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

2006, Vol.6, No.5 (October 20, 2006)



Contents


LETTERS

RESEARCH PAPERS

Extragalactic Astronomy


Solar System


The Sun


History of Astronomy




Abstract

Short-living Supermassive Magnetar Model for the Early X-ray Flares Following Short GRBs

Wei-Hong Gao1 and Yi-Zhong Fan2,3,4

1 Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, Nanjing 210008; gaoweihong@njnu.edu.cn
2 The Racah Inst. of Physics, Hebrew University, Jerusalem 91904, Israel; yizhongfan@phys.huji.ac.il
3 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008
4 Lady Davis Fellow

Abstract
We suggest a short-lived supermassive magnetar model to account for the X-ray flares following short $\gamma$ -ray bursts. In this model the central engine of the short $\gamma$ -ray bursts is a supermassive millisecond magnetar, formed in coalescence of double neutron stars. The X-ray flares are powered by the dipole radiation of the magnetar. When the magnetar has lost a significant part of its angular momentum, it collapses to a black hole and the X-ray flares cease abruptly.

Key Words:   Gamma-rays: bursts -- radiation mechanisms: nonthermal -- magnetic fields -- stars: neutron -- stars: rotation
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G328: A Small-Amplitude Red Variable with a Period Near One Day

Bao-An Yao1, Chang-Jun Shen2, Chun-Sheng Zhang2, Han-Ming Hu3 and Qing Lin1

1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030 yba@center.shao.ac.cn
2 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008
3 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011

Abstract
G328 = A65 = L3314 (V=13.83, B-V=1.91) is a field star in the direction of the globular cluster M4. If we take E(B-V) =0.40, then its (B-V)0 = 1.51, corresponding to a spectral type of K5III if it is a giant star; or of dM2 if it is a dwarf. Observations at both the MSSSO and Yunnan Observatory have shown that G328 is a new variable with peak to peak amplitude $\simeq$ 0.05 mag in V. While it is not unusual for so red a star to be a variable, special attention must be paid to its short period of about one day. If the variability is due to pulsation, the spectral type and luminosity as well as effective temperature should be determined in order to compare it with Xiong's theory.

Key Words:   stars: variables -- stars: individual (G328)
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Statistical Cluster-QSO Weak Lensing in the Sloan Digital Sky Survey

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

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; zlwen@vega.bac.pku.edu.cn
2 Department of Physics, Nanjing Normal University, Nanjing 210097

Abstract
We investigate the cross-correlation between galaxy clusters and QSOs using Sloan Digital Sky Survey (SDSS) DR4 $\sim
5000 \rm deg^2$ data. With photometric redshifts of galaxies, we select galaxy clusters based on the local projected densities of LRGs brighter than Mr'=-22. The QSOs are from the main sample of SDSS QSO spectroscopic survey to i'=19. A significant positive correlation is found between the clusters and QSOs. Under the assumption that the signal is caused by gravitational lensing, we fit the signal with singular isothermal sphere (SIS) model and NFW profile halo model. The velocity dispersion $\sigma_v=766 \rm
km s^{-1}$ is derived for the best-fit of SIS model. Best-fit for the NFW model requires the dark matter halo mass within 1.5 h-1 Mpc to be $4.6\times10^{14} h^{-1} M_{\odot}$ . The mass parameter $\Omega_{\rm cl}$ of the cluster sample is deduced as 0.077 with the SIS model and 0.083 with the NFW model. Our results of $\Omega_{\rm cl}$ are smaller than those given by Croom & Shanks and by Myers et al.

Key Words:   cosmology: observations -- galaxies: clusters: general -- quasars: general -- gravitational lensing
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High Brightness Temperatures in IDV Sources

