$\textstyle \parbox{150mm}{ {\LARGE\bf Chinese Journal of Astronomy and Astrophysics}\\ }$

2007, Vol.7, No.5 (October 20, 2007)


Contents
RESEARCH PAPERS

Extragalactic Astronomy
Stars
Instruments, Observational Techniques and Data Processing
The Sun




Abstract

Seven-frequency VLBI Observations of the GHz-Peaked-Spectrum Source OQ 208

Wen-Feng Luo1,3, Jun Yang2,3, Lang Cui1,3, Xiang Liu1 and Zhi-Qiang Shen2,4

1 National Astronomical Observatories / Urumqi Observatory, Chinese Academy of Sciences, Urumqi 830011; liux@ms.xjb.ac.cn

2 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030

3 Graduate School of Chinese Academy of Sciences, Beijing 100049

4 Joint Institute for Galaxy and Cosmology, SHAO and USTC

Abstract
We present quasi-simultaneous VLBI images of the GHz-Peaked-Spectrum radio source OQ 208 obtained with the Very Long Baseline Array at 1.4, 1.7, 2.3, 5.0, 8.4, 15.4GHz and the European VLBI Network at 6.7GHz. The low frequency (1.4, 1.7 and 2.3GHz) observations reveal a weak and extended steep-spectrum component at about 30mas away at position angle $- 110^\circ$, which may be a remnant emission. The radio structure of OQ 208 consists of two mini-lobes at 5.0, 6.7, 8.4 and 15.4GHz. Our spectral analysis further confirms that the southwest lobe undergoes free-free absorption and finds that the free-free absorption is stronger in the inner region. By fitting the 8.4GHz images from 1994 to 2005, we obtain a separation speed of 0.031$\pm$0.006mas yr-1 between the two mini-lobes. This indicates a jet proper motion of 0.105$\pm$0.020c and a kinematic age of 219$\pm$42yr for the radio source.

Key Words:   galaxies: individual (OQ 208) -- radio continuum: galaxies
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Luminosity Function of the Cluster of Galaxies Abell 566

Quan-Bao Xiao 1,2, Zheng-Yi Shao1,2 and Xu Zhou3

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

2 Joint Institute for Galaxy & Cosmology, Chinese Academy of Sciences, Shanghai 200030

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

Abstract
We investigate the Luminosity Function (LF) of the cluster of galaxies Abell 566. The photometric data of 15 intermediate-bands are obtained from the Beijing-Arizona-Taiwan-Connecticut (BATC) photometric sky survey. For each of the 15 wavebands, the LF of cluster galaxies is well modelled by the Schechter function, with characteristic luminosities from -18.0 to -21.9 magnitude, from the a- to the p-band. Morphological dependence of the LF is investigated by separating the cluster members into `red' and `blue' subsamples. It is clear that late type galaxies have a steeper shape of LF than the early type galaxies. We also divided the sample galaxies by their local environment. It was found that galaxies in the sparser region have steeper shape of LF than galaxies in the denser region. Combining the results of morphological and environmental dependence of LFs, we show that Abell 566 is a well relaxed cluster with positive evidence of galaxy interaction and merger, and excess number of bright early type galaxies located in its denser region.

Key Words:   galaxy: cluster: individual: Abell 566 -- galaxy: photometry -- galaxy: luminosity function
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A Statistical Study of XBLs, RBLs and FSRQs at 1.5 GHz

Ji-Liang Zhou1, Jun-Hui Fan1, Juan Li2 and Yi Liu1

1 Center for Astrophysics, Guangzhou University, Guangzhou 510006; jiliangzhou@163.com

2 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030

Abstract
BL Lac objects are similar to the flat spectrum radio quasars in many aspects except regarding the emission lines. In order to study their relationship, we selected 56 BL Lacertae objects (33 X-ray-selected, 23 radio-selected) and 45 flat spectrum radio quasars, analyzed their radio luminosities and core-dominance parameters. We found that the radio luminosities of the radio selected BL Lac objects located in between the X-ray selected BL Lac objects and the flat spectrum radio quasars. However, this intermediate position does not hold for the core-dominance parameter: the RBLs have the largest core-dominance parameters. This suggests that the core-dominance parameter can not be taken as a sequencing criterion. We also investigated the correlation between the luminosity and the core-dominance parameter for the three subclasses. We concluded that, here, the sequence XBL-RBL--FSRQ still exists.

