2001, Vol.1, No.1 (February 20, 2001)

Contents      

INVITED REVIEW

Gamma-Ray Bursts: Afterglows and Centeral Engines    
K. S. Cheng and T. Lu    ........................................................ p. 1
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

LETTER

Origin and Properties of Strong Mg II Quasar Absorption Line Syetems
Wei-Peng Lin and Zhen-Long Zou    ............................................... p.21
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

RESEARCH PAPERS

Testing the Universality of the TIS Model on Cluster Scales from 
the X-ray Surface Brightness Profiles
Yu-Ying Zhang    ................................................................ p.29
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Relations between the IR-UV-X-ray Continuum and Emission Lines for 
a Large Composite Sample of Narrow Line and Normal Seyfert 1 Galaxies
You-Jun Lu and Chong-Shan Zhao   ................................................ p.39
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Optical Identifications of X-ray Selected AGNs
Da-Wei Xu,Jian-Yan Wei and Jing-Yao Hu    ....................................... p.46
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

A Note on the Mixing Length Theory and Massive Star Evolution
Li-Cai Deng and Da-Run Xiong    ................................................. p.50
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Two Cataclysmic Variables Identified from ROSAT Bright Sources
Jiang-Hua Wu,Yang Chen,Xiang-Tao He,Xi-Zhen Zhang and Wolfgang Voges    ......... p.57
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

A Search for High-velocity Gas Associated H₂O Masers and Ultracompact 
H II Regions
Ye Xu, Dong-Rong Jiang, Zhi-Yao Yu, Xing-Wu Zheng, Chuan-Yi Yang,
Yan-Ping Zhang, Chun-Chuan Pei, Jun-Mei Ma, Jin-Jiang Sun, Li-Ming Wang, Deng-Rong Lu, Jie-Qing Wu, Yang Li and Cheng-Ming Lei   ....................................... p.60
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Anisotropy and Dissipation of Turbulence and Their Effects
on Solar Models 
Yan Li and Jia-Yuan Yang   ...................................................... p.66
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity 
You-Qiu Hu, Yan-Wei Jiang and Wei Liu    ........................................ p.77
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

A Filament-Associated Halo Coronal Mass Ejection
Jun Zhang, Jingxiu Wang and Nariaki Nitta   ..................................... p.85
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Abstract

K.S.Cheng and T.Lu

E-mail: hrspksc@hkucc.hku.hk

Abstract:

Gamma-ray bursts (GRBs) are the most intense transient gamma-ray events in the sky; this, together with the strong evidence (the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, makes them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to the total convertion into radiation of the mass energy of more than one solar mass. This is thousand times stronger than the energy of a supernova explosion. Some unconventional energy mechanism and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae at cosmological distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on their central engine. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands.

Keywords: gamma-rays: bursts --- shock waves --- ISM: jets and outflows --- radiation mechanisms: non-thermal

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Wei-Peng Lin and Zheng-long Zou

E-mail: linwp@bac.pku.edu.cn

Abstract:

Strong Mg II quasar absorption line systems provide us with a useful tool to understand the gas that plays an important role in galaxy formation. In this paper, placing the theories of galaxy formation in a cosmological context, we present semi-analytic models and Monte-Carlo simulations for strong Mg II absorbers produced in gaseous galactic haloes and/or galaxy discs. We investigate the redshift path density for the Mg II absorption lines and the properties of galaxy/absorber pairs, in particular the anti-correlation between the equivalent width of Mg II absorption line and the projected galaxy-to-sightline distance. The simulated result of the mean redshift path density of strong Mg II systems is consistent with the observational result. The fraction of strong Mg II systems arising from galaxy disks is predicted to be 10% of the total. There exists an anti-correlation between the absorption line equivalent and the projected distance of sightline to galaxy center and galaxy luminosity. We determined that the mean absorbing radius R_abs 29 h ^-1 kpc (LB/LB*) ^0.35. After taking selection effects into consideration, this becomes R _abs 38 h ^-1 kpc (LB/LB*)^0.18, which is in good agreement with the observational result. This shows the importance of considering selection effects when comparing models with observations.

