture Optical/infrared Telescope(LOT).Co-phasing errors including out-of-plane errors are simulated separately and comprehensively based on several software simulation platforms.PAOLA simulation results show that the Strehl Ratio(SR)of LOT is larger than 0.8 when the RMS value of tip-tilt obeying a normal distribution is less than 0.018 arcsec,and the SR of LOT is larger than 0.8 when the RMS value of piston obeying a normal distribution is less than 40 nm.Besides,simulation results of Zemax show that the SR of LOT is larger than 0.8 when the RMS value of tip-tilt obeying a normal distribution is less than 0.02 arcsec,and the SR of LOT is larger than 0.8 when the RMS value of piston obeying a normal distribution is less than 40 nm.These simulation results successfully lay a solid foundation for LOT(especially the segmented primary mirror with active optics).
摘要:In-situ observations from the FREJA magnetospheric research satellite and the Fast Auroral SnapshoT satellite have shown that plasma waves are frequently observed in the auroral plasma,which are believed to be fundamentally important in wave energy dissipation and particle energization.However,the effects of a displacement current on these waves have not been examined.Based on the two-fluid theory,we investigate the dispersion relation and polarization properties of fast,Alfven,and slow modes in the presence of a displacement current,and the effects of the displacement current on these waves are also considered.The results show that the wave frequency,polarization,magnetic helicity and other properties for the fast and Alfven modes are highly sensitive to the normalized Alfven velocity v_(A)/c,plasma beta β,and propagation angle θ,while for the slow mode the dependence is minor.In particular,for both fast and Alfven modes,the magnetic helicity is obviously different with and without the displacement current,especially for the Alfven mode with the helicity reversals from right-handed to left-handed when v_(A)/c increases from 0 to 0.3.The charge-neutral condition of both fast and Alfven modes with frequencies larger than the proton cyclotron frequency is invalid in the presence of the displacement current.Moreover,the presence of the displacement current leads to relatively large magnetic compressibility for the Alfven mode and relatively large electron compressibility for the fast mode.These results can be useful for a comprehensive understanding of the wave properties and the physics of particle energization phenomena in auroral plasmas.
摘要:The state of supranuclear matter in compact stars remains puzzling,and it is argued that pulsars could be strangeon stars.The consequences of merging double strangeon stars are worth exploring,especially in the new era of multi-messenger astronomy.To develop the"strangeon kilonova"scenario proposed in Paper I,we make a qualitative description about the evolution of ejecta and light curves for merging double strangeon stars.In the hot environment of the merger,the strangeon nuggets ejected by tidal disruption and hydrodynamical squeezing would suffer from evaporation,in which process particles,such as strangeons,neutrons and protons,are emitted.Taking into account both the evaporation of strangeon nuggets and the decay of strangeons,most of the strangeon nuggets would turn into neutrons and protons,within dozens of milliseconds after being ejected.The evaporation rates of different particles depend on temperature,and we find that the ejecta could end up with two components,with high and low opacity respectively.The high opacity component would be in the directions around the equatorial plane,and the low opacity component would be in a broad range of angular directions.The bolometric light curves show that the spin-down power of the long-lived remnant would account for the whole emission of kilonova AT2017gfo associated with GW170817,if the total ejected mass 10^(-3)M⊙.The detailed picture of merging double strangeon stars is expected to be tested by future numerical simulations.
摘要:Gamma-ray bursts(GRBs)are brief,intense,gamma-ray flashes in the universe,lasting from a few milliseconds to a few thousand seconds.For short gamma-ray bursts(sGRBs)with duration less than 2 seconds,the isotropic energy(E_(iso))function may be more scientifically meaningful and accurately measured than the luminosity(Lp)function.In this work we construct,for the first time,the isotropic energy function of s GRBs and estimate their formation rate.First,we derive the L_(p)-E_(p) correlation using 22 s GRBs with known redshifts and well-measured spectra and estimate the pseduo redshifts of 334 Fermi s GRBs.Then,we adopt the Lynden-Bell c-method to study isotropic energy functions and formation rate of s GRBs without any assumption.A strong evolution of isotropic energy E_(iso)∝(1+z)^(5.79) is found,which is comparable to that between L_(p) and z.After removing effect of the cosmic evolution,the isotropic energy function can be reasonably fitted by a broken power law,which is φ(E_(iso,0))∝E_(iso,0)^(-0.045) for dim sGRBs andφ(E_(iso,0))∝E_(iso,0)^(-1.11) for bright sGRBs,with the break energy 4.92×10^(49)erg.We obtain the local formation rate of s GRBs is about 17.43 events Gpc^(-3)yr^(-1).If assuming a beaming angle is 6° to 26°,the local formation rate including off-axis s GRBs is estimated as ρ_(0,all)=155.79-3202.35 events Gpc^(-3)yr^(-1).
