2003, Vol.3, No.3 (June 20, 2003)


Contents

Cosmology
Extragalactic Astronomy
  • Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273   p.203
    Qiu-Sheng Gu, Lei Shi, Shi-Jun Lei, Wen-Hao Liu and Jie-Hao Huang
    Abstract | | PDF file (238 KB) | PS file *.gz  (246 KB) 
  • On the Correlation between Radio Properties and Black Hole Mass of Quasar   p.212
    Xue-Guang Zhang, Ting-Gui Wang and You-Jun Lu
    Abstract | | PDF file (243 KB) | PS file *.gz  (399 KB) 
Stars
  • Cylindrical Jet -- Wind Interaction Model of Gamma-Ray Burst Afterglows  p.225
    Hai-Tao Ma, Yong-Feng Huang, Zi-Gao Dai and Tan Lu
     Abstract | | PDF file (233 KB) | PS file *.gz  (290 KB)  
  • Fibonacci Sequences and the Multiperiodicity of the Variable Star UW Herculis   p.235
    Juan Garcia Escudero 
    Abstract | | PDF file (145 KB) | PS file *.gz  (214 KB) 
The Sun
  • Coronal Flux Rope Equilibria in Closed Magnetic Fields   p.241
    Zhen Wang and You-Qiu Hu
    Abstract | | PDF file (245 KB) | PS file *.gz  (166 KB) 
  • A Fluid Dynamics Approach for the Computation of Nonlinear Force-Free Magnetic Field   p.247
    Jing-Qun Li, Jing-Xiu Wang and Feng-Si Wei   
    Abstract |  | PDF file (790 KB) |  PS file *.gz  (1554 KB) 
  • Distribution of Helical Properties of Solar Magnetic Fields   p.257
    Kirill M. Kuzanyan, Victor G. Lamburt, Hong-Qi Zhang and Shu-Dong Bao   
    Abstract |  | PDF file (182 KB) |  PS file *.gz  (125 KB) 
  • H$\alpha$ Line Polarization in the 2B/X4.8 Flare of 2002 July 23   p.266
    Zhi Xu, N. M. Firstova, Qing-Rong Chen and Cheng Fang   
    Abstract |  | PDF file (464 KB) |  PS file *.gz  (933 KB) 
Instruments, Observational Techniques and Data Processing
  • Automated Separation of Stars and Normal Galaxies Based on Statistical Mixture Modeling with RBF Neural Networks   p.277
    Dong-Mei Qin, Ping Guo, Zhan-Yi Hu and Yong-Heng Zhao
    Abstract | | PDF file (283 KB) | PS file *.gz  (457 KB) 


Abstract

Resolved SZE Cluster Count 

Jia-Yu Tang (1) and Zu-Hui Fan (1,2)

Affiliation:  (1) Department of Astronomy, Peking University, Beijing 100871
               (2) Chinese Academy of Sciences-Peking University Joint Beijing Astrophysics Center, Beijing 100871

E-mail:   tangjy@vega.bac.pku.edu.cn 

Abstract

We study the counts of resolved SZE (Sunyaev-Zel'dovich effect) clusters expected from an interferometric survey in different cosmological models under different conditions. The self-similar universal gas model and Press-Schechter mass function are used. We take the observing frequency to be 90 GHz, and consider two dish diameters, 1.2 m and 2.5 m. We calculate the number density of the galaxy clusters dN/(d$\Omega$ dz) at a high flux limit $S_{\nu}^{\rm lim}$ =100 mJy and at a relative low $S_{\nu}^{\rm lim}$ =10 mJy. The total numbers of SZE clusters N in two low-$\Omega$0 models are compared. The results show that the influence of the resolved effect depends not only on D, but also on $S_{\nu}^{\rm lim}$: at a given D, the effect is more significant for a high than for a low $S_{\nu}^{\rm lim}$. Also, the resolved effect for a flat universe is more impressive than that for an open universe. For D=1.2 m and $S_{\nu}^{\rm lim}$=10 mJy, the resolved effect is very weak. Considering the designed interferometers which will be used to survey SZE clusters, we find that the resolved effect is insignificant when estimating the expected yield of the SZE cluster surveys.

Key words: cosmology: theory --- galaxy: cluster: general ---large-scale structure of universe

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Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273   

Qiu-Sheng Gu, Lei Shi, Shi-Jun Lei, Wen-Hao Liu and Jie-Hao Huang

 Affiliation: Department of Astronomy, Nanjing University, Nanjing 210093

E-mail:  qsgu@nju.edu.cn     (corresponding author)

