47 research outputs found
Chandra Study of X-Ray Point Sources in the Early-Type Galaxy NGC 4552 (M89)
We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect
47 X-ray point sources, most of which are likely LMXBs, within 4 R_e. The
brightest X-ray source coincides with the optical/UV/radio center of the
galaxy, and shows variability on >1 hr timescales, indicating the possible
existence of a LLAGN. The 46 off-center sources and the unresolved point
sources contribute about 29% and 20% to the total luminosity of the galaxy,
respectively. The corrected cumulative XLF of the off-center sources is best
fit by a broken power-law model with a break at L_b=4.4+2.0-1.4 e38 ergs/s. We
identified 210 GCs candidates in a HST WFPC2 optical image of the galaxy's
central region. Of the 25 off-center LMXBs that fall within the WFPC2 FOV, 10
sources are coincident with a GC. Thus the fraction of the GCs hosting bright
LMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%,
respectively. In the V and I bands, the GCs hosting bright LMXBs are typically
1-2 magnitudes brighter than the GCs with no detected LMXBs. There are about
1.9+-0.4 times as many LMXBs in the red, metal-rich GCs as there are in the
blue, metal-poor ones. We find no obvious difference between the luminosity
distributions of LMXBs in GCs and in field, but the cumulative spectrum of the
LMXBs in GCs tends to be softer than that of the LMXBs in field. We detected 3
X-ray sources that have isotropic luminosities larger than 1e39 erg/s. The one
located in the joint Chandra-HST field is found to be associated with a GC. By
studying the ACIS spectra we infer that the this may be a candidate black hole
system with a mass of 15-135 M_sun. One of the other sources with a luminosity
brighter than 1e39 ergs/s reveals temporal variations in brightness on
timescales greater than an hour.Comment: Accepted for publication in Ap
Reconstructing dynamics of complex systems from noisy time series with hidden variables
Reconstructing the equation of motion and thus the network topology of a
system from time series is a very important problem. Although many powerful
methods have been developed, it remains a great challenge to deal with systems
in high dimensions with partial knowledge of the states. In this paper, we
propose a new framework based on a well-designed cost functional, the
minimization of which transforms the determination of both the unknown
parameters and the unknown state evolution into parameter learning. This method
can be conveniently used to reconstruct structures and dynamics of complex
networks, even in the presence of noisy disturbances or for intricate parameter
dependence. As a demonstration, we successfully apply it to the reconstruction
of different dynamics on complex networks such as coupled Lorenz oscillators,
neuronal networks, phase oscillators and gene regulation, from only a partial
measurement of the node behavior. The simplicity and efficiency of the new
framework makes it a powerful alternative to recover system dynamics even in
high dimensions, which expects diverse applications in real-world
reconstruction.Comment: 23 pages,23 figure
Improving Adaptive Real-Time Video Communication Via Cross-layer Optimization
Effective Adaptive BitRate (ABR) algorithm or policy is of paramount
importance for Real-Time Video Communication (RTVC) amid this pandemic to
pursue uncompromised quality of experience (QoE). Existing ABR methods mainly
separate the network bandwidth estimation and video encoder control, and
fine-tune video bitrate towards estimated bandwidth, assuming the maximization
of bandwidth utilization yields the optimal QoE. However, the QoE of a RTVC
system is jointly determined by the quality of compressed video, fluency of
video playback, and interaction delay. Solely maximizing the bandwidth
utilization without comprehensively considering compound impacts incurred by
both network and video application layers, does not assure the satisfactory
QoE. And the decoupling of network and video layer further exacerbates the user
experience due to network-codec incoordination. This work therefore proposes
the Palette, a reinforcement learning based ABR scheme that unifies the
processing of network and video application layers to directly maximize the QoE
formulated as the weighted function of video quality, stalling rate and delay.
To this aim, a cross-layer optimization is proposed to derive fine-grained
compression factor of upcoming frame(s) using cross-layer observations like
network conditions, video encoding parameters, and video content complexity. As
a result, Palette manages to resolve the network-codec incoordination and to
best catch up with the network fluctuation. Compared with state-of-the-art
schemes in real-world tests, Palette not only reduces 3.1%-46.3% of the
stalling rate, 20.2%-50.8% of the delay, but also improves 0.2%-7.2% of the
video quality with comparable bandwidth consumption, under a variety of
application scenarios
Chandra Observation of the Cluster of Galaxies MS 0839.9+2938 at z=0.194: the Central Excess Iron and SN Ia Enrichment
We present the Chandra study of the intermediately distant cluster of
galaxies MS 0839.9+2938. By performing both the projected and deprojected
spectral analyses, we find that the gas temperature is approximately constant
at about 4 keV in 130-444h_70^-1 kpc. In the inner regions, the gas temperature
descends towards the center, reaching <~ 3 keV in the central 37h_70^-1 kpc.
