190 research outputs found
Hydrodynamic Evolution of Sgr A East: The Imprint of A Supernova Remnant in the Galactic Center
We perform three-dimensional numerical simulations to study the hydrodynamic
evolution of Sgr A East, the only known supernova remnant (SNR) in the center
of our Galaxy, to infer its debated progenitor SN type and its potential impact
on the Galactic center environment. Three sets of simulations are performed,
each of which represents a represent a certain type of SN explosion (SN Iax, SN
Ia or core-collapse SN) expanding against a nuclear outflow of hot gas driven
by massive stars, whose thermodynamical properties have been well established
by previous work and fixed in the simulations. All three simulations can
simultaneously roughly reproduce the extent of Sgr A East and the position and
morphology of an arc-shaped thermal X-ray feature, known as the "ridge".
Confirming previous work, our simulations show that the ridge is the
manifestation of a strong collision between the expanding SN ejecta and the
nuclear outflow. The simulation of the core-collapse SN, with an assumed
explosion energy of 5x10^50 erg and an ejecta mass of 10 M_sun, can well match
the X-ray flux of the ridge, whereas the simulations of the SN Iax and SN Ia
explosions underpredict its X-ray emission, due to a smaller ejecta mass. All
three simulations constrain the age of Sgr A East to be <1500 yr and predict
that the ridge should fade out over the next few hundred years. We address the
implications of these results for our understanding of the Galactic center
environment.Comment: 21 pages, 18 figures. Accepted for publication on MNRA
A Statistical Study of Soft X-ray Flares on Solar-type Stars
The statistical characteristic of stellar flares at optical bands has
received an extensive study, but it remains to be studied at soft X-ray bands,
in particular for solar-type stars. Here, we present a statistical study of
soft X-ray flares on solar-type stars, which can help understand
multi-wavelength behaviors of stellar flares. We mainly use Chandra Source
Catalog Release 2.0, which includes a number of flaring stars with denoted
variability, and Gaia Data Release 3, which includes necessary information for
classifying stars. We also develop a set of methods for identifying and
classifying stellar soft X-ray flares and estimating their properties. A
detailed statistical investigation for 129 flare samples on 103 nearby
solar-type stars as selected yields the following main results. (1) The flare
energy emitted at the soft X-ray band in our sample ranges from
to , and the majority of them are superflares with
the most energetic one having energy of $6.0_{-4.7}^{+3.2} \times 10^{37} \
\mathrm{erg}T_\mathrm{duration,SXR} \propto E_\mathrm{flare,SXR}^{\ 0.201 \pm 0.024}\mathrm{d} N_\mathrm{flare} / \mathrm{d} E_\mathrm{flare,SXR} \propto
E_\mathrm{flare,SXR}^{\ -1.77}$, which is similar to the results found at other
bands and on other types of stars, indicating that the energy emitted at the
soft X-ray band could be a constant fraction of the full-band bolometric
energy
Chandra X-ray Measurement of Gas-phase Heavy Element Abundances in the Central Parsec of the Galaxy
Elemental abundances are key to our understanding of star formation and
evolution in the Galactic center. Previous work on this topic has been based on
infrared (IR) observations, but X-ray observations have the potential of
constraining the abundance of heavy elements, mainly through their K-shell
emission lines. Using 5.7 Ms Chandra observations, we provide the first
abundance measurement of Si, S, Ar, Ca and Fe, in four prominent diffuse X-ray
features located in the central parsec of the Galaxy, which are the
manifestation of shock-heated hot gas. A two-temperature, non-equilibrium
ionization spectral model is employed to derive the abundances of these five
elements. In this procedure, a degeneracy is introduced due to uncertainties in
the composition of light elements, in particular, H, C and N. Assuming that the
hot gas is H-depleted but C- and N-enriched, as would be expected for a
standard scenario in which the hot gas is dominated by Wolf-Rayet star winds,
the spectral fit finds a generally subsolar abundance for the heavy elements.
If, instead, the light elements had a solar-like abundance, the heavy elements
have a fitted abundance of 1--2 solar. The /Fe abundance ratio,
on the other hand, is mostly supersolar and insensitive to the exact
composition of the light elements. These results are robust against potential
biases due to either a moderate spectral S/N or the presence of non-thermal
components. Implications of the measured abundances for the Galactic center
environment are addressed.Comment: 13 pages, 6 figures, Accepted for publication in MNRA
Implementing a new fully stepwise decomposition-based sampling technique for the hybrid water level forecasting model in real-world application
Various time variant non-stationary signals need to be pre-processed properly
in hydrological time series forecasting in real world, for example, predictions
of water level. Decomposition method is a good candidate and widely used in
such a pre-processing problem. However, decomposition methods with an
inappropriate sampling technique may introduce future data which is not
available in practical applications, and result in incorrect
decomposition-based forecasting models. In this work, a novel Fully Stepwise
Decomposition-Based (FSDB) sampling technique is well designed for the
decomposition-based forecasting model, strictly avoiding introducing future
information. This sampling technique with decomposition methods, such as
Variational Mode Decomposition (VMD) and Singular spectrum analysis (SSA), is
applied to predict water level time series in three different stations of
Guoyang and Chaohu basins in China. Results of VMD-based hybrid model using
FSDB sampling technique show that Nash-Sutcliffe Efficiency (NSE) coefficient
is increased by 6.4%, 28.8% and 7.0% in three stations respectively, compared
with those obtained from the currently most advanced sampling technique. In the
meantime, for series of SSA-based experiments, NSE is increased by 3.2%, 3.1%
and 1.1% respectively. We conclude that the newly developed FSDB sampling
technique can be used to enhance the performance of decomposition-based hybrid
model in water level time series forecasting in real world
Promising derivatives of rutaecarpine with diverse pharmacological activities
Rutaecarpine (RUT) is a natural pentacyclic indolopyridoquinazolinone alkaloid first isolated from one of the most famous traditional Chinese herbs, Evodia rutaecarpa, which is used for treating a variety of ailments, including headaches, gastrointestinal disorders, postpartum hemorrhage, amenorrhea, difficult menstruation, and other diseases. Accumulating pharmacological studies showed that RUT possesses a wide range of pharmacological effects through different mechanisms. However, its poor physicochemical properties and moderate biological activities have hampered its clinical application. In this regard, the modification of RUT aimed at seeking its derivatives with better physicochemical properties and more potency has been extensively studied. These derivatives exhibit diverse pharmacological activities, including anti-inflammatory, anti-atherogenic, anti-Alzheimer’s disease, antitumor, and antifungal activities via a variety of mechanisms, such as inhibiting cyclooxygenase-2 (COX-2), acetylcholine (AChE), phosphodiesterase 4B (PDE4B), phosphodiesterase 5 (PDE5), or topoisomerases (Topos). From this perspective, this paper provides a comprehensive description of RUT derivatives by focusing on their diverse biological activities. This review aims to give an insight into the biological activities of RUT derivatives and encourage further exploration of RUT
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