Key Words:   techniques: statistical studies -- blazars: general -- luminosity: radio
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Local Environmental Dependence of Galaxy Properties in a Volume-Limited Sample of Main Galaxies

Xin-Fa Deng, Ji-Zhou He, Qun Zhang, Xiao-Xia Qian, Peng Jiang and Yong Xin

School of Science, Nanchang University, Nanchang 330031; xinfadeng@163.com

Abstract
Using a volume-limited sample of Main Galaxies from SDSS Data Release 5, we investigate the dependence of galaxy properties on local environment. For each galaxy, a local three-dimensional density is calculated. We find that the galaxy morphological type depends strongly on the local environment: galaxies in dense environments have predominantly early type morphologies. Galaxy colors have only a weak dependence on the environment. This puts an important constraint on the process of galaxy formation.

Key Words:  galaxy: distances and redshifts -- galaxies: statistics
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Are Type Ia Supernovae Reliable Distance Indicators?

Lian-Zhong Lü1,2,4, Yi-Ping Qin2,3 and Fu-Wen Zhang1,4

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

2 Physics Department, Guangxi University, Nanning 530004

3 Center for Astrophysics of Guangzhou University, Guangzhou 510006

4 Graduate School of Chinese Academy of Sciences, Beijing 100049

Abstract
Recent applications of type Ia supernovae (SNe Ia) in cosmology have successfully revealed the accelerating expansion of the universe. However, as distance indicators used in measuring the expansion history of the universe and probing the nature of dark energy, these objects must pass more strict tests. We propose a K-S test to investigate if there exists any systematic bias when deriving the luminosity distances under the standard candle assumption. Two samples, one comprising 71 high-redshift SNe Ia and the other, 44 nearby ones, are used in our investigation. We find that it is likely there exists a bias in the adopted samples, which is probably caused by a systematic error, e.g. in the color parameter used in the luminosity calibration and a bias may be caused by the SN evolution or by varying properties of the dust surrounding the SNe Ia.

Key Words:   cosmology: observations -- distance scale -- galaxies: distances and redshifts -- methods: statistical -- supernovae: general
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An Infrared Photometric Study of Galaxies with Extragalactic H2O Maser Sources

Pei-Sheng Chen, Yi-Fei Gao and Hong-Guang Shan

National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; iraspsc@yahoo.com.cn

Abstract
All galaxies with extragalactic H2O maser sources observed so far are collected. With the 2MASS and the IRAS photometric data an infrared study is performed on those galaxies. By a comparison between the H2O maser detected sources and non-detected sources in the infrared it is indicated that infrared properties in the IRAS 12-25$\mu $m and 60-100$\mu $m are important for producing H2O masers in galaxies. It is also found that the H2O maser galaxies with different nuclear activity types have rather different infrared properties mainly in the IRAS 12-60$\mu $m region.

Key Words:   galaxies: maser -- galaxies: nuclei -- galaxies: starburst -- infrared: galaxies
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Shallow Decay of X-ray Afterglows in Short GRBs: Energy Injection from a Millisecond Magnetar?

Yu Yu1 and Yong-Feng Huang2

1 Shanghai Astronomical Observatories, Chinese Academy of Sciences, Shanghai 200030 yuy@mail.ustc.edu.cn

2 Department of Astronomy, Nanjing University, Nanjing 210093

Abstract
With the successful launch of Swift satellite, more and more data of early X-ray afterglows from short gamma-ray bursts have been collected. Some interesting features such as unusual afterglow light curves and unexpected X-ray flares are revealed. Especially, in some cases, there is a flat segment in the X-ray afterglow light curve. Here we present a simplified model in which we believe that the flattening part is due to energy injection from the central engine. We assume that this energy injection arises from the magnetic dipole radiation of a millisecond pulsar formed after the merger of two neutron stars. We check this model with the short GRB 060313. Our numerical results suggest that energy injection from a millisecond magnetar could make part of the X-ray afterglow light curve flat.