This infers that the lower and upper limits of the mass deposit rate are 9-34
M_sun yr^-1 and 96-126 M_sun yr^-1, respectively within 74h_70^-1 kpc where the
gas is significantly colder. Along with the temperature drop, we detect a
significant inward iron abundance increase from about 0.4 solar in the outer
regions to about 1 solar within the central 37h_70^-1 kpc. Thus MS 0839.9+2938
is the cluster showing the most significant central iron excess at z>~ 0.2. We
argue that most of the excess iron should have been contributed by SNe Ia. By
utilizing the observed SN Ia rate and stellar mass loss rate, we estimate that
the time needed to enrich the central region with excess iron is 6.4-7.9 Gyr,
which is similar to those found for the nearby clusters. Coinciding with the
optical extension of the cD galaxy (up to about 30h_70^-1 kpc), the observed
X-ray surface brightness profile exhibits an excess beyond the distribution
expected by either the beta model or the NFW model, and can be well fitted with
an empirical two-beta model that leads to a relatively flatter mass profile in
the innermost region.Comment: Accepted for publication in Ap
The Precambrian Khondalite Belt in the Daqingshan area, North China Craton: evidence for multiple metamorphic events in the Palaeoproterozoic era
High-grade pelitic metasedimentary rocks (khondalites) are widely distributed in the northwestern part of the North China Craton and were named the âKhondalite Beltâ. Prior to the application of zircon geochronology, a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology. We report here SHRIMP UâPb zircon ages and Hf-isotope data on metamorphosed sedimentary and magmatic rocks at Daqingshan, a typical area of the Khondalite Belt. The main conclusions are as follows: (1) The early Precambrian supracrustal rocks belong to three sequences: a 2.56â2.51 Ga supracrustal unit (the previous Sanggan âgroupâ), a 2.51â2.45 Ga supracrustal unit (a portion of the previous upper Wulashan âgroupâ) and a 2.0â1.95 Ga supracrustal unit (including the previous lower Wulashan âgroupâ, a portion of original upper Wulashan âgroupâ and the original Meidaizhao âgroupâ) the units thus do not represent a true stratigraphy; (2) Strong tectono-thermal events occurred during the late Neoarchaean to late Palaeoproterozoic, with four episodes recognized: 2.6â2.5, 2.45â2.37, 2.3â2.0 and 1.95â1.85 Ga, with the latest event being consistent with the assembly of the Palaeoproterozoic supercontinent Columbia; (3) During the late Neoarchaean to late Palaeoproterozoic (2.55â2.5, 2.37 and 2.06 Ga) juvenile, mantle-derived material was added to the crust
Genesis of the worldâs largest rare earth element deposit, Bayan Obo, China:Protracted mineralization evolution over ~1 b.y.
The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the worldâs largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361â913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower than the recrystallized dolomite (0.7038â0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks.ΔNd(1.3Ga) values (0.3 ± 0.6) close87Sr/86Sr ratios (0.7024â0.7030
Origin of heavy rare earth mineralization in South China
Heavy rare earth elements (HREE) are dominantly mined from the weathering crusts of granites in South China. Although weathering processes occur globally, no economic HREE resources of this type have yet been found outside China. Here, we report the occurrence of unidentified REE minerals in the granites from South Chinese deposits. They contain high levels of both HREE and light REE, but are strongly depleted in Ce, implying high oxidation state. These REE minerals show higher initial Nd isotope than primary REE-rich minerals (eNd(t) = 0.9 +/- 0.8 versus -11.5 +/- 0.5). The mineralized weathering crusts inherited REE signature of the granites, but show more Ce depletion and more overall concentration of the REE. We propose, therefore, that highly oxidized, REE-rich fluids, derived from external, isotopically depleted sources, metasomatized the granites, which resulted in Ce depletion as Ce4+ and enrichment of the remaining REE, especially the HREE, contributing to formation of a globally important REE resource.Chinese National Science Foundation [41573033, 41222022]; European Union's Horizon [689909]; Czechic Project CEITEC [LQ1601]SCI(E)ARTICLE