摘要:We present Arecibo 327 MHz confirmation and follow-up studies of seven new pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope(FAST).These pulsars are discovered in a pilot program of the Commensal Radio Astronomy FAST Survey(CRAFTS)with the ultra-widebandwidth commissioning receiver.Five of them are normal pulsars and two are extreme nulling slow pulsars.PSR J2111+2132’s dispersion measure(DM:78.5 pc cm^(-3))is above the upper limits of the two Galactic free electron density models,NE2001 and YMW16,and PSR J2057+2133’s position is out of the Scutum-Crux Arm,making them uniquely useful for improving the Galactic free electron density model in their directions.We present a detailed single pulse analysis for the slow nulling pulsars.We show evidence that PSR J2323+1214’s main pulse component follows a non-Poisson distribution and marginal evidence for a sub-pulse-drift or recurrent period of 32.3±0.4 rotations from PSR J0539+0013.We discuss the implication of our finding to the pulsar radiation mechanism.
摘要:The tension between luminous matter and dynamical matter has long been an interesting and controversial topic in the investigation of galaxies.This is particularly true when we study spiral galaxies for which we have high quality observations of rotation curves.The solutions to the tension are proposed in two different approaches,one is the dark matter hypothesis and the other is MOdified Newtonian Dynamics(MOND)theory.When we test the solutions by using observational data of rotation curves,the controversy arises when we apply them to both low surface brightness(LSB)galaxies and high surface brightness(HSB)galaxies.Usually one likes to use the rotation curves of LSB galaxies,since dark matter is needed or the Newtonian acceleration falls below the characteristic acceleration a0 in most regions of such galaxies,even near their centers.But for HSB galaxies,dark matter is needed or Newtonian acceleration falls below the characteristic acceleration a0 only in their outer regions so it is helpful to single out HSB galaxies from some large sample to test the solutions.To this end,we employ a sub-sample of the rotation curves consisting of45 non-bulgy HSB galaxies selected from the Spitzer Photometry and Accurate Rotation Curves(SPARC)database to test two dark halo models(NFW and Burkert)and MOND.We find that,among the three models,the core-dominated Burkert halo model(χ_(v)^(2)=1.00)provides a better description of the observed data than the NFW model(χ_(v)^(2)=1.44)or MOND model(χ_(v)^(2)=1.87).This is not consistent with the most recent numerical simulations,which tend to favor some cuspy density profiles for HSB galaxies.For MOND,when we take a0 as a free parameter,there is no obvious correlation between a0 and disk central surface brightness at 3.6μm of these HSB spiral galaxies,which is in line with the basic assumption of MOND that a0 should be a universal constant,but is surprisingly not consistent with the results when LSB galaxies are included.Furthermore,our fittings give a0 an average value of(0.74±0.45)×10^(-8)cm s^(-2),which only marginally supports the standard value of a0(1.21×10^(-8)cm s^(-2)).Since the standard value of a0 is strongly supported when both HSB and LSB galaxies are included in the large SPARC sample,we conclude that our slightly smaller value of a0 cannot be explained by the so called external field effect in MOND theory.
摘要:A simple method for measuring grating groove density as well as its position and orientation is proposed based on the idea of ERT(Experimental Ray Tracing).Conventional methods only measure grating groove density with accuracy limited by its rotary stage and goniometer.The method proposed in the paper utilizes linear guides which could be calibrated to much higher accuracy.It is applicable to gratings of arbitrary surface profile or mosaic of a group of various gratings.Various measurement error sources are simulated by the Monte Carlo method and the results show high accuracy capability of grating parameters identification.A verification testing is performed.The accuracy dependency on main configuration parameters is evaluated.A method to expand measurement range by double wavelength is also discussed.