Abstract

We present spectrophotometric results of the Seyfert~2 galaxy NGC~2273. The presence of high-order Balmer absorption lines (H8, H9, H10) and weak equivalent widths of CaII K $\lambda$3933, CN $\lambda$4200, G-band $\lambda$4300 and MgIb $\lambda$5173 clearly indicate recent star-forming activity in the nuclear region. Using a simple stellar population synthesis model, we find that for the best fit, the contributions of a power-law featureless continuum, an intermediate-age ($\sim$ 108 yr) and an old (>109 yr) stellar population to the total light at the reference normalization wavelength are 10.0%, 33.4% and 56.6%, respectively. The existence of recent starburst activity is also consistent with its high far-infrared luminosity ( $\log L_{\rm FIR}/L_{\odot} = 9.9$), its infrared color indexes [$\alpha$(25, 60) = 1.81 and $\alpha$(60, 100)= 0.79, typical values for Seyfert galaxies with circumnuclear starburst], and its q-value (2.23, ratio of infrared to radio flux, very similar to that of normal spirals and starburst galaxies). Byrd et al. have suggested that NGC 2273 might have interacted with NGC 2273B in less than 109 yr ago, so the starburst activity in this galaxy could have been triggered by tidal interaction, as indicated in recent numerical simulations.

Key words: galaxies: Seyfert --- galaxies: starburst --- galaxies: stellar content

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On the Correlation between Radio Properties and Black Hole Mass of Quasars

Xue-Guang Zhang, Ting-Gui Wang and You-Jun Lu 

 Affiliation:  Center for Astrophysics, and Department of Astronomy and Applied Physics,
University of Science and Technology of China, Hefei 230026

E-mail:  zxg@mail.ustc.edu.cn  

Abstract

The question whether the radio properties of quasars are related to the mass of the central black hole or the accretion rate is important for our understanding of the formation of relativistic jets, but no consensus has been reached from statistical analyses. Using two large quasar samples, one radio-selected, one optical-selected, we re-examined these relations and find that previous differences between radio- and optical- selected samples can be ascribed, at least partly, to the effect of the narrow line component. All previous claimed correlations are much weaker, if exist at all.

Key words: galaxies: active --- galaxies: nuclei --- quasars --- black hole

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Cylindrical Jet -- Wind Interaction Model of Gamma-Ray Burst Afterglows

Hai-Tao Ma, Yong-Feng Huang, Zi-Gao Dai and Tan Lu

 Affiliation: Department of Astronomy, Nanjing University, Nanjing 210093
                          
E-mail:   hyf@nju.edu.cn    (corresponding author)

Abstract

Observations on relativistic jets in radio galaxies, active galactic nuclei, and ``microquasars'' revealed that many of these outflows are cylindrical, not conical. So it is worthwhile to investigate the evolution of cylindrical jets in gamma-ray bursts. We discuss afterglows from cylindrical jets in a wind environment. Numerical results as well as analytic solutions in some special cases are presented. Our light curves are steeper compared to those in the homogeneous interstellar medium case, carefully considered by Cheng, Huang & Lu. We conclude that some afterglows, used to be interpreted as isotropic fireballs in a wind environment, can be fitted as well by cylindrical jets interacting with a wind.

Key words: gamma-rays: bursts --- ISM: jets and outflows --- radiation mechanisms: non-thermal --- stars: neutron

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Fibonacci Sequences and the Multiperiodicity of the Variable Star UW Herculis
Juan Garcia Escudero

Affiliation:   Departamento de F\'{\i}sica, Universidad de Oviedo, 33007 Oviedo, Spain

E-mail:   jjge@pinon.ccu.uniovi.es 
  
Abstract

We present an application of the methods recently developed for the study of quasicrystal structures to the analysis of multiperiodicity of semiregular variables. A light curve analysis of UW Her shows frequencies that can be included within the general scheme characterizing the Fourier spectra of Fibonacci quasiperiodic sequences. The analysed data come from the BAA Variable Star Section computerised archive.

Key words: stars: variables

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Coronal Flux Rope Equilibria in Closed Magnetic Fields

Zhen Wang and You-Qiu Hu

Affiliation: School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026
                         
E-mail:  huyq@ustc.edu.cn    (corresponding author)

Abstract

Using a 2.5-dimensional ideal MHD model in Cartesian coordinates, we investigate the equilibrium properties of coronal magnetic flux ropes in background magnetic fields that are completely closed. The background fields are produced by a dipole, a quadrupole, and an octapole, respectively, located below the photosphere at the same depth. A magnetic flux rope is then launched from below the photosphere, and its magnetic properties, i.e., the annular magnetic flux $\Phi_p$ and the axial magnetic flux $\Phi_z$, are controlled by a single emergence parameter. The whole system eventually evolves into equilibrium, and the resultant flux rope is characterized by three geometrical parameters: the height of the rope axis, the half-width of the rope, and the length of the vertical current sheet below the rope. It is found that the geometrical parameters increase monotonically and continuously with increasing $\Phi_p$ and $\Phi_z$: no catastrophe occurs. Moreover, there exists a steep segment in the profiles of the geometrical parameters versus either $\Phi_p$ or $\Phi_z$, and the faster the background field decays with height, the larger both the gradient and the growth amplitude within the steep segment will be.

