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{\LARGE\bf Chinese Journal of Astronomy and Astrophysics}\\

2007, Vol.7, No.1 (February 20, 2007)





Extragalactic Astronomy


The Sun

Celestial Mechanics


Gamma-Ray Bursts in the Swift Era

Bing Zhang

Department of Physics & Astronomy, University of Nevada, Las Vegas, NV 89154-4002, USA; bzhang@physics.unlv.edu

Since the successful launch of NASA's dedicated gamma-ray burst (GRB) mission, Swift, the study of cosmological GRBs has entered a new era. Here I review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs. Besides summarizing how Swift helps to solve some pre-Swift mysteries, I also list some outstanding problems raised by the Swift observations. An outlook of GRB science in the future, especially in the GLAST era, is briefly discussed.

Key Words:   gamma-rays: bursts
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Note on Redshift Distortion in Fourier Space

Yan-Chuan Cai1,2 and Jun Pan1

1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; jpan@pmo.ac.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

We explore features of redshift distortion in Fourier analysis of N-body simulations. The phases of the Fourier modes of dark matter density fluctuation are generally shifted by the peculiar motion along the line of sight, the induced phase shift is stochastic and has a probability distribution function (PDF) that is symmetric about the peak at zero shift and whose exact shape depends on the wave vector, except on very large scales where phases are invariant by linear perturbation theory. Analysis of the phase shifts motivates our phenomenological models for the bispectrum in redshift space. Comparison with simulations shows that our toy models are very successful in modeling bispectrum of equilateral and isosceles triangles at large scales. In the second part we compare the monopole of the power spectrum and bispectrum in the radial and plane-parallel distortion to test the plane-parallel approximation. We confirm the results of Scoccimarro that difference of power spectrum is at the level of $10\%$, and, in the reduced bispectrum, the difference is as small as a few percent. However, on the plane perpendicular to the line of sight of kz=0, the difference in power spectrum between the radial and plane-parallel approximation can be more than $\sim
10\%$, and even worse on very small scales. Such difference is prominent for bispectrum, especially for configurations of tilted triangles. Non-Gaussian signals under the radial distortion on small scales are systematically biased downside than are in the plane-parallel approximation, with amplitudes depending on the opening angle of the sample point to the observer. This observation gives warning to the practice of using the power spectrum and bispectrum measured on the kz=0 plane as estimates of the real space statistics.

Key Words:   cosmology: theory -- large-scale structure of universe -- methods: statistical
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Evidence of the Link between Broad Emission Line Regions and Accretion Disks in Active Galactic Nuclei

Yun Xu1,2 and Xin-Wu Cao1

1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030; xuyun@shao.ac.cn
2 Graduate School of Chinese Academy of Sciences, Beijing 100049

There is observational evidence that broad-line regions (BLRs) exist in most active galactic nuclei (AGNs), but their origin is still unclear. One scenario is that the BLRs originate from winds accelerated from the hot coronae of the disks, and the winds are suppressed when the black hole is accreting at low rates. This model predicts a relation between $\dot m$ ( $\dot m =
{\dot M}/{\dot M}_{\rm Edd}$) and the FWHM of broad emission lines. We estimate the central black hole masses for a sample of bright AGNs by using their broad H$_\beta$ line-widths and optical luminosities. The dimensionless accretion rates $\dot{m}=\dot{M}/\dot{M}_{\rm Edd}$ are derived from the optical continuum luminosities by using two different models: using an empirical relation between the bolometric luminosity $L_{\rm bol}$ and the optical luminosity ( $\dot{m}={L_{\rm bol}}/{L_{\rm Edd}}$, a fixed radiative efficiency is adopted); and calculating the optical spectra of accretion disks as a function of $\dot{m}$. We find a significant correlation between the derived $\dot{m}$ and the observed line width of H$_\beta$, FWHM $\propto\dot{m}^{-0.37}$, which almost overlaps the disk-corona model calculations, if the viscosity $\alpha\approx0.1
- 0.2$ is adopted. Our results provide strong evidence for the physical link between the BLRs and accretion disks in AGNs.

