Fast radio bursts (FRBs) are millisecond-duration radio transients
of unknown origin. Two possible mechanisms that could generate extremely coherent emission from FRBs invoke neutron star ...magnetospheres
or relativistic shocks far from the central energy source
. Detailed polarization observations may help us to understand the emission mechanism. However, the available FRB polarization data have been perplexing, because they show a host of polarimetric properties, including either a constant polarization angle during each burst for some repeaters
or variable polarization angles in some other apparently one-off events
. Here we report observations of 15 bursts from FRB 180301 and find various polarization angle swings in seven of them. The diversity of the polarization angle features of these bursts is consistent with a magnetospheric origin of the radio emission, and disfavours the radiation models invoking relativistic shocks.
In a genomewide association study, variants at three loci —
EBF1,
EEFSEC,
and
AGTR2
— were associated with both gestational duration and preterm birth.
Spatial pattern information of carbon density in forest ecosystem including forest litter carbon (FLC) plays an important role in evaluating carbon sequestration potentials. The spatial variation of ...FLC density in the typical subtropical forests in southeastern China was investigated using Moran's I, geostatistics and a geographical information system (GIS). A total of 839 forest litter samples were collected based on a 12 km (south–north) × 6 km (east–west) grid system in Zhejiang province. Forest litter carbon density values were very variable, ranging from 10.2 kg ha−1 to 8841.3 kg ha−1, with an average of 1786.7 kg ha−1. The aboveground biomass had the strongest positive correlation with FLC density, followed by forest age and elevation. Global Moran's I revealed that FLC density had significant positive spatial autocorrelation. Clear spatial patterns were observed using local Moran's I. A spherical model was chosen to fit the experimental semivariogram. The moderate "nugget-to-sill" (0.536) value revealed that both natural and anthropogenic factors played a key role in spatial heterogeneity of FLC density. High FLC density values were mainly distributed in northwestern and western part of Zhejiang province, which were related to adopting long-term policy of forest conservation in these areas, while Hang-Jia-Hu (HJH) Plain, Jin-Qu (JQ) Basin and coastal areas had low FLC density due to low forest coverage and intensive management of economic forests. These spatial patterns were in line with the spatial-cluster map described by local Moran's I. Therefore, Moran's I, combined with geostatistics and GIS, could be used to study spatial patterns of environmental variables related to forest ecosystem.
This paper reports probably the first systematic experimental investigation of the influences of lateral inertia confinement, end friction confinement and aggregates in high-speed impact tests on ...dynamic compressive properties of concrete. The influences of the concerned factors on the dynamic increase factor (DIF) are discussed. The proposed DIF relations in Hao and Hao 36 are verified by the experimental data in this study. The results confirm the observations made before based on numerical simulations that lateral inertia confinement effect is dependent on the specimen diameter and strain rate. The influences of aggregate size and volume to DIF are considerable, and aggregates cannot be neglected in experimental or numerical studies. It also confirms that the end friction confinement effect is sensitive to the specimen geometry and strain rate. The equation proposed in Hao et al. 40 based on numerical simulations satisfactorily removes the friction confinement effect on the DIF of concrete material strength obtained in impact test.
Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown physical origin observed at extragalactic distances
. It has long been speculated that magnetars are the engine powering ...repeating bursts from FRB sources
, but no convincing evidence has been collected so far
. Recently, the Galactic magnetar SRG 1935+2154 entered an active phase by emitting intense soft γ-ray bursts
. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft γ-ray/hard-X-ray flare
. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard-X-ray) data. During the third session, 29 soft-γ-ray repeater (SGR) bursts were detected in γ-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB-SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.
•A model-driven learning method is proposed to trace fluctuations dynamically.•The fluctuation forecasting under uncertainties is made possible using the HMM.•Case studies of different scenarios are ...presented to show the validity of the HMM.•Two forecasting formulas are proposed to cope with two scenarios separately.