Key words: Sun: magnetic fields --- Sun: corona

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A Fluid Dynamics Approach for the Computation of Nonlinear Force-Free Magnetic Field

Jing-Qun Li (1,2), Jing-Xiu Wang (1) and Feng-Si Wei (2)
Affiliation: (1)  National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
             (2) Center for Space Science and Applied Researches, Chinese Academy of Sciences, Beijing 100080
                    
E-mail:   lee@ourstar.bao.ac.cn  (corresponding author)

Abstract

Inspired by the analogy between the magnetic field and velocity field of incompressible fluid flow, we propose a fluid dynamics approach for computing nonlinear force-free magnetic fields. This method has the advantage that the divergence-free condition is automatically satisfied, which is a sticky issue for many other algorithms, and we can take advantage of modern high resolution algorithms to process the force-free magnetic field. Several tests have been made based on the well-known analytic solution proposed by Low & Lou. The numerical results are in satisfactory agreement with the analytic ones. It is suggested that the newly proposed method is promising in extrapolating the active region or the whole sun magnetic fields in the solar atmosphere based on the observed vector magnetic field on the photosphere.

Key words: Sun: magnetic fields --- MHD --- methods: numberical

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Distribution of Helical Properties of Solar Magnetic Fields

Kirill M. Kuzanyan (1,2),Victor G. Lamburt (2,3), Hong-Qi Zhang (1) and Shu-Dong Bao(1)

 Affiliation:  (1) National Astronomical Observatories, CAS (NAOC), Beijing 100012, China
              (2) IZMIRAN, Heliophysics Lab. Troitsk, Moscow Region, 142190 Russia
              (3) Moscow State University, Faculty of Mathematics and Mechanics, 119899, Moscow, Russia
                    
E-mail:    kuzanyan@dnttm.ru ,  (corresponding author), lamburt@yandex.ru 

Abstract

We summarize studies of helical properties of solar magnetic fields such as current helicity and twist of magnetic fields in solar active regions (ARs), that are observational tracers of the alpha-effect in the solar convective zone (SCZ). Information on their spatial distribution is obtained by analysis of systematic magnetographic observations of active regions taken at Huairou Solar Observing Station of National Astronomical Observatories of Chinese Academy of Sciences. The main property is that the tracers of the alpha-effect are antisymmetric about the solar equator. Identifying longitudinal migration of active regions with their  individual rotation rates and taking into account the internal differential rotation law within the SCZ known from helioseismology, we deduce the distribution of the effect over depth. We have found evidence that the alpha-effect changes its value and sign near the bottom of the SCZ, and this is in accord with the theoretical studies and numerical simulations. We discuss other regularities which can be revealed by further analysis such as possible dependence on longitude, time, and magnetic field strength, etc.

Key words: Sun: activity --- Sun: magnetic fields

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H$\alpha$Line Polarization in the 2B/X4.8 Flare of 2002 July 23

Zhi Xu (1), N. M. Firstova (2), Qing-Rong Chen (1) and Cheng Fang (1) 

 Affiliation:  (1) Department of Astronomy, Nanjing University, Nanjing 210093
              (2) Institute of Solar-Terrestrial Physics, Russian Academy of Sciences, Siberian Division, Russia
                    
E-mail:    xuzhi@nju.edu.cn  (corresponding author)

Abstract

On 2002 July 23, a 2B/X4.8 flare was observed in the H$\alpha$ line spectropolarimetrically by the Large Solar Vacuum Telescope of Baikal Astrophysical Observatory. Linear polarization of 3%10% was detected in the H$\alpha$ line, particularly where the line showed central reversal. The linear polarization is mainly radial on the solar disk and appears at the impulsive phase of the hard X-ray and $\gamma$-ray bursts. It is limited to some relatively small regions of the flare. The polarization in a limited small region ($\sim$ 4'' 5'') changed its direction within a short period of time ($\sim$ 10s).

Key words: Polarization --- line: profile --- Sun: flare

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Automated Separation of Stars and Normal Galaxies Based on Statistical Mixture Modeling with RBF Neural Networks

Dong-Mei Qin (1), Ping Guo (2), Zhan-Yi Hu (1) and Yong-Heng Zhao (3) 

 Affiliation:  (1) National Laboratory of Pattern Recognition Laboratory, Institute of Automation, Chinese Academy of Sciences, Beijing 100080
              (2) Department of Computer Sciences, Beijing Normal University, Beijing 100875
              (3) National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
                    
E-mail:    dmqin@nlpr.ia.ac.cn (corresponding author)

Abstract

For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra---a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of E0, S0, Sa, Sb types with redshift ranging from 0 to 1.2, and the test set of stellar spectra from Pickles' atlas and SDSS spectra of normal galaxies with SNR of 13. Experiments show that our SMM-RBFNN is more efficient in both the training and testing stages than the BPNN (back propagation neural networks), and more importantly, it can achieve a good classification accuracy of 99.22% and 96.52%, respectively for stars and normal galaxies.

Key words: methods: data analysis --- techniques: spectroscopic --- stars: general --- galaxies: stellar content

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ChJAA, 2003, Vol.3, No.3

Chinese Journal of Astronomy and Astrophysics: Online Edition

http://www.chjaa.org 

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