Key Words:   accretion, accretion disks--galaxies: active--galaxies: theory-- radiation mechanisms: emission lines-- black hole physics
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A Fourteen-Band Photometric Study of A2443

Zhong-Lue Wen 1, Yan-Bin Yang1, Qi-Rong Yuan2, Xu Zhou1, Jun Ma 1 and Zhao-Ji Jiang 1

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; zhonglue@bao.ac.cn
2 Department of Physics, Nanjing Normal University, Nanjing 210097

We present a multi-color photometric study of the galaxy cluster A2443 (z=0.108) with the Beijing-Arizona-Taiwan-Connecticut (BATC) system. The spectral energy distributions (SEDs) in 14 intermediate bands are obtained for 5975 detected from $\sim$1$\rm deg^2$ of the BATC images. Color-color diagrams are used for star-galaxy separation, then a photometric redshift technique is applied to the galaxy sample for cluster membership determination. There are 301 galaxies with photometric redshifts between 0.08 and 0.14 determined as member candidates of A2443, including 289 new ones. Based on this enlarged sample, the luminosity function and color magnitude relation of the cluster are studied. With an evolutionary synthesis model, we find that the fainter galaxies tend to have longer time scales of star formation than the brighter ones. Morphologically, we show an elongated spatial distribution associating with the galaxy cluster ZwCl 2224.2+1651, which contains more blue galaxies. This result indicates that galaxy cluster ZwCl 2224.2+1651 may be falling into A2443, and cluster-cluster interaction could have triggered star formation activities in ZwCl 2224.2+1651.

Key Words:   galaxies: clusters: individual: A2443 -- galaxies: distances and redshifts
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A Statistical Analysis of Point-like Sources in the Chandra Galactic Center Survey

J. F. Wu1, S. N. Zhang1,2, F. J. Lu2 and Y. K. Jin3

1 Department of Physics and Center for Astrophysics, Tsinghua University, Beijing 100084; jfwu03@mails.tsinghua.edu.cn
2 Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049
3 Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084

The Chandra Galactic Center Survey detected $\sim 800$ X-ray point-like sources in the $2^{\circ} \times 0.8^{\circ}$ sky region around the Galactic Center. We study the spatial and luminosity distributions of these sources according to their spectral properties. Fourteen bright sources detected are used to fit jointly an absorbed power-law model, from which the power-law photon index is determined to be $\sim$2.5. Assuming that all other sources have the same power-law form, the relation between hardness ratio and HI column density $N_{\rm H}$ is used to estimate the $N_{\rm H}$ values for all sources. Monte Carlo simulations show that these sources are more likely concentrated in the Galactic center region, rather than distributed throughout the Galactic disk. We also find that the luminosities of the sources are positively correlated with their HI column densities, i.e., a more luminous source has a higher HI column density. From this relation, we suggest that the X-ray luminosity comes from the interaction between an isolated old neutron star and interstellar medium (mainly dense molecular clouds). Using the standard Bondi accretion theory and the statistical information of molecular clouds in the Galactic center, we confirm this positive correlation and calculate the luminosity range in this scenario, which is consistent with the observation ( 1032-1035 erg s-1).

Key Words:   methods: data analysis -- Galaxy: center -- X-ray: stars
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Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

Xu Zhang1,2, Ye Lu1,3 and Yong-Heng Zhao1

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; ly@bao.ac.cn
2 Department of Physics, Shandong University, Jinan 250100
3 Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong

We suggest a two-dimensional time dependent analytic model to describe the accretion of matter onto a neutron star moving at a high speed across the ejecta left in the aftermath of a supernova explosion. The formation of a strange star resulting from the accretion is also addressed. The newborn neutron star is assumed to move outward at a kick velocity of $v_{\rm ns}\sim 
10^{3}$ km s-1, and the accretion flow is treated as a dust flow. When the neutron star travels across the ejecta with high speed, it sweeps up material, and when the accreted mass has reached a critical value, the neutron star will undergo a phase transition, for instance, to become a strange star. Our results show that the accretion rate decreases in a complicated way in time, not just a power law dependence: it drops much faster than the power law derived by Colpi et al. We also found that the total accreted mass and the phase transition of the neutron star depend sensitively on the velocity of supernova ejecta.

