2004, Vol.4, No.6 (December 20, 2004)


Contents

RESEARCH PAPERS
Cosmology
  • Pairwise Velocity Statistics of Dark Halos     p.507
    Hai-Yan Zhang and Yi-Peng Jing  
    Abstract | | PDF file (403 KB) | PS file *.gz  (353 KB) 
Extragalactic Astronomy
  • The Correlation between Optical Spectral Index and
    Continuum Luminosity Variation in the Seyfert 1 Galaxy NGC 5548   
    p.518
    Min-Zhi Kong, Xue-Bing Wu, Jin-Lin Han and Yu-Feng Mao  
    Abstract | | PDF file (402 KB) | PS file *.gz  (311 KB) 
  • A Peculiar Broad Line Radio Galaxy 1938--155: VLA Observation at 90 cm    p.527
    Tao An, Xiao-Yu Hong, Wen-Xia Feng and Jun-Hui Zhao  
    Abstract | | PDF file (362 KB) | PS file *.gz  (179 KB) 
  • Superluminal Motion and Polarization in Blazars    p.533
    Jun-Hui Fan, Yong-Jiu Wang, Jiang-He Yang and Cheng-Yue Su  
    Abstract | | PDF file (331 KB) | PS file *.gz  (127 KB) 
Stars
  • Global Axisymmetric Stability Analysis for a Composite System of Two Gravitationally Coupled Scale-Free Discs    p.541
    Yue Shen and Yu-Qing Lou 
    Abstract | | PDF file (401 KB) | PS file *.gz  (107 KB) 
  • RT Leonis Minoris: an Unstable W Ursae Majoris System with a Spotted
    Component   
    p.553
    Yu-Lan Yang and Qing-Yao Liu
    Abstract | | PDF file (203 KB) | PS file *.gz  (190 KB)  
The Sun
  • Formation of Solar Delta Active Regions: Twist and Writhe of Magnetic Ropes   p.563
    Hong-Qi Zhang
    Abstract | | PDF file (2936 KB) | PS file *.gz  (3594 KB)  
  • Wavelet Analysis of the Schwabe Cycle Properties in Solar Activity   p.578
    Gui-Ming Le
    Abstract | | PDF file (154 KB) | PS file *.gz  (269 KB)  
Instruments, observational techniques and data processing
  • Timescale Analysis of Spectral Lags   p.583
    Ti-Pei Li, Jin-Lu Qu, Hua Feng, Li-Ming Song, Guo-Qiang Ding and Li Chen 
    Abstract | | PDF file (561 KB) | PS file *.gz  (545 KB)  
  • Algorithm Design and Test of the Solar Guide Telescope   p.599
    Wei-Bin Wen and Sheng-Zhen Jin
    Abstract | | PDF file (192 KB) | PS file *.gz  (200 KB)  

 


Abstract

 


Pairwise Velocity Statistics of Dark Halos

Hai-Yan Zhang1 and Yi-Peng Jing2

1 Department of Astronomy, Peking University, Beijing, 100871; proudzhy@126.com 

2 Shanghai Astronomy Observatory, the Partner Group of MPI für Astrophysik,
Nandan Road 80, Shanghai, 200030

Abstract
We have accurately evaluated the halo pairwise velocity dispersion and the halo mean streaming velocity in the LCDM model (the flat $\omega_{0}$ = 0.3 model) using a set of high-resolution N-body simulations. Based on the simulation results, we have developed a model for the pairwise velocity dispersion of halos. Our model agrees with the simulation results over all scales we studied. We have also tested the model of Sheth et al. for the mean streaming motion of halos derived from the pair-conservation equation. We found that their model reproduces the simulation data very well on large scale, but under-predicts the streaming motion on scales $r<10\,h^{-1}$ Mpc. We have introduced an empirical relation to improve their model. These improved models are useful for predicting the redshift correlation functions and the redshift power spectrum of galaxies if the halo occupation number model, e.g. the cluster weighted model, is given for the galaxies.

