7,651 research outputs found

### An Imaging and Spectral Study of Ten X-Ray Filaments around the Galactic Center

We report the detection of 10 new X-ray filaments using the data from the
{\sl Chandra} X-ray satellite for the inner $6^{\prime}$ ($\sim 15$ parsec)
around the Galactic center (GC). All these X-ray filaments are characterized by
non-thermal energy spectra, and most of them have point-like features at their
heads that point inward. Fitted with the simple absorbed power-law model, the
measured X-ray flux from an individual filament in the 2-10 keV band is $\sim
2.8\times10^{-14}$ to $10^{-13}$ ergs cm$^{-2}$ s$^{-1}$ and the
absorption-corrected X-ray luminosity is $\sim 10^{32}-10^{33}$ ergs s$^{-1}$
at a presumed distance of 8 kpc to the GC. We speculate the origin(s) of these
filaments by morphologies and by comparing their X-ray images with the
corresponding radio and infrared images. On the basis of combined information
available, we suspect that these X-ray filaments might be pulsar wind nebulae
(PWNe) associated with pulsars of age $10^3 \sim 3\times 10^5$ yr. The fact
that most of the filament tails point outward may further suggest a high
velocity wind blowing away form the GC.Comment: 29 pages with 7 figures and 3 pages included. Accepted to Ap

### Axisymmetric stability criterion for two gravitationally coupled singular isothermal discs

Using the two-fluid formalism with the polytropic approximation, we examine
the axisymmetric stability criterion for a composite system of gravitationally
coupled stellar and gaseous singular isothermal discs (SIDs).Comment: 12 pages, 11 figures, accepted by MNRA

### Envelope Expansion with Core Collapse. III. Similarity Isothermal Shocks in a Magnetofluid

We explore MHD solutions for envelope expansions with core collapse (EECC)
with isothermal MHD shocks in a quasi-spherical symmetry and outline potential
astrophysical applications of such magnetized shock flows. MHD shock solutions
are classified into three classes according to the downstream characteristics
near the core. Class I solutions are those characterized by free-fall collapses
towards the core downstream of an MHD shock, while Class II solutions are those
characterized by Larson-Penston (LP) type near the core downstream of an MHD
shock. Class III solutions are novel, sharing both features of Class I and II
solutions with the presence of a sufficiently strong magnetic field as a
prerequisite. Various MHD processes may occur within the regime of these
isothermal MHD shock similarity solutions, such as sub-magnetosonic
oscillations, free-fall core collapses, radial contractions and expansions. We
can also construct families of twin MHD shock solutions as well as an
`isothermal MHD shock' separating two magnetofluid regions of two different yet
constant temperatures. The versatile behaviours of such MHD shock solutions may
be utilized to model a wide range of astrophysical problems, including star
formation in magnetized molecular clouds, MHD link between the asymptotic giant
branch phase to the proto-planetary nebula phase with a hot central magnetized
white dwarf, relativistic MHD pulsar winds in supernova remnants, radio
afterglows of soft gamma-ray repeaters and so forth.Comment: 21 pages, 33 figures, accepted by MNRA

### Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs

In a composite system of gravitationally coupled stellar and gaseous discs,
we perform linear stability analysis for axisymmetric coplanar perturbations
using the two-fluid formalism. The background stellar and gaseous discs are
taken to be scale-free with all physical variables varying as powers of
cylindrical radius $r$ with compatible exponents. The unstable modes set in as
neutral modes or stationary perturbation configurations with angular frequency
$\omega=0$.Comment: 7 pages using AAS styl

### Periodicities in Solar Coronal Mass Ejections

Mid-term quasi-periodicities in solar coronal mass ejections (CMEs) during
the most recent solar maximum cycle 23 are reported here for the first time
using the four-year data (February 5, 1999 to February 10, 2003) of the Large
Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric
Observatory (SOHO). In parallel, mid-term quasi-periodicities in solar X-ray
flares (class >M5.0) from the Geosynchronous Operational Environment Satellites
(GOES) and in daily averages of Ap index for geomagnetic disturbances from the
World Data Center (WDC) at the International Association for Geomagnetism and
Aeronomy (IAGA) are also examined for the same four-year time span. Several
conceptual aspects of possible equatorially trapped Rossby-type waves at and
beneath the solar photosphere are discussed.Comment: Accepted by MNRAS, 6 figure

### Distribution of Spectral Lags in Gamma Ray Bursts

Using the data acquired in the Time To Spill (TTS) mode for long gamma-ray
bursts (GRBs) collected by the Burst and Transient Source Experiment on board
the Compton Gamma Ray Observatory (BATSE/CGRO), we have carefully measured
spectral lags in time between the low (25-55 keV) and high (110-320 keV) energy
bands of individual pulses contained in 64 multi-peak GRBs. We find that the
temporal lead by higher-energy gamma-ray photons (i.e., positive lags) is the
norm in this selected sample set of long GRBs. While relatively few in number,
some pulses of several long GRBs do show negative lags. This distribution of
spectral lags in long GRBs is in contrast to that in short GRBs. This apparent
difference poses challenges and constraints on the physical mechanism(s) of
producing long and short GRBs. The relation between the pulse peak count rates
and the spectral lags is also examined. Observationally, there seems to be no
clear evidence for systematic spectral lag-luminosity connection for pulses
within a given long GRB.Comment: 20 pages, 4 figure

