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k-Nearest Neighbors for automated classification of celestial objects

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Science china(Physics,Mechanics & Astronomy) 2008年第7期51卷 916-922页

作者： LI LiLi1,2.3, ZhanG YanXia1 & ZHAO YongHeng1 1 national astronomical observatories, Chinese Academy of Sciences,{2.,{2. 2.Department of Physics, Hebei Normal University, Shijiazhuang 050016,{2. 3 Weishanlu Middle School, Tianjin 3002.2.{2. 1. national astronomical observatories Chinese Academy of Sciences Beijing 100012 China 2. Department of Physics Hebei Normal University Shijiazhuang 050016 China 3. Weishanlu Middle School Tianjin 300222 China

The nearest neighbors (NNs) classifiers, especially the k-Nearest Neighbors (kNNs) algorithm, are among the simplest and yet most efficient classification rules and widely used in practice. It is a nonparametric method of pattern recognition. In this paper, k-Nearest Neighbors, one of the most commonly used machine learning methods, work in automatic classification of multi-wavelength astronomical objects. Through the experiment, we conclude that the running speed of the kNN classier is rather fast and the classification accuracy is up to 97.73%. As a result, it is efficient and applicable to discriminate active objects from stars and normal galaxies with this method. The classifiers trained by the kNN method can be used to solve the automated classification problem faced by astronomy and the virtual observatory (VO).

关键词： k-Nearest Neighbors data analysis classification astronomical catalogues

Do Globular Clusters Harbor Black Holes?

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Chinese Journal of Astronomy and Astrophysics 2001年第4期1卷 291-295页

作者： Xian-Zhong Zheng (national astronomical {2., Chinese Academy of Sciences, beijing 100012)E-mail:zxz@*** national astronomical {2. Chinese Academy of Sciences Beijing 100012

It has been firmly established that there exists a tight correlation be- tween the mass of the central black hole and velocity dispersion (or luminosity) in elliptical galaxies, 'pseudobulges' and bulges of galaxies, although the nature of this correlation still remains nuclear. We explore the possibility of extrapolating such a correlation to less massive, spherical systems like globular clusters. In particular, motivated by the apparent success in the globular cluster M15, we present an estimate of the central black hole mass for a number of globular clusters with available velocity dispersion data.

关键词： black hole physics-globular clusters: general - galaxies: general

Evolution of the Level of Sunspot Activity in Solar Cycles I. Evolution in the Descending Phase

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Chinese Journal of Astronomy and Astrophysics 2006年第3期6卷 354-362页

作者： Jia-Long Wang national astronomical {2. Chinese Academy of Sciences Beijing 100012

Taking the 13-point smoothed monthly sunspot number,Ri, and the deviation of the 13 associated monthly sunspot numbers from the smoothed one, Di, as a number-pair describing the global level of sunspot activity, the evolution of the level is statistically studied for the period from the month which is just 48 months before the minimum to the minimum in the descending phase, using the observed data of Solar Cycles 10 to 2.. Our results show （1） for 46 months （94%） of the studied 49 months it is found that for a given month, the distribution of the 13 pairs which come from the 13 solar cycles on a log Ri - Di plane may be fitted by a straight line with a correlation coefficient larger than the critical one at confidence level a = 5%, and for 36 months （73%） the fitting is even better, for a = 1%; （2. time variations of these two parameters and their correlations in the studied period can be described respectively by functions of time, whose main trends may be expressed by a linear or simple curvilinear function; （3） the evolutionary path of the level of sunspot activity may be represented by a logarithmic function as log/~ = 0.704 lnDi - 0.2.1.

关键词： Sun - sunspots - active level evolution

Formation of Solar Delta Active Regions:Twist and Writhe of Magnetic Ropes

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Chinese Journal of Astronomy and Astrophysics 2004年第6期4卷 563-577页

作者： Hong-QiZhang nationalastronomical{2. ChineseAcademyofSciencesBeijing100012

We analyze the process of formation of delta configuration in some well-known super active regions based on photospheric vector magnetogram observations. It is found that the magnetic field in the initial developing stage of some delta active regions shows a potential-like configuration in the solar atmosphere, the magnetic shear develops mainly near the magnetic neutral line with magnetic islands of opposite polarities, and the large-scale photospheric twisted field forming gradually later. Some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2. The magnetic model of kink magnetic ropes, supposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and not entirely consistent with the relationship with magnetic shear in some delta active regions. (3) The proposition is that the large-scale delta active regions are formed from contribution by small-scale non-potential magnetic flux bundles generated in the subatmosphere.

关键词： sun activity sun flares sun magnetic fields

Properties of Broad Band Continuum of Narrow Line Seyfert 1 Galaxies

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Chinese Journal of Astronomy and Astrophysics 2002年第6期2卷 501-524页

作者： Hong-Yan Zhou and Ting-Gui WangCenter for Astrophysics, University of Science and Technology of china, Hefei 2.002.national {2. observatories, Chinese Academic of Science, beijing 100012 national astronomical {2. Chinese Academic of Science Beijing 100012