Key words:   cosmology: theory -- dark matter halo -- large-scale structure of universe

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The Correlation between Optical Spectral Index and Continuum Luminosity Variation in the Seyfert 1 Galaxy NGC 5548

Min-Zhi Kong1, 2, Xue-Bing Wu2, Jin-Lin Han1 and Yu-Feng Mao1

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012;  kmz@bao.ac.cn 

2 Department of Astronomy, Peking University, Beijing 100871


Abstract
Using the archived optical spectra of NGC 5548 between 1989 and 2001, we derived the optical spectral index by fitting the spectra in wavelength windows unaffected by strong emission lines. We found that the index is anti-correlated with the continuum luminosity at 5100Å with a correlation coefficient of -0.8. Based on the standard thin accretion disk model, we investigated whether the correlation is related to the variations of the dimensionless accretion rate $\dot{m}$ (mass accretion rate in Eddington unit), or the inner radius of the accretion disk $R_{\rm in}$, or both. The correlation can be modeled well using a co-variable mode of $R_{\rm
in}/R_{\rm s}=12.5 {\dot{m}^{-0.8}}$ ($R_{\rm s}$ is Schwarzschild radius). As luminosity increases, $\dot{m}$ increases from 0.05 to 0.16 and at the same time $R_{\rm in}$ decreases from 133.9$R_{\rm s}$ to 55.5$R_{\rm s}$, consistent with the prediction for a transition radius within which an ADAF structure exists. We concluded that the change of both inner accretion radius and the dimensionless accretion rate are key factors for the variations of spectral index and luminosity in the optical band for NGC 5548.

Key words:   accretion: accretion disks - black hole physics - galaxies: individual (NGC 5548)

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A Peculiar Broad Line Radio Galaxy 1938-155: VLA Observation at 90cm

Tao An1, 2, Xiao-Yu Hong1, 2, Wen-Xia Feng1, 2 and Jun-Hui Zhao3

1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030;
antao@center.shao.ac.cn
 

2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge,
MA 02138


Abstract
We observed 1938-155, a broad line radio galaxy (BLRG), with the Very Large Array (VLA) at 90cm with an angular resolution of 6.8'' x 4.0''. The source consists of two components separated by 4 arcsec ($\sim 20$kpc, for H0 = 65km s-1 Mpc-1, q0=0.5) along the SE-NW direction. Both components show steep-spectra with a similar spectral index $\alpha\sim 0.83 \pm
0.07$ ( $S_\nu\propto\nu^{-\alpha}$). The bright double components are surrounded by a low-brightness cocoon. The radio properties of the two bright components are consistent with the hot spots produced by twin jets. An upper limit of $\sim 0.0008$ for the core dominance parameter (R) is inferred, suggesting there is no prominent radio core in the source. Assuming a modest viewing angle 30, a jet velocity is estimated $\sim 0.07\,c$, based on the jet to counter-jet brightness ratio (J). The lower limit in jet speed inferred is consistent with no Doppler beaming effect on the jet. The radio galaxy 1938-155 could be an exceptional BLRG with no prominent radio core or jet.

Key words:   Galaxy: quasar: individual -- radio continuum: VLA

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Superluminal Motion and Polarization in Blazars