Shan-Jie Qian1,2, T. P. Krichbaum2, A. Witzel2, J. A. Zensus2 and Xi-Zhen Zhang1

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; rqsj@bao.ac.cn
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Abstract
High brightness temperatures are a characteristic feature of IntraDay Variability (IDV) of extragalactic radio sources. Recent studies of the polarization properties of some IDV sources (e.g., 1150+812, PKS 0405-385 and 0716+714) have shown that these sources harbor several compact IDV components with angular sizes of $\sim$ 10-30$\mu$ as and very high polarizations (of up to $\sim$ 50%-70$\%$ ). These results indicate the possibility of the existence of uniform magnetic fields in the IDV components. We investigate the incoherent synchrotron and self- Compton radiation of an anisotropic distribution of relativistic electrons which spin around the magnetic field lines at small pitch angles. The brightness temperature limit caused by second-order Compton losses is discussed and compared to the brightness temperatures derived from energy equipartition arguments. It is found that anisotropic distributions of electrons moving in ordered magnetic fields can raise the equipartition and Compton brightness temperatures by a factor of up to $\sim$ 3-5. This would remove some of the difficulties in the interpretation of extremely high intrinsic brightness temperatures of >1012 K (or apparent brightness temperatures of $\sim10^{14}$  K with a Doppler factor of $\sim$ 30).

Key Words:   radio continuum: galaxies -- Quasars: individual: PKS 0405-385
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Radio Spectra of Three Supernova Remnants: G114.3+0.3, G116.5+1.1 and G116.9+0.2

Wen-Wu Tian1,2 and Denis Leahy2

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; tww@bao.ac.cn
2 Department of Physics & Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada

Abstract
Presented are new images of supernova remnants G114.3+0.3, G116.5+1.1 and G116.9+0.2 (CTB 1) at 408 MHz from the Canadian Galactic Plane Survey (CGPS). We also use the 1420 MHz images from the CGPS in a study of their 408-1420 MHz spectral indices. The flux densities at 408 MHz and 1420 MHz, corrected for flux densities from compact sources within the SNRs, are 12$\pm$ 6 Jy and 9.8$\pm$ 0.8 Jy for G114.3+0.3, 15.0$\pm$ 1.5 Jy and 10.6$\pm$ 0.6 Jy for G116.5+1.1, 15.0$\pm$ 1.5 Jy and 8.1$\pm$ 0.4 Jy for G116.9+0.2. The integrated flux density-based spectral indices (S$_{\nu}$ $\propto$ $\nu$ $^{-\alpha}$ ) are $\alpha$ =0.16$\pm$ 0.41, 0.28$\pm$ 0.09 and 0.49$\pm$ 0.09 for G114.3+0.3, G116.5+1.1 and G116.9+0.2, respectively. Their T-T plot-based spectral indices are 0.68$\pm$ 0.48, 0.28$\pm$ 0.15, and 0.48$\pm$ 0.04, in agreement with the integrated flux density-based spectral indices. New flux densities are derived at 2695 MHz which are significantly larger than previous values. The new 408, 1420 and 2695 MHz flux densities and published values at other frequencies, where images are not available, are fitted after correcting for contributions from compact sources, to derive their multi-frequency spectral indices.

Key Words:   ISM: individual G114.3+0.3, G116.5+1.1,G116.9+0.2 -- radio continuum: ISM
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Tail Emission from a Ring-like Jet: Its Application to Shallow Decays of Early Afterglows and GRB 050709

Yuan-Chuan Zou and Zi-Gao Dai

Department of Astronomy, Nanjing University, Nanjing 210093; zouyc@hust.edu.cn, dzg@nju.edu.cn

Abstract
Similar to the case of pulsars the magnetic axis and the spin axis of gamma-ray burst sources may not lie on the same line. This may cause the formation of a ring-like jet due to collimation of the precessing magnetic axis. We analyze the tail emission from such a jet, and find that it has a shallow decay phase with a temporal index of -1/2 if the Lorentz factor of the ejecta is not very high, which is consistent with the shallow decay phase of some early X-ray afterglow detected by Swift. The ring-like jet has a tail cusp with sharp rising and very sharp decay. This effect can provide an explanation for the re-brightening and sharp decay of the X-ray afterglow of GRB 050709.

