The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium
, which is assumed to dominate the total extragalactic dispersion. ...Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs
, in at least one case there is evidence for an extreme magneto-ionic local environment
and a compact persistent radio source
. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately Formula: see text parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies
, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications.
The event rate, energy distribution and time-domain behaviour of repeating fast radio bursts (FRBs) contain essential information regarding their physical nature and central engine, which are as yet ...unknown
. As the first precisely localized source, FRB 121102 (refs.
) has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution
. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1,652 independent bursts with a peak burst rate of 122 h
, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be approximately 4.8 × 10
erg at 1.25 GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allows sensitive periodicity searches between 1 ms and 1,000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavouring emission mechanisms with large energy requirements or contrived triggering conditions.
The mineral and organic phases of mineralized dentin contribute co-operatively to its strength and toughness. This study tested the null hypothesis that there is no difference in nano-dynamic ...mechanical behavior (complex modulus-E*; loss modulus-E′′; storage modulus-E′; in GPa) of dentin hybrid layers (baseline: E*, 3.86 ± 0.24; E′′, 0.23 ± 0.05; E′, 3.85 ± 0.24) created by an etch-and-rinse adhesive in the presence or absence of biomimetic remineralization after in vitro aging. Using scanning probe microscopy and nano-dynamic mechanical analysis, we demonstrated that biomimetic remineralization restored the nano-dynamic mechanical behavior of heavily remineralized, resin-sparse regions of dentin hybrid layers (E*, 19.73 ± 3.85; E′′, 8.75 ± 3.97; E′, 16.02 ± 2.58) to those of the mineralized dentin base (E*, 19.20 ± 2.42; E′′, 6.57 ± 1.96; E′, 17.39 ± 2.0) p > 0.05. Conversely, those resin-sparse, water-rich regions degraded in the absence of biomimetic remineralization, with significant decline p < 0.05 in their complex and storage moduli (E*, 0.83 ± 0.35; E′′, 0.88 ± 0.24; E′, 0.62 ± 0.32). Intrafibrillar apatite deposition preserves the integrity of resin-sparse regions of hybrid layers by restoring their nanomechanical properties to those exhibited by mineralized dentin.
Recent studies suggest that most cases of myelodysplastic syndrome (MDS) are clonally heterogeneous, with a founding clone and multiple subclones. It is not known whether specific gene mutations ...typically occur in founding clones or subclones. We screened a panel of 94 candidate genes in a cohort of 157 patients with MDS or secondary acute myeloid leukemia (sAML). This included 150 cases with samples obtained at MDS diagnosis and 15 cases with samples obtained at sAML transformation (8 were also analyzed at the MDS stage). We performed whole-genome sequencing (WGS) to define the clonal architecture in eight sAML genomes and identified the range of variant allele frequencies (VAFs) for founding clone mutations. At least one mutation or cytogenetic abnormality was detected in 83% of the 150 MDS patients and 17 genes were significantly mutated (false discovery rate ≤0.05). Individual genes and patient samples displayed a wide range of VAFs for recurrently mutated genes, indicating that no single gene is exclusively mutated in the founding clone. The VAFs of recurrently mutated genes did not fully recapitulate the clonal architecture defined by WGS, suggesting that comprehensive sequencing may be required to accurately assess the clonal status of recurrently mutated genes in MDS.
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.
The Community Land Model version 3 (CLM3) is the land component of the Community Climate System Model (CCSM). CLM3 has energy and water biases resulting from deficiencies in some of its canopy and ...soil parameterizations related to hydrological processes. Recent research by the community that utilizes CLM3 and the family of CCSM models has indicated several promising approaches to alleviating these biases. This paper describes the implementation of a selected set of these parameterizations and their effects on the simulated hydrological cycle. The modifications consist of surface data sets based on Moderate Resolution Imaging Spectroradiometer products, new parameterizations for canopy integration, canopy interception, frozen soil, soil water availability, and soil evaporation, a TOPMODEL‐based model for surface and subsurface runoff, a groundwater model for determining water table depth, and the introduction of a factor to simulate nitrogen limitation on plant productivity. The results from a set of offline simulations were compared with observed data for runoff, river discharge, soil moisture, and total water storage to assess the performance of the new model (referred to as CLM3.5). CLM3.5 exhibits significant improvements in its partitioning of global evapotranspiration (ET) which result in wetter soils, less plant water stress, increased transpiration and photosynthesis, and an improved annual cycle of total water storage. Phase and amplitude of the runoff annual cycle is generally improved. Dramatic improvements in vegetation biogeography result when CLM3.5 is coupled to a dynamic global vegetation model. Lower than observed soil moisture variability in the rooting zone is noted as a remaining deficiency.
Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both ...internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed in regions with near horizontal magnetic fields and under low solar wind (SW) dynamic pressures. We also find that below 160 km, the occurrence increases with increasing CO2 density, a trend mostly visible in weakly magnetized regions. These observations have important implications on the formation of electron depletions: (1) Near strong magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; and (2) in weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered, but at sufficiently low altitudes, they are either “absorbed” due to inelastic collisions with ambient neutrals or shielded again in response to a change in magnetic connectivity from open to closed. Our analysis further reveals that both the ionospheric plasma content and thermal electron temperature are reduced in regions with depletions compared to regions without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere.
Key Points
Electron depletions in the nightside Martian ionosphere are more easily observed in regions with near horizontal magnetic fields
Electron depletions are more easily observed under low solar wind dynamic pressures
Substantially reduced ionospheric plasma content and thermal electron temperature are observed in regions with depletions
Land models, which have been developed by the modeling community in the past few decades to predict future states of ecosystems and climate, have to be critically evaluated for their performance ...skills of simulating ecosystem responses and feedback to climate change. Benchmarking is an emerging procedure to measure performance of models against a set of defined standards. This paper proposes a benchmarking framework for evaluation of land model performances and, meanwhile, highlights major challenges at this infant stage of benchmark analysis. The framework includes (1) targeted aspects of model performance to be evaluated, (2) a set of benchmarks as defined references to test model performance, (3) metrics to measure and compare performance skills among models so as to identify model strengths and deficiencies, and (4) model improvement. Land models are required to simulate exchange of water, energy, carbon and sometimes other trace gases between the atmosphere and land surface, and should be evaluated for their simulations of biophysical processes, biogeochemical cycles, and vegetation dynamics in response to climate change across broad temporal and spatial scales. Thus, one major challenge is to select and define a limited number of benchmarks to effectively evaluate land model performance. The second challenge is to develop metrics of measuring mismatches between models and benchmarks. The metrics may include (1) a priori thresholds of acceptable model performance and (2) a scoring system to combine data-model mismatches for various processes at different temporal and spatial scales. The benchmark analyses should identify clues of weak model performance to guide future development, thus enabling improved predictions of future states of ecosystems and climate. The near-future research effort should be on development of a set of widely acceptable benchmarks that can be used to objectively, effectively, and reliably evaluate fundamental properties of land models to improve their prediction performance skills.
The equimolar AlCoCrFeNi high entropy alloy (HEA) was printed by selective laser melting (SLM), with emphasis on its densification, phase identification, non-equilibrium microstructure and ...properties. The density of SLM samples increase gradually with the increase of volumetric energy density (VED) with the maximal relative density 98.4%. SLM samples are consisted of disordered body-center cubic phases (A2) and ordered body-center cubic phases (B2) which are different from the conventional cast or deformation with A2 and fcc phase; the content of B2 phase increases with the VED. Interestingly, the Fe-Cr precipitate, which has never been found in the cast one, is observed in the SLM one without heat treatment. The microstructure of SLM samples shows epitaxial growth of columnar A2 grain bundles perpendicular to the melt pool boundary with an average grain size of about 1.5 μm; the B2 phase is between the columnar A2 grains. At low VED, the SLM sample shows preferred orientation, and is gradually transformed to the crystallographic direction families at a higher VED. Besides, there exists the {100}A2∥{100}B2 orientation relationship in A2 and B2 phase. The maximal micro-hardness of 632.8Hv of SLM sample is higher than the cast one and the electron beam melting printed one.
•The equiatomic AlCoCrFeNi alloy was successfully printed by SLM.•The A2 and B2 phase were identified by EBSD.•The Fe-Cr precipitate was found in the SLM samples.•The micro-hardness of SLM samples reaches 632Hv.
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