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

2008, Vol.8, No.2 (April 20, 2008)

Contents

RESEARCH PAPERS

The Sun

Astrophysical Processes

Cosmology

Extragalactic Astronomy

Stars




Abstract

Determination of the Topology Skeleton of Magnetic Fields in a Solar Active Region

Hui Zhao 1, Jing-Xiu Wang 1, Jun Zhang 1, Chi-Jie Xiao 1, Hai-Min Wang 2

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

2 Big Bear Solar Observatory, 40386 North Shore Lane, Big Bear City, CA 92314, USA

Abstract
Magnetic topology has been a key to the understanding of magnetic energy release mechanism. Based on observed vector magnetograms, we have determined the three-dimensional (3D) topology skeleton of the magnetic fields in the active region NOAA 10720. The skeleton consists of six 3D magnetic nulls and a network of c time, we have identified a spiral magnetic null in Sun's corona. The magnetic lines of force twisted around the spine of the null, forming a `magnetic wreath' with excess of free magnetic energy and resembling observed brightening structures at extra-ultraviolet (EUV) wavebands. We found clear evidence of topology eruptions which are referred to as catastrophic changes of topology skeleton associated with a coronal mass ejection (CME) and an explosive X-ray flare. These results shed new lights on the structural complexity and its role in explosive magnetic activity. The concept of flux rope associated with the 3D spiral null is likely an important class of the physical entity of flux ropes.

Key Words:   Sun: magnetic fields -- Sun: solar corona -- Sun: activity


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Speed Distributions of CMEs in Cycle 23 at Low and High Latitudes 

Peng-Xin Gao 1,2 and Ke-Jun Li 1

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

2 Graduate School of Chinese Academy of Sciences, Beijing 100049

Abstract
We analyzed the speed (v) distributions of 11584 coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment on board the Solar and Heliospheric Observatory (SOHO/LASCO) in cycle 23 from 1996 to 2006. We find that the speed distributions for high-latitude (HL) and low-latitude (LL) CME events are nearly identical and to a good approximation they can be fitted with a lognormal distribution. This finding implies that statistically the same driving mechanism of a nonlinear nature is acting in both HL and LL CME events, and CMEs are intrinsically associated with the source's magnetic structure on large spatial scales. Statistically, the HL CMEs are slightly slower than the LL CMEs. For HL and LL CME events respectively, the speed distributions for accelerating and decelerating events are nearly identical and also to a good approximation they can be both fitted with a lognormal distribution, thus supplementing the results obtained by Yurchyshyn et al.

Key Words:   Sun: activity -- Sun: coronal mass ejections (CMEs)


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Constraining Galactic $p\gamma$ Interactions with Cosmic Ray Electron and Positron Spectra

Nayantara Gupta and Bing Zhang

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

Abstract
High energy protons produced by various sources of cosmic rays, e.g., supernova remnants, pulsar wind nebulae, active galactic nuclei and gamma-ray bursts, participate in $p\gamma$ and pp interactions. Although pp interactions may be the dominant mechanism in our Galaxy, it is unclear how important $p\gamma$ process is. We show that the upper bound on the fraction of total number of protons participating in $p\gamma$ interactions inside all Galactic astrophysical sources of cosmic rays is $10\%$.

Key Words:   Cosmic rays: protons, electrons, positrons


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DWT Power Spectrum of the Two-Degree Field Galaxy Redshift Survey