Key Words:   gamma rays: bursts - X-rays: general
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The Early X-ray Afterglows of Optically Bright and Dark Gamma-Ray Bursts

Yi-Qing Lin

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

Abstract
A systematic study on the early X-ray afterglows of both optically bright and dark gamma-ray bursts (B-GRBs and D-GRBs) observed by Swift is presented. Our sample includes 25 GRBs of which 13 are B-GRBs and 12 are D-GRBs. Our results show that the distributions of the X-ray afterglow fluxes ($F_{\rm X}$ ), the gamma-ray fluxes ($S_{\gamma}$ ), and the ratio ( $R_{\gamma,
\rm X}$ ) are similar for the two kinds of GRBs, that any observed differences should be simply statistical fluctuation. These results indicate that the progenitors of the two kinds of GRBs are of the same population with comparable total energies of explosion. The suppression of optical emission in the D-GRBs should result from circumburst but not from their central engine.

Key Words:   gamma rays: bursts
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Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae Science Foundation of China.

Fa-Yin Wang and Zi-Gao Dai

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

Abstract
The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae (SNe Ia) and anisotropy of cosmic microwave background (CMB). The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``Gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$ , which is consistent with Riess et al. The transition redshift is $z_{\rm
T}=0.60_{-0.08}^{+0.06}$ . We also discuss several dark energy models that define w(z) of the parameterized equation of state of dark energy including one parameter and two parameters (w(z) being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from $z_{\rm T}=0.29_{-0.06}^{+0.07}$ to $z_{\rm
T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum redshift zc at which the current observations need the universe to accelerate.

Key Words:   cosmology: observations -- distance scale -- supernovae: general
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Toward an Understanding of the Periastron Puzzle of PSR B1259-63  

Ren-Xin Xu

Department of Astronomy, School of Physics, Peking University, Beijing 100871; r.x.xu@pku.edu.cn

Abstract
Efforts are made to understand the timing behaviors (e.g., the jumps in the projected pulsar semimajor axis at the periastron passages) observed in the 13-year monitoring of PSR~B1259--63. Planet-like objects are suggested to orbit around the Be star, which may gravitationally perturb the (probably low mass) pulsar when it passes through periastron. An accretion disk should exist outside the pulsar's light cylinder, which creates a spindown torque on the pulsar due to the propeller effect. The observed negative braking index and the discrepant timing residuals close to periastron could be related to the existence of a disk with a varying accretion rate. A speculation is presented that the accretion rate may increase on a long timescale in order to explain the negative braking index.

Key Words:   pulsars: individual: PSR~B1259-63 --- binaries: general --- stars: neutron
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High Resolution Spectroscopy of Halo Stars in the Near UV and Blue Region  
I. Spectra in the Wavelength Region 3550-5000Å

V. G. Klochkova 1,2, Gang Zhao1, S. V. Ermakov1,2 and V. E. Panchuk1,2

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012; gzhao@bao.ac.cn
2 Special Astrophysical Observatory, Russian Academy of Sciences, Nighnij Arkhyz, 369167 Karachai-Cherkessia, Russia

Abstract
An atlas of high resolution ($ R = 60 000$ ) CCD-spectra in the wavelength range 3500-5000Å is presented for four objects in metallicity range $\rm -3.0 <[Fe/H] < -0.6$ , temperature range $4750 < T_{\rm eff} < 5900  \rm K$ , and surface gravity range $1.6 < \lg{g} < 5.0$ . We describe the calibration of the stellar atmospheric parameters using Alonso's formula based on the method of infrared flux and outline the determination of the abundances of a total number of 25 chemical elements. An analysis of the abundance determination errors for different chemical elements is carried out, and a method is provided for the observations and reduction of spectral material. Properties of the method of producing an atlas of spectra and line identifications are described.

Key Words:   stars: halo stars - stars: G, K-type - stars: spectra - stars: UV-spectra
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Formation of the Kuiper Belt by Long Time-Scale Migration of Jovian Planets

Jian Li, Li-Yong Zhou and Yi-Sui Sun

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

Abstract
The orbital migration of Jovian planets is believed to have played an important role in shaping the Kuiper Belt. We investigate the effects of the long time-scale (2 x 107 yr) migration of Jovian planets on the orbital evolution of massless test particles that are initially located beyond 28AU. Because of the slowness of the migration, Neptune's mean motion resonances capture test particles very efficiently. Taking into account the stochastic behavior during the planetary migration and for proper parameter values, the resulting concentration of objects in the 3:2 resonance is prominent, while very few objects enter the 2:1 resonance, thus matching the observed Kuiper Belt objects very well. We also find that such a long time-scale migration is favorable for exciting the inclinations of the test particles, because it makes the secular resonance possible to operate during the migration. Our analyses show that the $\nu_8$ secular resonance excites the eccentricities of some test particles, so decreasing their perihelion distances, leading to close encounters with Neptune, which can then pump the inclinations up to 20$^{\circ}$ .