Jun-Hui Fan1,2, Yong-Jiu Wang2, Jiang-He Yang3 and Cheng-Yue Su4

1 Center for Astrophysics, Guangzhou University, Guangzhou 510400; fjh@gzhu.edu.cn 

2 Physics Institute, Hunan Normal University, Changsha

3 Department of Electronics and Physics, Hunan University of Arts and Science, Changde 415000

4 Department of Physics, Guangdong Industry University, Guangzhou

Abstract
A relativistic beaming model has been successfully used to explain the observed properties of active galactic nuclei (AGNs). In this model there are two emission components, a boosted one and an unbeamed one, shown up in the radio band as the core and lobe components. The luminosity ratio of the core to the lobe is defined as the core-dominance parameter ( $R = {\frac{L_{\rm Core}}{L_{\rm
Lobe}}}$). The de-beamed radio luminosity ( $L^{\rm db}_{\rm jet}$) in the jet is assumed to be proportional to the unbeamed luminosity ($L_{\rm ub}$) in the co-moving frame, i.e., $f = {\frac{L^{\rm
db}_{\rm jet}}{L_{\rm ub}}}$, and f is determined in our previous paper. We further discuss the relationship between BL Lacertae objects (BLs) and flat spectrum radio quasars (FSRQs), which are subclasses of blazars with different degrees of polarization, using the calculated values of the ratio f for a sample of superluminal blazars. We found 1) that the BLs show smaller averaged Doppler factors and Lorentz factors, larger viewing angles and higher core-dominance parameters than do the FSRQs, and 2) that in the polarization-core dominance parameter plot ($P-\log R$) the BLs and FSRQs occupy a scattered region, but in a revised plot ( $\log {\frac{P}{c(m)}}-\log R$), they gather around two different lines, suggesting that they have some different intrinsic properties.


Key words:   active galactic nuclei -- superluminal motion -- jets -- relativistic beaming model

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Global Axisymmetric Stability Analysis for a Composite System of Two Gravitationally Coupled Scale-Free Discs

Yue Shen1 and Yu-Qing Lou1, 2, 3

1 Department of Physics & Center for Astrophysics, Tsinghua University, Beijing 100084;
shenyue98@mails.tsinghua.edu.cn ; louyq@tsinghua.edu.cn 

2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

3 Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 USA; lou@oddjob.uchicago.edu 

Abstract
For a composite system of gravitationally coupled stellar and gaseous discs, we have carried out a linear stability analysis for axisymmetric coplanar perturbations using the two-fluid formalism. The background stellar and gaseous discs are taken to be scale-free with all physical variables varying as powers of the cylindrical radius r with compatible exponents. The unstable modes set in as neutral modes or stationary perturbation configurations with angular frequency $\omega=0$. The axisymmetric stable range is bounded by two marginal stability curves derived from stationary perturbation configurations. Because of the gravitational coupling between the stellar and the gaseous discs, one only needs to consider the parameter regime of the stellar disc. There exist two unstable regimes in general: a collapse regime corresponding to large-scale perturbations and a ring-fragmentation regime corresponding to short-wavelength perturbations. The composite system will collapse if it rotates too slowly and will succumb to ring-fragmentation instabilities if it rotates sufficiently fast. The overall stable range against axisymmetric perturbations is determined by a necessary D-criterion involving the effective Mach number squared Ds2 (the squared ratio of the stellar disc rotation speed to the stellar velocity dispersion up to a numerical factor). Different mass ratio $\delta$ and sound speed ratio $\eta$ of the gaseous and stellar disc components will alter the overall stability. For spiral galaxies or circumnuclear discs, we further include the dynamical effect of a massive dark matter halo. Astrophysical applications to disc galaxies, proto-stellar discs and circumnuclear discs are given as examples.


Key words:   hydrodynamics -- ISM: general -- galaxies: kinematics and dynamics -- galaxies: spiral -- galaxies: structure -- waves

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RT Leonis Minoris: an Unstable W Ursae Majoris System with a Spotted Component

Yu-Lan Yang and Qing-Yao Liu

National Astronomical Observatories, Yunnan Observatory, Chinese Academy of Sciences,  Kunming 650011; bily@public.km.yn.cn 
United Laboratory of Optical Astronomy, Chinese Academy of Sciences