Keywords: galaxies:formation---galaxies: fundamental parameters--- galaxies: haloes---quasars: absorption lines

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Yu-Ying Zhang

E-mail: zyy@class2.bao.ac.cn

Abstract:

The truncated isothermal sphere (TIS) model has been recently suggested as an alternative for virialized dark halos (Shapiro et al. 1999). Both its profound theoretical motivation and its successful explanations for the galactic rotation curves and the gravitational scaling laws of clusters indicate that the TIS model is a promising candidate among other prevailing models such as the NFW profile and the Burkert profile. This promotes us to re-examine the universality of the TIS model on cluster scales from a different angle. Using an ensemble of X-ray surface brightness profiles of 45 clusters, we test the goodness of fit of the TIS predicted gas distributions to the X-ray data under the assumption of isothermal, hydrostatic equilibrium. Unlike the conventional model or the NFW/Burkert profile, for which about half of the clusters have the reduced values smaller than 2, the TIS model fails in the fitting of the X-ray surface brightness profiles of clusters in the sense that 38 out of the 45 clusters show >2. This may constitute a challenge for the universality of the TIS model unless the present analysis is seriously contaminated by other uncertainties including the negligence of non-gravitational heating processes and the unconventional sampling of the X-ray data.

Keywords: cosmology: theory --- dark matter --- galaxies: clusters:  general --- X-rays: galaxies

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You-Jun Lu and Chong-Shan Zhao

E-mail: cszhao@mail.ustc.edu.cn

Abstract:

We report on our research on the UV, optical line parameters and the infrared through UV to soft X-ray continuum parameters for  a composite sample of narrow line and normal Seyfert 1 galaxies.  The strong correlations among the line width of H, optical line strength of FeII and the soft X-ray slope are confirmed. We found no correlations between the UV line parameters, the equivalent widths of Ly and CIV and their ratio on one hand and the optical line parameters on the other. The UV and X-ray luminosities strongly correlate with the line widths of H and the FeII/H ratio. No significant correlation is found between the infrared-soft X-ray continuum slope _ix and the line width of H.

Keywords: galaxies: active galactic nuclei (AGN) --- emission lines --- Ultraviolet: galaxies --- galaxies: X-rays

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Da-Wei Xu,Jian-Yan Wei and Jing-Yao Hu

E-mail: dwxu@bao.ac.cn

Abstract:

For investigating the statistical properties of X-ray selected  Active Galactic Nuclei (AGNs), we have carried out a program of  optical identification of a selection of X-ray sources from ROSAT All Sky Survey Bright Source Catalogue (RASS-BSC) using the 2.16 m telescope of Beijing Astronomical Observatory (BAO). In the preliminary observations, 23 new AGNs were discovered, of which 9 are quasars, and 14 are Seyfert galaxies.

Keywords:  X-rays: galaxies --- galaxies: quasar --- galaxies: Seyfert

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Li-Cai Deng and Da-Run Xiong

E-mail: licai@alpha.bao.ac.cn

Abstract:

In this paper, we investigate the problem of supersonic convection caused by the application of the usual Mixing Length Theory (MLT), and give a modification to the original expression of the MLT. In the case of the usual MLT, a zone of supersonic convection exists when the stellar model goes into yellow--red supergiant phase. When the modification is applied, the models of late type supergiants possess shallower convective envelopes for a given temperature compared with the normal formalism of the MLT. Therefore the stellar models made with our new formalism move to lower effective temperature by 100--400 K depending on the luminosity of the star. Such a modification does not affect the convective envelope structure of lower luminosity late type stars, as both expressions of the MLT give the same results in these stars.