Key Words:   pulsars: general-pulsars -- stars: neutron -- X-rays: stars -- accretion disk -- instability
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Mira Symbiotic Stars

Guo-Liang Lü1,2, Chun-Hua Zhu2 and Zhan-Wen Han1

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; ytlgl@yahoo.com.cn
2 Department of Physics, Xinjiang University, Urumqi 830046

We have carried out a detailed study of Mira symbiotic stars by means of a population synthesis code. We estimate the number of Mira symbiotic stars in the Galaxy as 1700 - 3100 and the Galactic occurrence rate of Mira symbiotic novae as from $\sim$ 0.9 to 6.0 yr-1, depending on the model assumptions. The distributions of the orbital periods, the masses of the components, mass-loss rates of cool components, mass-accretion rates of hot components and Mira pulsation periods in Mira symbiotic stars are simulated. By a comparison of the number ratio of Mira symbiotic stars to all symbiotic stars, we find the model with the stellar wind model of Winters et al. to be reasonable.

Key Words:   binaries: symbiotic -- Galaxy: stellar content -- stars: Mira variables
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Possible Streams of the Globular Clusters in the Galaxy

Shuang Gao1, Bi-Wei Jiang1 and Yong-Heng Zhao2

1 Department of Astronomy, Beijing Normal University, Beijing 100875; bjiang@bnu.edu.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

We aim to retrieve ghost streams under the assumption that all the globular clusters in our Galaxy were formed in very early merge events. Our results are based on two speculations: that the specific energy and angular momentum of the globular clusters after merge are not changed in the course of evolution and that the globular clusters with a common origin would stay in the same orbit plane as the parent galaxy. After taking into account the apogalacticum distance of the orbits, we suggest with some confidence five possible streams. The number of streams is consistent with the previous results. Three of the four well established members of the Sagittarius stream were found to be in one of our streams. Several other globular clusters in our result were also thought to come from accretion by previous researchers. The orbital parameters of the streams are derived, which provide a way to test whether these streams are true with the help of more accurate measurement of proper motions of the globular clusters.

Key Words:   globular clusters: general -- Galaxy: formation -- Galaxy: halo
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A Kinematical Calibration of the Galactocentric Distance

Ming Shen and Zi Zhu

Department of Astronomy, Nanjing University, Nanjing 210093; shenming@nju.org.cn

We present a new determination of the Galactocentric distance by a pure kinematical model. Two subgroups of components from the Galactic thin disk, the O-B5 stars and the Galactic open clusters, were selected for our analysis. On the basis of kinematical data of around 1200 O-B5 stars, we obtained an estimated value of R0=8.25$\pm0.79$ kpc, while a similar evaluation from 270 Galactic open clusters gives R0=7.95$\pm0.62$ kpc. Considering the scatter of R0 given by individual investigators with different methods, our present determinations agree well with the best value proposed by Reid.

Key Words:   Galaxy: kinematics and dynamics -- stars: early-type -- open clusters and associations
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Eruption of an Active-Region Filament Driven by an Emerging Bipole

Yun-Chun Jiang1, Yuan-Deng Shen1 and Jing-Xiu Wang2

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; jyc@ynao.ac.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