Abstract  
Complete BV light curves of the W UMa type binary RT LMi are presented. From the observations, four times of minimum light were determined. Based on the new times of minimum light and those collected from the literature, changes in the orbital period of the system were found and analyzed with Kalimeris et al.'s method. The result shows that the orbital period possibly oscillates with a cycle of about 64 years and an amplitude of 1.2 x 10-6 days. The present CCD photometric observations reveal that the light curves are obviously asymmetrical, and show a positive O'Connell effect, while the light curves obtained in 1982 exhibit a negative O'Connell effect. The present light curves were analyzed by means of the latest version of the Wilson-Devinney code, which was also used to correct the photometric effects, including the distortion on the radial-velocity curves obtained by Rucinski et al. The following absolute dimensions have been derived: M1 = $1.28\pm{0.08}~M_{\odot}$, M2 = $0.48\pm{0.06}~M_{\odot}$, R1 = $1.28\pm{0.06}~R_{\odot}$, R2 = $0.83\pm{0.05}~R_{\odot}$, L1 = $1.88\pm{0.12}~L_{\odot}$, $L_{2} = 0.77
\pm{0.08}~L_{\odot}$, and $A = 2.64\pm{0.02}~R_{\odot}$. The asymmetry of the light curves can be explained by a model with a cool spot on the secondary component. The orbital period modulation can be reproduced by a magnetic activity cycle model of the secondary component with $\Delta{J} = 5.6\times 10^{46}$ g cm2 s-1, $\Delta{\Omega}/\Omega = 8.8 \times 10^{-4}$, and B = 5.1 x 103G.

Key words:   star: contact binary -- star: individuals: RT LMi - star: magnetic activity

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

Hong-Qi Zhang

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012;  hzhang@bao.ac.cn

Abstract   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.

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

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Wavelet Analysis of the Schwabe Cycle Properties in Solar Activity

Gui-Ming Le

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; kjzhxsge@263.net 
Center for Space Science and Applied Research Center, Chinese Academy of Sciences, Beijing 100080

Abstract  
Properties of the Schwabe cycles in solar activity are investigated by using wavelet transform. We study the main range of the Schwabe cycles of the solar activity recorded by relative sunspot numbers, and find that the main range of the Schwabe cycles is the periodic span from 8-year to 14-year. We make the comparison of 11-year's phase between relative sunspot numbers and sunspot group numbers. The results show that there is some difference between two phases for the interval from 1710 to 1810, while the two phases are almost the same for the interval from 1810 to 1990.

Key words:   Sun: sunspot -- Sun: activity -- method: data analysis

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Timescale Analysis of Spectral Lags

Ti-Pei Li1, 2, 3, Jin-Lu Qu2, Hua Feng3, Li-Ming Song2, Guo-Qiang Ding2 and Li Chen4

1 Department of Physics & Center for Astrophysics, Tsinghua University, Beijing 100084; litp@mail.tsinghua.edu.cn 
2 Particle Astrophysics Lab., Institute of High Energy Physics, Chinese Academy of Scinces
3 Department of Engineering Physics & Center for Astrophysics, Tsinghua University
4 Department of Astronomy, Beijing Normal University

Abstract  
A technique for timescale analysis of spectral lags performed directly in the time domain is developed. Simulation studies are made to compare the time domain technique with the Fourier frequency analysis for spectral time lags. The time domain technique is applied to studying rapid variabilities of X-ray binaries and $\gamma$-ray bursts. The results indicate that in comparison with the Fourier analysis the timescale analysis technique is more powerful for the study of spectral lags in rapid variabilities on short time scales and short duration flaring phenomena.

Key words:   methods: data analysis -- binaries: general -- X-rays: stars -- gamma rays: bursts -- X-rays: bursts

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Algorithm Design and Test of the Solar Guide Telescope

Wei-Bin Wen and Sheng-Zhen Jin

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; wwbolder@sst.bao.ac.cn 

Abstract  
The Solar Guide Telescope (SGT), an important solar attitude sensor of the SST (Space Solar Telescope, a space solar observing instrument being developed in China), can accurately produce pointing error signals of the SST for attitude control at high speed. We analyze in detail the error algorithm of the heliocentric coordinates and the edge judging of solar images. The measuring accuracy of $\pm $0.5 arcsec of the SGT is verified by experiments on the tracking of the Sun and by testing a sun simulator. Some factors causing the pointing errors are examined.

Key words:   Solar Guide Telescope -- telescopes -- techniques: image processing

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ChJAA, 2004, Vol.4, No.6

Chinese Journal of Astronomy and Astrophysics: Online Edition

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