Key Words:   gamma rays: bursts -- X-rays: individual (GRB 060313) -- ISM -- stars: neutron
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Observational Constraints on Quark Matter in Neutron Stars

Na-Na Pan and Xiao-Ping Zheng

Institute of Astrophysics, Huazhong Normal University, Wuhan 430079; Pannana@phy.ccnu.edu.cn

Abstract
We study the observational constraints of mass and redshift on the properties of the equation of state (EOS) for quark matter in compact stars based on the quasi-particle description. We discuss two scenarios: strange stars and hybrid stars. We construct the equations of state utilizing an extended MIT bag model taking the medium effect into account for quark matter and the relativistic mean field theory for hadron matter. We show that quark matter may exist in strange stars and in the interior of neutron stars. The bag constant is a key parameter that affects strongly the mass of strange stars. The medium effect can lead to the stiffer hybrid-star EOS approaching the pure hadronic EOS, due to the reduction of quark matter, and hence the existence of heavy hybrid stars. We find that a middle range coupling constant may be the best choice for the hybrid stars being compatible with the observational constraints .

Key Words:   dense matter -- gravitation -- stars: neutron -- stars: rotation -- stars: oscillations
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Effects of Magnetic Fields on Neutrino-dominated Accretion Model for Gamma-ray Bursts

Yi Xie 1,2, Chang-Yin Huang 1 and Wei-Hua Lei 1

1 Department of Physics, Huazhong University of Science and Technology, Wuhan 430074 sourcexieyi@gmail.com

2 School of Physics and Information Engineering, Shanxi Normal University, Linfen 041004

Abstract
Many models of gamma-ray bursts suggest a common central engine: a black hole of several solar masses accreting matter from a disk at an accretion rate from 0.01 to 10  $M_\odot {\rm s}^{-1}$, the inner region of the disk is cooled by neutrino emission and large amounts of its binding energy are liberated, which could trigger the fireball. We improve the neutrino-dominated accreting flows by including the effects of magnetic fields. We find that more than half of the liberated energy can be extracted directly by the large-scale magnetic fields in the disk, and it turns out that the temperature of the disk is a bit lower than the neutrino-dominated accreting flows without magnetic field. Therefore, the outflows are magnetically-dominated rather than neutrino dominated. In our model, the neutrino mechanism can fuel some GRBs (not the brightest ones), but cannot fuel X-ray flares. The magnetic processes (both BZ and electromagnetic luminosity from a disk) are viable mechanisms for most of GRBs and their following X-ray flares.

Key Words:   magnetic fields -- accretion, accretion disks-- neutrinos -- gamma rays: bursts
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X-ray Spectroscopy of Dips of Cir X-1

Ya-Juan Lei1, Fang-Jun Lu1, Jin-Lu Qu1, Li-Ming Song1 and Cheng-Min Zhang2

1 Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049; leiyj@mail.ihep.ac.cn

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

Abstract
We present X-ray spectral analyses of the low-mass X-ray binary Cir X-1 during X-ray dips, using the Rossi X-ray Timing Explorer (RXTE) data. Each dip was divided into several segments, and the spectrum of each segment was fitted with a three-component blackbody model, in which the first two components are affected by partial covering and the third one is unaffected. A Gaussian emission line is also included in the spectral model to represent the Fe K$\alpha$ line at $\sim$ 6.4keV. The fitted temperatures of the two partially covered components are about 2keV and 1keV, while the uncovered component has a temperature of $\sim$0.5-0.6keV. The equivalent blackbody emission radius of the hottest component is the smallest and that of the coolest component is the largest. During the dips the fluxes of the two hot components are linearly correlated, while that of the third component does not show any significant variation. The Fe line flux remains constant, within the errors, during the short dips. However, during the long dips the line flux varies significantly and is positively correlated with the fluxes of the two hot components. These results suggest: (1) that the temperature of the X-ray emitting region decreases with radius, (2) that the Fe K$\alpha$ line emitting region is close to the hot continuum emitting region, and (3) that the size of the Fe line emitting region is larger than that of the obscuring matter causing the short dips but smaller than the region of that causing the long dips.

Key Words:   stars: individual (Circinus X-1) -- stars: neutron -- X-rays: stars
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CO Clouds around SNR G21.8-0.6 and G32.8-0.1

Jian-Jun Zhou1,2,3, Xi-Zheng Zhang1, Hong-Bo Zhang1, Jarken Esimbek2, Ju-Yong Zhang1 and Bing-Gang Ju4

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; zhoujj@ms.xjb.ac.cn

2 National Astronomical Observatories / Urumqi Observatory, Chinese Academy of Sciences, Urumqi 830011

3 Graduate School of Chinese Academy of Sciences, Beijing 100049

4 Qinghai Station of Purple Mountain Observatory, Chinese Academy of Sciences, Delingha 817000

Abstract
We made the first CO(1-0) mapping to SNR G21.8-0.6 and SNR G32.8-0.1, both associated with OH 1720MHz maser. Based on the morphological correspondence and velocity and position agreement between the radio remnant and the CO clouds, we tentatively identify the clouds that are respectively interacting with the two SNRs.