Yan-Chuan Cai 1,2, Jun Pan 1, Yong-Heng Zhao 2, Long-Long Feng 1, Li-Zhi Fang 3

1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; y.c.cai@durham.ac.uk

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

3 Department of Physics, University of Arizona, Tucson, AZ 85721, US

Abstract
The power spectrum of the two-degree Field Galaxy Redshift Survey (2dFGRS) sample is estimated with the discrete wavelet transform (DWT) method. The DWT power spectra within $0.035 <k< 2.2\,h$Mpc-1 are measured for three volume-limited samples defined in consecutive absolute magnitude bins $-19 \sim -18$, $-20 \sim -19$ and $-21 \sim -20$. We show that the DWT power spectrum can effectively distinguish $\Lambda$CDM models of $\sigma_8=0.84$ and $\sigma_8=0.74$. We adopt maximum likelihood method to perform three-parameter fitting of the bias parameter b, pairwise velocity dispersion $\sigma_{\rm pv}$ and redshift distortion parameter $\beta=\Omega_m^{0.6}/b$ to the measured DWT power spectrum. The fitting results state that in a $\sigma_8=0.84$ universe the best-fit values of $\Omega_m$ given by the three samples are mutually consistent within the range $0.28 \sim 0.36$, and the best fitted values of $\sigma_{\rm pv}$ are 398+35-27, 475+37-29 and $550 \pm 20$km s-1 for the three samples, respectively. In the model of $\sigma_8=0.74$, our three samples give very different values of $\Omega_m$. We repeated the fitting using the empirical formula of redshift distortion. The result of the model of low $\sigma_8$ is still poor, especially, one of the best-fit values of $\sigma_{\rm pv}$ is as large as 103km s-1. We also repeated our fitting by incorporating a scale-dependent galaxy bias. This gave a slightly lower value of $\Omega_m$. Differences between the models of $\sigma_8=0.84$ and $\sigma_8=0.74$ still exist in the fitting results. The power spectrum of 2dFGRS seems to disfavor models with low amplitude of density fluctuations if the bias parameter is assumed to be scale independent. For the fitting value of $\Omega_m$ to be consistent with that given by WMAP3, strong scale dependence of the bias parameters is needed.

Key Words:   methods: data analysis -- methods: statistical -- (cosmology:) cosmological parameters --(cosmology:) large-scale structure of universe


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Bending of Jets in the QSO NRAO 530

Xiao-Yu Hong 1, Chuan-Hao Sun 1,2, Jun-Hui Zhao 3, Dong-Rong Jiang 1, Zhi-Qiang Shen 1,4, Tao An 1, Wei-Hua Wang 1, Jun Yang 1,2

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

2 Graduate University of Chinese Academy of Sciences, Beijing 100049

3 Harvard-Smithsonian Center for Astrophysics, 60 Garden street, MS 72, Cambridge, MA02138

4 Joint Institute for Galaxy and Cosmology

Abstract
We present radio images of NRAO 530 on scales ranging from pc to kpc. The observations include the EVN at 5GHz, the VLBA at 1.6, 8.6 and 15GHz, the MERLIN at 1.6 and 5GHz, and the VLA at 5, 8.4, 15, 22, and 43GHz. The VLBI images show a core-jet structure with an oscillating trajectory on a scale of about 30 mas north of the strongest compact component (core). Superluminal motions are detected in five of the jet components with apparent velocities in the range of 13.6 to 25.2c. A new component is detected at 15GHz with the VLBA observations, which appears to be associated with the outburst in 2002. Significant polarized emission is detected around the core with the VLBA observations at 15GHz. Rapid variations of the polarization intensity and angle are found between the epochs in 2002 and 2004. On the kpc-scale, a distant component (labelled as WL) located 11 arcsec west (PA=$-86\dg$) of the core is detected beyond the core-jet structure which extended to several hundreds of mas in the north-west direction ($-50\dg$). A significant emission between the core-jet structure and the WL is revealed. A clump of diffuse emission (labelled EL, 12 arcsec long) at PA $70\dg$ to the core, is also detected in the VLA observations, suggesting the presence of double lobes in the source. The core component shows a flat spectrum, while the distant components WL and EL have steep spectra. The steep spectra of the distant components and the detection of the arched emission suggest that the distant components are lobes or hot-spots powered by the core of NRAO 530. The morphologies from pc- to kpc-scales and the bending of jets are investigated. The observed radio morphology from pc to kcp appears to favor the model in which precession or wobbling of the nuclear disk drives the helical motion of the radio plasma and produces the S-shaped structure on kpc scale.