Waste generation forecasting is a complex process that is found to be influenced by some latent influencing parameters and their uncertainties, such as economic growth, demography, individual behaviors, activities and events, and management policies. These hidden features play an important role in forecasting the fluctuations of waste generation. We therefore focus on revealing the trend of waste generation in megacities which face significant influences of social and economic changes to achieve urban sustainable development. To dynamically trace fluctuations caused by these uncertainties, we propose a probability model-driven statistical learning approach which hybridizes a wavelet de-noising, a Gaussian mixture model, and a hidden Markov model. First, to gain the actual underlying trend, wavelet de-noising is used to eliminate the noise of data. Next, the Expectation–Maximization and the Viterbi algorithms are employed for learning parameters and discerning the most probable sequence of hidden states, respectively. Subsequently, the state transition matrix is updated by fractional predictable changes of influencing parameters to perform non-periodic fluctuation problem forecasting, and the forward algorithm is utilized to search the most similar data pattern for the current pattern from historical data in order to forecast the future trend of the periodic fluctuation problem. Finally, we apply the approaches into two kinds of case studies that test both a small dataset and a large dataset. How uncertainty factors influence forecasted results is analyzed in the subsection of results and discussion. The computational results demonstrate that the proposed approaches are effective in solving the municipal waste generation forecasting problem.
Using the first-principles pseudopotential method and Boltzmann transport theory, we give a comprehensive understanding of the electronic and phonon transport properties of the thermoelectric ...material BiCuSeO. By choosing an appropriate hybrid functional for the exchange-correlation energy, we find that the system is a semiconductor with a direct band gap of ∼0.8 eV, which is quite different from those obtained previously using standard functionals. Detailed analysis of a three-dimensional energy band structure indicates that there is a valley degeneracy of eight around the valence band maximum, which leads to a sharp density of states and is responsible for a large p-type Seebeck coefficient. Moreover, we find that the density of states effective mass is much larger and results in a very low hole mobility for BiCuSeO. On the other hand, we discover two flat phonon branches contributed by the Cu and Se atoms, which can effectively block heat transfer. Combined with large atomic displacement parameters of the Cu atom, we believe that the intrinsically low lattice thermal conductivity in BiCuSeO is mainly caused by the Cu atoms, instead of the prevailingly believed Bi atoms. The thermoelectric figure-of-merit is also predicted and compared with available experimental results.
Using the first-principles pseudopotential method and Boltzmann transport theory, we give a comprehensive understanding of the electronic and phonon transport properties of a thermoelectric material BiCuSeO.
The purpose of this research is to provide a new understanding of the turbulence dynamics in a heated flow of fluid at supercritical pressure. A unified explanation has been established for the ...laminarisation mechanisms due to the variations of thermophysical properties, buoyancy and inertia, the last of which plays a significant role in a developing flow. In the new understanding, the various factors can all be treated similarly as (pseudo-)body forces, the effect of which is to cause a reduction in the so-called apparent Reynolds number. The partially laminarising flow is represented by an equivalent-pressure-gradient reference flow plus a perturbation flow. Full laminarisation is used in the paper referring to a region where no new vortical structures are generated. This region is akin to the pre-transition region of a boundary layer bypass transition, and in both cases, the free-stream or pipe-core turbulence decays exponentially, but elongated streaks are formed in the boundary layer. Turbulence kinetic energy in this region may still be significant due to the decaying turbulence as well as newly generated streaks. The latter lead to an increase in streamwise velocity fluctuations near the wall. Later, re-transition occurs when the streaks break down and multi-scale vortices are generated, leading to an increase in the radial and circumferential velocity fluctuations. The structural effect of buoyancy on turbulence is weak and negative in the partially laminarising flow, but is dominant in the full laminarisation and re-transition regions.
Pd nanowire arrays (NWAs) with high electrochemically active surface area (see figure) are successfully fabricated using anodized aluminum oxide electrodeposition. The electrocatalytic activity and ...stability of the Pd NWAs for ethanol electrooxidation are not only significantly higher that of conventional Pd film electrodes, but also higher than that of well‐established commercial PtRu/C electrocatalysts. The Pd NWAs show great potential as electrocatalysts for ethanol electrooxidation in alkaline media in direct ethanol fuel cells.
PDF