Key Words:  ISM: clouds -- ISM: individual (G21.8-0.6, G32.8-0.1) -- ISM: molecules -- masers -- supernova remnants
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A Study on the Technique of Observing Interplanetary Scintillation with Simultaneous Dual-Frequency Measurements

Xi-Zhen Zhang

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

Abstract
Ground-based observation of Interplanetary Scintillation (IPS) is an important approach of monitoring solar wind speed. We describe both the principle and method of observing the solar wind speed by using the normalized cross-spectrum of simultaneous dual-frequency IPS measurement. The effects of the solar wind properties and the angular size of the scintillation source on the measurement of solar wind speed are investigated by numerical analysis. We carry out a comparison of this method with the traditional single station-single frequency method. We outline a new IPS observation system using this method now under construction at the National Astronomical Observatories, CAS (NAOC).

Key Words:   interplanetary scintillation -- method -- instrument -- telescope
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Small-Scale Brightenings in the UV Continuum of an M9.1 Solar Flare

Lin Wang, Cheng Fang and Ming-De Ding

Department of Astronomy, Nanjing University, Nanjing 210093; wangl@nju.edu.cn

Abstract
We analyze an M9.1 two-ribbon solar flare which occurred on 2004 July 22 using the TRACE white-light and 1700 Å images, the RHESSI, and the SOHO/MDI data. We find many small-scale fast-varying brightenings that appeared in the white-light and 1700 Å images along the flare ribbons. Some of them underwent rapid motions in weak magnetic field regions. We identify these short-lived brightenings as UV continuum enhancement. Our preliminary result shows that the brightenings are closely related to the HXR emission. They have a lifetime of 30-60s and a typical size of about 1''-2''. The intensity enhancement is about 150-200 times the mean value of the quiet-Sun. According to previous works, we infer that the 1700Å enhancement may be dominated by the increased emission of 1680Å continuum coming from the temperature minimum region. The impulsive feature in the 1700Å light curves of the small-scale brightenings may be due to the irradiation of the impulsive CIV line intensity caused by the bombardment of non-thermal electron beams.

Key Words:   Sun: activity -- Sun: flares -- Sun: UV radiation
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A Statistical Study of Rapid Sunspot Structure Change Associated with Flares

Wei-Zhong Chen, Chang Liu, Hui Song, Na Deng, Chang-Yi Tan and Hai-Min Wang

Center for Solar-Terrestrial Research, New Jersey Institute of Technology, University Heights, Newark, NJ, 07102-1982, USA; haimin@flare.njit.edu

Abstract
We reported recently some rapid changes of sunspot structure in white-light (WL) associated with major flares. We extend the study to smaller events and present here results of a statistical study of this phenomenon. In total, we investigate 403 events from 1998 May 9 to 2004 July 17, including 40 X-class, 174 M-class, and 189 C-class flares. By monitoring the structure of the flaring active regions using the WL observations from the Transition Region and Coronal Explorer (TRACE), we find that segments in the outer sunspot structure decayed rapidly right after many flares; and that, on the other hand, the central part of sunspots near the flare-associated magnetic neutral line became darkened. These rapid and permanent changes are evidenced in the time profiles of WL mean intensity and are not likely resulted from the flare emissions. Our study further shows that the outer sunspot structure decay as well as the central structure darkening are more likely to be detected in larger solar flares. For X-class flares, over 40% events show distinct sunspot structure change. For M- and C-class flares, this percentage drops to 17% and 10%, respectively. The results of this statistical study support our previously proposed reconnection picture, i.e., the flare-related magnetic fields evolve from a highly inclined to a more vertical configuration.

Key Words:   Sun: activity -- Sun: flares -- Sun: magnetic fields -- sunspots
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ChJAA, 2007, Vol.7, No.5

Chinese Journal of Astronomy and Astrophysics: Online Edition

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