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
Abstract
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
PDF file (943 KB) |
PS file *.gz (1327 KB) |
Back to Contents
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
Abstract
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

, 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

, 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
PDF file (417 KB) |
PS file *.gz (186 KB) |
Back to Contents
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
Abstract
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

(

) 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

line-widths and optical
luminosities. The dimensionless accretion rates

are derived from the optical
continuum luminosities by using two different models: using an
empirical relation between the bolometric luminosity

and the optical luminosity (

,
a fixed radiative efficiency is adopted); and calculating the
optical spectra of accretion disks as a function of

. We
find a significant correlation between the derived

and
the observed line width of H

,
FWHM

, which almost overlaps the
disk-corona model calculations, if the viscosity

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
PDF file (250 KB) |
PS file *.gz (130 KB) |
Back to Contents
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
Abstract
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

1

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
PDF file (716 KB) |
PS file *.gz (164 KB) |
Back to Contents
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
Abstract
The
Chandra Galactic Center Survey detected

X-ray point-like sources in the

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

2.5. Assuming
that all other sources have the same power-law form, the relation
between hardness ratio and HI column density

is used
to estimate the

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
(
10
32-10
35 erg s
-1).
Key Words: methods: data analysis -- Galaxy: center --
X-ray: stars
PDF file (305 KB) |
PS file *.gz (199 KB) |
Back to Contents
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
Abstract
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

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
PDF file (203 KB) |
PS file *.gz (730 KB) |
Back to Contents
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
Abstract
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

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
PDF file (233 KB) |
PS file *.gz (115 KB) |
Back to Contents
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
Abstract
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
PDF file (182 KB) |
PS file *.gz (148 KB) |
Back to Contents
A Kinematical Calibration of the Galactocentric Distance
Ming Shen and Zi Zhu
Department of Astronomy, Nanjing University, Nanjing
210093;
shenming@nju.org.cn
Abstract
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

kpc, while a similar evaluation from 270
Galactic open clusters gives
R0=7.95

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
PDF file (281 KB) |
PS file *.gz (113 KB) |
Back to Contents
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
Abstract
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
PDF file (691 KB) |
PS file *.gz (2528 KB) |
Back to Contents
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
Abstract
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 (

0.1-1

) 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

MeV nucl
-1 by the ion-ion hybrid waves and be
further accelerated to the energy of

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
PDF file (209 KB) |
PS file *.gz (180 KB) |
Back to Contents
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
Abstract
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

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

and

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
PDF file (179 KB) |
PS file *.gz (315 KB) |
Back to Contents
ChJAA, 2007, Vol.7,
No.1
Chinese Journal of Astronomy and Astrophysics:
Online Edition
http://www.chjaa.org
Copyright 2001--2007 All rights reserved. The National Astronomical
Observatories of
Chinese Academy of Sciences, P.R. China. This Journal or parts thereof, may
not be
reproduced in any form or by any means without written permission from the
Copyright owner.
*****
The web version of ChJAA is created and maintained by Aiying Zhou