Key Words:   celestial mechanics -- Kuiper Belt -- methods: numerical -- solar system: formation
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Spectral Analysis and Atmospheric Models of Microflares

Cheng Fang1, Yu-Hua Tang1, and Zhi Xu1,2

1 Department of Astronomy, Nanjing University, Nanjing 210093; fangc@nju.edu.cn
2 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011

Abstract
By use of the high-resolution spectral data obtained with THEMIS on 2002 September 5, the spectra and characteristics of five well-observed microflares have been analyzed. Our results indicate that some of them are located near the longitudinal magnetic polarity inversion lines. All the microflares are accompanied by mass motions. The most obvious characteristic of the H$\alpha$ microflare spectra is the emission at the center of both H$\alpha$ and CaII 8542Å lines. For the first time both thermal and non-thermal semi-empirical atmospheric models for the conspicuous and faint microflares are computed. In computing the non-thermal models, we assume that the electron beam resulting from magnetic reconnection is produced in the chromosphere, because it requires lower energies for the injected particles. It is found there is obvious heating in the low chromosphere. The temperature enhancement is about 1000-2200 K in the thermal models. If the non-thermal effects are included, then the required temperature increase can be reduced by 100-150 K. These imply that the H$\alpha$ microflares can probably be produced by magnetic reconnection in the solar lower atmosphere. The radiative and kinetic energies of the H$\alpha$ microflares are estimated and the total energy is found to be 1027-4 x 1028 erg.

Key Words:   Sun: Microflares - Sun:spectrum - Sun: semi-empirical modelling
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An Impulsive Heating Model for the Evolution of Coronal Loops

Li Feng1,2 and Wei-Qun Gan1

1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; lfeng@pmo.ac.cn
2 Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191, Katlenburg-Lindau, Germany

Abstract
It was suggested by Parker that the solar corona is heated by many small energy release events generally called microflares or nanoflares. More and more observations showed flows and intensity variations in nonflaring loops. Both theories and observations have indicated that the heating of coronal loops should actually be unsteady. Using SOLFTM (Solar Flux Tube Model), we investigate the hydrodynamics of coronal loops undergoing different manners of impulsive heating with the same total energy deposition. The half length of the loops is 110 Mm, a typical length of active region loops. We divide the loops into two categories: loops that experience catastrophic cooling and loops that do not. It is found that when the nanoflare heating sources are in the coronal part, the loops are in non-catastrophic-cooling state and their evolutions are similar. When the heating is localized below the transition region, the loops evolve in quite different ways. It is shown that with increasing number of heating pulses and inter-pulse time, the catastrophic cooling is weakened, delayed, or even disappears altogether.

Key Words:   hydrodynamics -- Sun: corona -- Sun: transition region
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Derivation of the Spiral Motion of an Eruptive Prominence and Its Explanation

Xiao-Ma Gu1, Shu-Hua Zhong1, Hong-Fei Liang1 and La-Sheng Zhan2

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; xmgu@ynao.ac.cn
2 Jingdezhen Ceramic Institute, Jingdezhen 333000

Abstract
A 2D velocity field of the eruptive prominence (EP) of 1991 March 5 is obtained from its spectral data observed at the Yunnan Observatory and the velocity distributions along the entrance slit are derived for different observing frames. Under the assumption that matter in the EP undergoes axial, radial and possible rotational motions, we construct a theoretical velocity distribution of the EP along the entrance slit, to derive, by fitting, the angular velocity of rotation $\omega$ and the other three parameters (axial velocity v0, radial velocity vr and the angle between the EP plane and the line of sight $\phi$ ). We found: an averaged angular velocity $\omega$ of 3.0 x 10-3 arc s-1 and the variation of $\omega$ with the height above the solar limb. As the EP rises, the matter within it in fact moves along a spiral path around its axis. The spiral motion may be explained by the theory of plasma `double pole diffusion' (DPD) caused by a sharp density gradient between the eruptive prominence and the surrounding corona. A theoretical angular velocity $\omega'$ is estimated based on the DPD and basically coincides with $\omega$ obtained from the optimal velocity fitting.