### Scale-Free Thin Discs with an Isopedic Magnetic Field

Global stationary configurations of both aligned and logarithmic spiral MHD
perturbations are constructed analytically within an axisymmetric background
ofrazor-thin scale-free gas disc, which is embedded in an axisymmetric
gravitational potential of a dark matter halo and involves an isopedic magnetic
field almost vertically threaded through the disc plane. The scale-free index
$\beta$ of the disc rotation speed $v_{\theta}\propto R^{-\beta}$ falls in the
rangeof $(-1/2 , 1/2)$ where $R$ is the cylindrical radius. With the
holding-back of a deep background dark matter halo potential, the isopedic
magnetic field may be strong enough to allow for the magnetic tension force
overtaking the disc self-gravity, which can significantly influence global
stationary MHD perturbation configurations and stability properties of the
scale-free disc system. Only for stationary logarithmic spiral MHD
perturbations with a perturbation scale-free index $\beta_1=1/4$ or for aligned
stationary MHD perturbations, can the MHD disc maintain a constant radial flux
of angularmomentum. The variable radial flux of angular momentum in the radial
direction corresponds to a complex dispersion relation. The marginal
instabilities foraxisymmetric MHD disturbances are also examined for a special
case as an example. When the magnetic tension force overtakes the disc
self-gravity, the scale-free disc can be completely stable against axisymmetric
MHD disturbances of all wavelengths. We predict the possible existence of an
isopedicallymagnetized gas disc system in rotation primarily confined by a
massive dark matter halo potential.Comment: 31 pages, 7 figures, Accepted for publication in MNRA

### Coupled KdV equations derived from atmospherical dynamics

Some types of coupled Korteweg de-Vries (KdV) equations are derived from an
atmospheric dynamical system. In the derivation procedure, an unreasonable
$y$-average trick (which is usually adopted in literature) is removed. The
derived models are classified via Painlev\'e test. Three types of
$\tau$-function solutions and multiple soliton solutions of the models are
explicitly given by means of the exact solutions of the usual KdV equation. It
is also interesting that for a non-Painlev\'e integrable coupled KdV system
there may be multiple soliton solutions.Comment: 19 pages, 2 figure

### Global structures in a composite system of two scale-free discs with a coplanar magnetic field

We investigate a theoretical MHD disc problem involving a composite disc
system of gravitationally coupled stellar and gaseous discs with a coplanar
magnetic field in the presence of an axisymmetric dark matter halo. The two
discs are expediently approximated as razor-thin, a ring-like magnetic field,
and a power-law rotation curve in radius . By imposing the scale-free
condition, we construct analytically stationary global MHD perturbation
configurations for both aligned and logarithmic spiral patterns. MHD
perturbation configurations in a composite system of partial discs in the
presence of an axisymmetric dark matter halo are also considered. We derive
analytically the stationary MHD dispersion relations for both aligned and
unaligned perturbation structures and analyze the corresponding phase
relationships between surface mass densities and the magnetic field. Compared
with earlier results, we obtain three solution branches corresponding to super
fast MHD density waves, fast MHD density waves and slow MHD density waves,
respectively. By evaluating the unaligned $m=0$ case, we determine the marginal
stability curves where the two unstable regimes corresponding to Jeans collapse
instability and ring fragmentation instability are identified. We find that the
aligned $m=0$ case is simply the limit of the unaligned $m=0$ case with the
radial wavenumber $\xi\to0$. We further show that a composite system of partial
discs behaves much differently from a composite system of full discs in certain
aspects. Our formalism provides a useful theoretical framework in the study of
stationary global perturbation configurations for MHD disc galaxies with bars,
spirals and barred spirals.Comment: 35 pages, 24 figures, Accepted for publication in MNRA

### Dynamic Evolution of a Quasi-Spherical General Polytropic Magnetofluid with Self-Gravity

In various astrophysical contexts, we analyze self-similar behaviours of
magnetohydrodynamic (MHD) evolution of a quasi-spherical polytropic magnetized
gas under self-gravity with the specific entropy conserved along streamlines.
In particular, this MHD model analysis frees the scaling parameter $n$ in the
conventional polytropic self-similar transformation from the constraint of
$n+\gamma=2$ with $\gamma$ being the polytropic index and therefore
substantially generalizes earlier analysis results on polytropic gas dynamics
that has a constant specific entropy everywhere in space at all time. On the
basis of the self-similar nonlinear MHD ordinary differential equations, we
examine behaviours of the magnetosonic critical curves, the MHD shock
conditions, and various asymptotic solutions. We then construct global
semi-complete self-similar MHD solutions using a combination of analytical and
numerical means and indicate plausible astrophysical applications of these
magnetized flow solutions with or without MHD shocks.Comment: 21 pages, 7 figures, accepted for publication in APS

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