摘要:We present Chandra observations of the nearby polar ring galaxy NGC 660 to study its X-ray source population.Based on our analysis,we detected a total of 23 X-ray sources in the 0.5-8 keV band,with luminosities ranging from~1037 to~1039 erg s-1.Twenty-two of these sources are located offnuclear and have luminosities below the ultraluminous X-ray source(ULX)threshold value of L0.5-8 keV<1039 erg s-1,suggesting that they are likely to be X-ray binary(XRB)candidates.The remaining source is located at the center of the galaxy,suggesting it is an active galactic nucleus(AGN).However,we estimated that four of the detected sources could be associated with background objects.Based on the source count rates in each of the Chandra observations,we found evidence for variability in nine of the 23 sources,including the AGN.However,further investigation with spectral analysis suggested no significant differences in the AGN luminosities between the observations.The X-ray luminosity distribution of the galaxy was found to be generally lower than that expected from previous studies on star forming and collisional ring galaxies.No ULX was also detected in the galaxy,in contrast with what was expected from the galaxy’s SFR and metallicity(i.e.,SFR=14.43±0.19 M⊙yr-1 and Z=0.94±0.01 Z⊙,respectively).These results suggest a deficit in the X-ray sources detected.Based on source hardness ratio distribution,we found evidence that the fainter sources have a harder source spectrum,indicating higher absorption.This further suggests that there could be more X-ray sources that were not detected in the galaxy due to significant obscuration.
摘要:We present the first evidence for the binarity of four targets in the TESS field.The temperatures are estimated by SED analysis and the orbital periods are determined.The TESS light curves of the systems are analyzed and the orbital and the absolute parameters are derived.The targets are also compared to wellstudied binary systems with the same morphological type and their evolutionary states are discussed.Our results indicate that the stars belong to the class of eclipsing detached binary systems.
摘要:Estimating ages for a large number of giants is of great importance for studying Galactic evolution.In this work,we determine stellar ages and masses for 556007 giants from LAMOST Data Release 5(DR5)with empirical relations estimated from chemical[C/N]abundance ratios.Our sample reveals the two wellknown sequences in the age-[α/M]relation.The high-αsequence is composed of stars older than 8 Gyr and low-αsequence is composed of stars with age ranging from 0 Gyr to 13.8 Gyr.Our sample also shows a flat age-[M/H]relation up until 12 Gyr.We compare these distributions with Galactic Chemical Evolution models for reference.When looking at the spatial distribution of stars in 2 Gyr age bins,we find that young stars are concentrated towards the Galactic plane and older stars extend to higher height above and below the disk.We find a smooth transition of median Galactic height for different age bins,which suggests a strong age-dependence on Galactic scale height.
摘要:Abstract:Binary stars play an important role in the evolution of stellar populations.The intrinsic binary fraction(fbin)of O and B-type(OB)stars in LAMOST DR5 was investigated in this work.We employed a cross-correlation approach to estimate relative radial velocities for each of the stellar spectra.The algorithm described by Sana et al.(2013)was implemented and several simulations were made to assess the performance of the approach.The binary fraction of the OB stars is estimated through comparing the unidistribution between observations and simulations with the Kolmogorov-Smirnov tests.Simulations show that it is reliable for stars most of whom have six,seven and eight repeated observations.The uncertainty of orbital parameters of binarity becomes larger when observational frequencies decrease.By adopting the fixed power exponents ofπ=-0.45 andκ=-1 for period and mass ratio distributions,respectively,we obtain that fbin=0.4-0.06+0.05 for the samples with more than three observations.When we consider the full samples with at least two observations,the binary fraction turns out to be 0.37-0.03+0.03.These two results are consistent with each other in 1σ.
摘要:We present a GPU-accelerated cosmological simulation code,PhotoNs-GPU,based on an algorithm of Particle Mesh Fast Multipole Method(PM-FMM),and focus on the GPU utilization and optimization.A proper interpolated method for truncated gravity is introduced to speed up the special functions in kernels.We verify the GPU code in mixed precision and different levels of the interpolated method on GPU.A run with single precision is roughly two times faster than double precision for current practical cosmological simulations.But it could induce an unbiased small noise in power spectrum.Compared with the CPU version of PhotoNs and Gadget-2,the efficiency of the new code is significantly improved.Activated all the optimizations on the memory access,kernel functions and concurrency management,the peak performance of our test runs achieves 48%of the theoretical speed and the average performance approaches to~35%on GPU.