Keywords: convection --- stars: supergiants --- stars: massive stars --- stars: evolution

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Jiang-Hua Wu,Yang Chen,Xiang-Tao He,Xi-Zhen Zhang and Wolfgang Voges

E-mail: jhwu@bao.ac.cn

Abstract:

This paper presents the results of optical spectroscopic observations of two ROSAT bright sources, 1RXS J020928.9+283243 and 1RXS J042332.8+745300. The low-dispersion spectra suggest the cataclysmic variable classification for the two objects. Further photometric observations are expected to reveal the variable features and to confirm the classifications.

Keywords: X-rays: stars --- stars: cataclysmic variable --- X-rays

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Abstract

With the objective of studying the relationships between high-velocity gas and water maser emissions the results of a search from 95 IRAS sources for high-velocity gas associated with star forming molecular clouds are reported. 21 sources have been identified as molecular outflow candidates.

Key words: H II regions --- ISM: jets and outflows --- stars: formation --- masers 

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Abstract

Based on a dynamic model for turbulent convection, we investigate the effects of dissipation and anisotropy of the turbulence on the convective energy transport. We introduce two time scales to describe the dissipation of the turbulence, and approximate the anisotropy of the turbulence by Rotta's proposal of ``return to isotropy''. The improved turbulence model results in an equation to determine the temperature gradient in the convection zone, which is of similar form as that of the MLT. We apply the improved MLT to solar models, and find that the increases of the anisotropy and decreases of the dissipation of the turbulence reduce the value of the convection parameter , because these processes enhance the convective energy transfer rate. Compared with the observed solar p-mode frequencies, it is plausible that the dissipation of the turbulence in the solar convection zone  should be fairly strong, while the degree of anisotropy of the turbulence  plays a less significant role on the structure of the solar convection zone.

Key words: Sun: interior --- turbulent convection --- Sun: oscillations 

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Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity 

You-Qiu Hu, Yan-Wei Jiang and Wei Liu

E-mail: huyq@ustc.edu.cn

Abstract

Using a 2.5-dimensional (2.5-D) ideal MHD model, this paper analyzes the equilibrium properties of coronal magnetic flux ropes in a bipolar ambient magnetic field. It is found that the geometrical features of the magnetic flux rope, including the height of the rope axis, the half-width of the rope, and the length of the vertical current sheet below the rope, are determined by a single magnetic parameter, the magnetic helicity, which is the sum of the self-helicity of the rope and the mutual helicity between the rope field and the ambient magnetic field. All the geometrical parameters increase monotonically with increasing magnetic helicity. The implication of this result in solar active phenomena is briefly discussed.

Key words: Sun: magnetic fields --- Sun: magnetic flux rope --- Sun: magnetic helicity

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A Filament-Associated Halo Coronal Mass Ejection

Jun Zhang, Jingxiu Wang, and Nariaki Nitta

E-mail: zjun@ourstar.bao.ac.cn

Abstract

There are only a few observations published so far that show the initiation of a coronal mass ejection (CME) and illustrate the magnetic changes in the surface origin of a CME. Any attempt to connect a CME with its local solar activities is meaningful. In this paper we present a clear instance of a halo CME initiation. A careful analysis of magnetograms shows that the only obvious magnetic changes in the surface region of the CME is a magnetic flux cancellation underneath a quiescent filament. The early disturbance was seen as the slow upward motion in segments of the quiescent filament. Four hours later, the filament was accelerated to about 50 km s^-1 and erupted. While a small part of the material in the filament was ejected into the upper corona, most of the mass was transported to a nearby region. About forty minutes later, the transported mass was also ejected partially to the upper corona. The eruption of the filament triggered a two-ribbon flare, with post-flare loops connecting the flare ribbons. A halo CME, which is inferred to be associated with the eruptive filament, was observed from LASCO/C2 and C3. The halo CME contained two CME events, each event corresponded to a partial mass ejection of the filament. We suggest that the magnetic reconnection at the lower atmosphere is responsible for the filament eruption and the halo CME.

Key words: Sun: filament --- Sun: magnetic fields ---Sun: coronal mass ejection

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