A section of an S-shaped filament underwent an eruption in a sigmoidal active region (AR8027) with S-shaped coronal structure, which was clearly driven by a bipole emerging below the NW end of the filament. The bipole with two separating poles showed typical characteristics of emerging flux region (EFR) and its axis rotated counterclockwise. Two cancelling magnetic features (CMFs) were formed between the two poles and adjacent flux with opposite polarity and substantial flux cancellation occurred in them. Along with the bipole emergence the filament was strongly disturbed. Just before the filament eruption, two X-ray loops overlying the filament brightened, an axial X-ray structure and then a cusp structure appeared. During the eruption first the whole filament rose and then its SE end broke away from the chromosphere, while its NW end remained stationary. Helical structure and motion were observed in the filament body and downward mass motion in the two ends. After the eruption, a major part of the filament remained and slowly returned to quiescence, and an X-ray arcade and an axial structure formed. These observations suggest that the eruption resulted from the interaction between the bipole and the overlying loops. We provide evidence that steady photospheric reconnection between their footprints took place in the two CMFs during the bipole emergence.

Key Words:   Sun: filaments -- Sun: flares -- Sun: magnetic fields -- Sun: corona -- Sun: X-rays
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The Origin and Acceleration of 3He and Heavy Ions in the 2000 July 14 Event

Gui-Ping Wu1, Guang-Li Huang2, Yu-Hua Tang3 and Yu Dai3

1 Department of Physics, Southeast University, Nanjing 210096; wuguiping@seu.edu.cn
2 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008
3 Department of Astronomy, Nanjing University, Nanjing 210093

According to the evolutionary properties of the flare, halo coronal mass ejection (CME), enrichments of 3He ions in the energy range of 3.5-26 MeV nucl-1 and Ne, Mg, Si and Fe ions in the energy range of 8.5-15 MeV nucl-1, we argue that the 3He and heavy ions originate in the middle corona ($\sim$0.1-1 $R_{\sun}$) with well-connected open field lines to the Earth, where the magnetic reconnection leads to acceleration of the electrons and the production of type-I$\!$I$\!$I burst during the decay phase of the soft X-ray emission. The acceleration of 3He and heavy ions may have been accomplished in two stages: first H-He ion-ion hybrid waves may be easily excited by the energetic electron beams produced in the middle corona, and these waves are preferentially absorbed by 3He and heavy ions due to their frequency being near the fundamental gyro frequency of the 3He ions and harmonic gyro frequency of Ne, Mg, Si and Fe ions. These preheated ions escape into interplanetary space along the open field lines and may be further accelerated to tens of MeV nucl-1 by CME-driven shock. The theoretical calculations show that the 3He and heavy ions may be accelerated to the energy of $\sim$ MeV nucl-1 by the ion-ion hybrid waves and be further accelerated to the energy of $\sim$ 100 MeV nucl-1 by the shock wave: these are basically consistent with the observations.

Key Words:   Sun: abundances -- waves -- shock wave -- acceleration of particles
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Revisit of Dynamical Mechanisms of Transporting Asteroids in the 3:1 Resonance to the Near-Earth Space

Jiang-Hui Ji1,3 and Lin Liu2,3

1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; jijh@pmo.ac.cn
2 Department of Astronomy, Nanjing University, Nanjing 210093
3 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

It is well-known that the asteroids in the main belt trapped in the 3:1 Mean Motion Resonance (MMR) with Jupiter (at semi-major axes $\sim$2.5AU) are few in number, forming one of the so-called Kirkwood Gaps. Wisdom pointed out that chaotic motion of such asteroids can increase their eccentricities and make them approach and cross the orbit of Mars (or even the Earth). We numerically investigated the orbital evolution of the asteroids involved in 3:1 MMR (NEOs) over millions of years and revisited the dynamical mechanisms of transporting such asteroids into the NEO region. The results show that the dynamical evolution of the asteroids around 2.5AU is mainly dominated by the 3:1 resonance, the $\nu_5$ and $\nu _{6}$ secular resonances and the Kozai resonance, and these bodies can evolve into NEOs through several of the dynamical mechanisms, so indicating possible dynamical origin of the NEOs.

Key Words:   celestial mechanics -- near-Earth Objects -- mean motion resonance -- secular resonance
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ChJAA, 2007, Vol.7, No.1

Chinese Journal of Astronomy and Astrophysics: Online Edition


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