Key Words:   Sun: eruptive prominence -- Sun: Spiral motion
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Historical Evidence for the Birth of the Newly Discovered Pulsar PSR J1833-1034

Zhen-Ru Wang1, Min Li1 and Yi Zhao2

1 Astronomy Department, Nanjing University, Nanjing 210093; zrwang@nju.edu.cn
2 Institute of Chinese Classics, Nanjing University, Nanjing 210093

Abstract
Stimulated by the recent discovery of PSR J1833-1034 in SNR G21.5-0.9 and its age parameters presented by two groups of discovery, we demonstrate that the PSR J1833-1034 was born 2053 years ago from a supernova explosion, the BC 48 guest star observed in the Western Han (Early Han) Dynasty by ancient Chinese. Based on a detailed analysis of the Chinese ancient record of the BC 48 guest star and the new detected physical parameters of PSR J1833-1034, agreements on the visual position, age and distance between PSR J1833-1034 and the BC 48 guest star are obtained. The initial period P0 of PSR J1833-1034 is now derived from its historical and current observed data without any other extra assumption on P0 itself, except that the factor $P\dot{P}$ is a constant in its evolution until now.

Key Words:   stars: pulsars: individual (PSR J 1833-1034) -- ISM: supernova remnants: individual (G21.5-0.9) -- supernovae: individual (BC 48 guest star) -- history of astronomy
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Observational Accuracy of Sunrise and Sunset Times in the Sixth Century China

Yong Li

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

Abstract
The Daye Calendar was compiled in AD 597 in the Sui Dynasty. We investigate the records of sunrise and sunset times on the 24 solar-term days in the calendar. By converting the ancient Chinese time units, Chen, Ke and Fen to hour, minute and second, and carrying out a comparison between the ancient records and values computed with modern astronomical theory, we find that the accuracy of solar measurements in the Sui period is remarkably high: for sunrise times, the average absolute deviation is 3.63 min (this value can be further reduced to 3.03 min when erroneous data are excluded), and for sunset times it is 3.48 min. We also find that the observed sunrise and sunset times are strictly symmetrically distributed with respect to both the Winter Solstice and the Summer Solstice, with their deviations showing a similar symmetrical distribution as well. We give a discussion on the date of observation, the feature of the data, and possible reasons of the deviation.

Key Words:   history of astronomy -- astrometry -- solar-terrestrial relations -- methods: statistical
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The Guest Star of AD185 must have been a Supernova

Fu-Yuan Zhao1, R. G. Strom2 and Shi-Yang Jiang1

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 fyzhao@bao.ac.cn
2 ASTRON, Radiosterrenwacht, Postbus 2, 7990 AA Dwingeloo, The Netherlands; and Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Abstract
The ``guest star'' of AD185, recorded in the ancient Chinese history the Houhanshu, has been widely regarded as a supernova. However, some authors have suggested that the guest star might have been a comet. It has also been proposed that the record is the concatenation of a nova with a comet made by an early compiler. We have checked the record of the guest star, comparing it with records of comets in the same history. We find that most descriptions of comets clearly indicate motion, whereas the record of the guest star does not. We further argue that the term ``yan'' used to describe the star's ``size'' might be short for yan-chuang (seat bed), and ``half a yan'' would be simply as an imaginary figuration of the ancient observer. Moreover, we show that the term ``hou -year" (hou-nian) most probably means the year after next. We argue that the asterism Southern Gate consisted of the stars $\alpha$ and $\beta $  Cen. We conclude that the record describing the guest star of AD 185 is completely different from any comet record in the same history, and that it almost certainly was a supernova.

Key Words:   history of astronomy -- guest star -- stars: supernovae: individual (SN185)
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ChJAA, 2006, Vol.6, No.5

Chinese Journal of Astronomy and Astrophysics: Online Edition

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