Key Words:   galaxies: nuclei -- galaxies: jets -- quasars: individual: NRAO 530


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Time Lags between the 22 and 37GHz Bursts of 48 Radio-loud AGNs 

Wen-Guo Deng 1,2, Jin-Ming Bai 1, Li Zhang 2, Xian Yang 1,2

1 National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; dwg906@ynao.ac.cn

2 Physics Department, Yunnan University, Kunming 650091

Abstract
Based on the light curves at 22 and 37GHz from the Metsahovi monitoring program, we investigate the time lags between the two radio bands for 48 radio-loud AGNs. DCF and ZDCF analyses are applied to the data. Our results show that there is a strong correlation between the two radio frequencies for all the sources, with the variations in the light curves at 37GHz leading the ones at 22GHz in general. There is no obvious differences between different sub-class AGNs as regards the time lag. In two sources, it was found that the bursts at the lower frequency lead the ones at the higher frequency. One possible explanation is that electron acceleration dominates the light curve until the radiation reaches the maximum. Some sources, such as 3C 273, 3C 279, 3C 345 and 3C 454.3, have good enough data, so we can calculate their lags burst-by-burst. Our calculations show that different outbursts have different lags. Some bursts have positive lags, most of bursts have no clear lags, and a few have negative lags. This result means that different bursts are triggered by different mechanisms, and the interpretation for the result involves both an intrinsic and a geometric mechanism. The positive lags are well consistent with the shock model, and we use these lags to calculate the typical magnetic field strength of the radiating region.

Key Words:   galaxies: active -- galaxies: individual (3C 273, 3C 279, 3C 345, 3C 454.3) -- methods: data analysis


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Are Seyfert 2 Galaxies without Polarized Broad Emission Lines More Obscured?

Xin-Wen Shu, Jun-Xian Wang and Peng Jiang

Center for Astrophysics, University of Science and Technology of China (USTC), Hefei 230026; xwshu@mail.ustc.edu.cn
Joint Institute for Galaxy and Cosmology, USTC and Shanghai Astronomical Observatory, Chinese Academy of Sciences, Hefei 230026

Abstract
New XMM-Newton data of seven Seyfert 2 galaxies with optical spectropolarimetric observations are presented. An analysis of the 0.5 - 10keV spectra shows that four Seyfert 2 galaxies with polarized broad lines (PBLs) are absorbed by $N_{\rm
H}<10^{24}$cm-2, while two of three Seyfert 2 galaxies without PBLs show evidence of Compton-thick obscuration, supporting the conclusion that Seyfert 2 galaxies without PBLs are more obscured than those with PBLs. Adding the measured obscuration indicators ($N_{\rm H}$, T ratio, and Fe K$\alpha$ line EW) of six luminous AGNs to our previous sample improves the significance level of the difference in absorption from 92.3% to 96.3% for $N_{\rm H}$, 99.1% to 99.4% for T ratio, and 95.3% to 97.4% for Fe K$\alpha$ line EW. The present results support and enhance the suggestions that the absence of PBLs in Seyfert 2 galaxies can be explained by larger viewing angles of the line of sight to the putative dusty torus, which leads to the obscuration of the broad-line scattering screen, as expected in the unification model.

Key Words:   galaxies: active -- galaxies: individual (NGC 513, NGC 1144, NGC 6890, NGC 7682, MCG -3-58-7, F02581-1136, UGC 6100) -- galaxies: Seyfert -- X-rays: galaxies -- polarization


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Clustering Property of Wolf-Rayet Galaxies in the SDSS

Wei Zhang, Xu Kong and Fu-Zhen Cheng

Center for Astrophysics, University of Science and Technology of China, Hefei 230026; xtwfn@mail.ustc.edu.cn

Abstract
We have analysed, for the first time, the clustering properties of Wolf-Rayet (W-R) galaxies, using a large sample of 846 W-R galaxies selected from the Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We compute the cross-correlation function between W-R galaxies and a reference sample of galaxies drawn from the DR4. We compare the function to the results for control samples of non-W-R star-forming galaxies that are matched closely in redshift, luminosity, concentration, 4000-Å break strength and specific star formation rate (SSFR). On scales larger than a few Mpc, W-R galaxies have almost the same clustering amplitude as the control samples, indicating that W-R galaxies and non-W-R control galaxies populate dark matter haloes of similar masses. On scales between 0.1-1h-1 Mpc, W-R galaxies are less clustered than the control samples, and the size of the difference depends on the SSFR. Based on both observational and theoretical considerations, we speculate that this ne preferentially at the centers of their dark matter haloes. We examine the distribution of W-R galaxies more closely using the SDSS galaxy group catalogue of Yang et al., and find that $\sim$82% of our W-R galaxies are the central galaxies of groups, compared to $\sim$74% for the corresponding control galaxies. We find that W-R galaxies are hosted, on average, by dark matter haloes of masses of $10^{12.3}M_\odot$, compared to $10^{12.1}M_\odot$ for centrally-located W-R galaxies and $10^{12.7}M_\odot$ for satellite ones. We would like to point out that this finding, which provides a direct observational support to our conjecture, is really very crude due to the small number of W-R galaxies and the incompleteness of the group catalogue, and needs more work in future with larger samples.