We have performed a statistical study of the properties of the broad band continuum of Narrow Line Seyfert 1 galaxies (NLS1s) by collecting ratio, infrared, optical and X-ray continuum data from various databases and compared the results with control samples of Broad Line Seyfert 1 galaxies (BLS1s). We find that the fraction (similar to 6%) of Radio Loud (RL) NLS1s is significantly less than that of BLS1s (similar to 13%), which is caused by the lack of radio-very-loud sources in the former. The rarity of RL NLS1s, especially radio-very-loud ones, is consistent with the scenario of small black hole and high accretion rate for NLS1s. Six new radio loud NLS1s are found and five RL NLS1 candidates are presented. In comparison with the BLS1s, the NLS1s tend to have stronger far infrared emission, cooler infrared colors and redder B - K color, which suggests that NLS1s are hosted by dust-richer nuclei. The NLS1s also show steeper soft X-ray spectrum and large soft X-ray to optical flux ratio, while a significant fraction show flat soft X-ray spectra. At least two factors can account for this, absorption and spectral variability. We also perform a correlation analysis between various broad band data. It is found that most correlations identified for NLS1s are also valid for radio quiet BLS1s: (1) the optical colors are anti-correlated with X-ray spectral index; (2. higher optical, Xray and NIR luminosity objects show bluer optical colors and red H - K color; (3) higher luminosity objects show warmer IRAS color; (4) the radio loudness correlates with B - K and X-ray to optical flux ratio. Radio loud objects behave somewhat differently in a few correlations.

关键词： galaxies : active galaxies : Seyfert radiation : continuum

Expectation for the X-ray Galactic Halo

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Chinese Journal of Astronomy and Astrophysics 2002年第2期2卷 112-116页

作者： Zhi-Ying Huo national astronomical observatories, Chinese Academy of Sciences, beijing 100012 Department of Physics, Tianjin Normal University, Tianjin 300073 national astronomical {2. Chinese Academy of Sciences Beijing 100012 []Department of Physics Tianjin Normal University Tianjin 300073

We present an estimate of the strength and spectrum of the X-ray background from the warm gas associated with the Galactic halo. This investigation is motivated primarily by the recent detection of a spatially variable soft X-ray component towards the north Galactic polar cap by Kuntz et al. (2.01), suggesting that the warm gas heated by gravitational shocks of the Galactic halo may produce a significant contribution to the soft X-ray sky. Another purpose of the study is to refine the recent theoretical prediction of the X-ray spectrum from the Galactic halo by Xue (2.01) who adopted an ideal and simple isothermal model for the gas and dark matter distributions of the Galactic halo. We use the universal density profile for the dark matter distributions of the Galactic halo to evaluate the X- ray properties of the warm gas either in hydrostatic equilibrium with, or tracing the underlying gravitational potential of the Galaxy. It has been shown that our prediction is consistent with the measured soft X-ray component towards the north Galactic polar cap if the gas fraction is taken to be^0.005.

关键词： cosmology: diffuse radiation -Galaxy: formation - X-rays: galaxies

Relativist ic Corrections to the Thermal Sunyaev-Zel'dovich Power Spectrum

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Chinese Journal of Astronomy and Astrophysics 2003年第5期3卷 410-414页

作者： Hai-Ning Linational astronomical {2., Chinese Academy of Sciences, beijing 100012 lhnDepartment of Astronomy, beijing Normal University, beijing 100875 national astronomical {2. CAS NAOC Beijing 100012 China []Department of Astronomy Beijing Normal University Beijing 100875

We present a quantitative estimate of the relativistic corrections to thethermal SZ power spectrum produced by the energetic electrons in massive clusters. The correctionsare well within 10% for current experiments with working frequencies below ν 350 GHz. Moreover, the corrections appear to be slightlysmaller at higher l or smaller angular scales. We conclude that there is no need to include therelativistic corrections in the theoretical study of the SZ power spectrum especially at lowfrequencies unless the SZ power spectrum is used for precision cosmology.

关键词： cosmic microwave background cosmology: theory galaxies: clusters: general

Magnetic and Velocity Fields in Flare Regions

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天文研究与技术 1999年第S1期 175-181页

作者： Li Wei 1(beijing astronomical {2., The Chinese Academy of Sciences, beijing 100012,china) 2.national astronomical observatories, The Chinese Academy of Sciences, beijing 100012, china)

This paper reviews observations of the solar magnetic and velocity fields in active regions where flares produce. Our study includes (1) the relationship among the magnetic fields, Doppler velocity fields and flares; (2. the relationship between magnetic shear and flares; (3) the relationship among electric currents, electric helicity and flares; (4) the evidence for non-potential magnetic field.

关键词： Magnetic and Velocity Fields in Flare Regions

Is the Variable X-ray Source in M82.due to Gravitational Lensing?

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Chinese Journal of Astronomy and Astrophysics 2001年第4期1卷 287-290页

作者： Da-Ming Chen (national astronomical {2., Chinese Academy of Sciences, beijing 100012)E-mail:dmchen66@*** national astronomical {2. Chinese Academy of Sciences Beijing 100012

We explore the possibility of attributing the recent discovery of the variable hard X-ray source CXO M82.J095550.2.694047 in M82.to the gravitational magnification by an intervening stellar object along the line of sight acting as a microlens. The duration of the event (> 84 days) allows us to set robust constraints on the mass and location of the microlensing object when combined with the dynamical properties of the Galactic halo, M82.and typical globular clusters. Except for the extremely low probability, the microlensing magnification by MACHO in either the Galactic halo or M82.halo is able to explain the X-ray variability of CXO M82.J095550.2.694047. It is hoped that the lensing hypothesis can be tested soon by measurement of the light curve.

关键词： X-rays: stars - Galaxies: individual: M82.- gravitational lensing