Key Words:   galaxies: distances and redshifts -- galaxies: starburst -- stars: Wolf-Rayet


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Correlated Radio-Optical Variations on Intraday Timescales

Shan-Jie Qian

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

Abstract
Correlated radio-optical variations on intraday timescales have been observed (e.g. in BLO 0716+714) and such radio intraday variability is suggested to have an intrinsic origin. Recently, multi-wavelength observations, simultaneous at radio, mm-submm, optical and hard X-rays, of 0716+714, show that during a period of intraday/interday variations at radio and mm wavelengths, the apparent brightness temperature of the source exceeded the Compton-limit ($\sim$1012K) by 2-4 orders of magnitude, but no Compton catastrophe (or no high luminosity of inverse-Compton radiation) was detected. It is also found that the intraday/interday variations at mm-submm wavelengths are consistent with the evolutionary behavior of a standard synchrotron source and for the intraday/interday variations at centimeter wavelengths opacity effects can play a significant role, which is consistent with the interpretation suggested previously by Qian et al. Thus the apparent high brightness temperatures may probably be explained in terms of Doppler boosting effects due to bulk relativistic motion of the source. We will argue a scenario to simulate the correlations between the radio and optical variations on intraday timescales observed in BLO 0716+714 in terms of a relativistic shock propagating through a jet with a dual structure.

Key Words:   optical and radio continuum: galaxies -- galaxies: intraday variability -- quasars: individual: BLO0716+71


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Detection of CO Outflow in Rotating Cores

Xin Guan and Yue-Fang Wu

Department of Astronomy, Peking University, Beijing 100871; guanx.bac@gmail.com

Abstract
We investigate the effect of bulk motion on the detection of molecular outflows in the sources S146, GGD27, and IRAS 22566+5830. The traditional techniques do allow for bulk motions or systematic $V_{\rm LSR}$ shifts of the core emissions, which may cause contamination of the high velocity gas emissions, and outflows may either fail to be detected or have their properties miscalculated. We used a program to follow the systematic shift of $V_{\rm LSR}$ and better results have been obtained.

Key Words:   ISM: jets and outflows -- ISM: kinematics and dynamics -- ISM: molecules -- stars: formation
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Variability Study of the SX Phoenicis Star BL Camelopardalis

Jian-Ning Fu 1, Chao Zhang 1, Kanokwan Marak 2, Chayan Boonyarak 2, Pongsak Khokhuntod 2, Shi-Yang Jiang 3

1 Department of Astronomy, Beijing Normal University, Beijing 100875; jnfu@bnu.edu.cn

2 Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand

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

Abstract
New time-series photometric observations of BL Cam in the V band and white light were made during 2005 to 2007 at the Xinglong Station of China. The frequency analysis confirms two closely separated frequencies, 25.181 d-1 and 25.571 d-1, but the frequency of 31-32 d-1 reported in the literature was not detected in the new data. New times of maximum light were determined from both our light curves and those available on Internet, allowing a more comprehensive study of the O-C diagram, together with the times of maximum light in the literature. A new interpretation, including the period increasing before 1988 and decreasing since 1992 of BL Cam and the light-time effect in a binary system, looks plausible.

Key Words:   techniques: photometric -- stars: variables: delta Scuti -- stars: individual: BL Cam


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ChJAA, 2008, Vol.8, No.2

Chinese Journal of Astronomy and Astrophysics: Online Edition

http://www.chjaa.org 

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