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.
Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer ...hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.
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.
We present a study of the infrared properties of X-ray selected, moderate-luminosity (i.e. L
X= 1042-1044 erg s−1) active galactic nuclei (AGNs) up to z ≈ 3, in order to explore the links between ...star formation in galaxies and accretion on to their central black holes. We use 100 and 160 μ m fluxes from GOODS-Herschel - the deepest survey yet undertaken by the Herschel telescope - and show that in the vast majority of cases (i.e. >94 per cent) these fluxes are dominated by emission from the host galaxy. As such, these far-infrared bands provide an uncontaminated view of star formation in the AGN host galaxies. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that global star formation is decoupled from nuclear (i.e. AGN) activity in these galaxies. On the other hand, we confirm that the star formation rates of AGN hosts increase strongly with redshift, by a factor of 43+27
− 18 from z < 0.1 to z = 2-3 for AGNs with the same range of X-ray luminosities. This increase is entirely consistent with the factor of 25-50 increase in the specific star formation rates (SSFRs) of normal, star-forming (i.e. main-sequence) galaxies over the same redshift range. Indeed, the average SSFRs of AGN hosts are only marginally (i.e. ≈20 per cent) lower than those of main-sequence galaxies at all surveyed redshifts, with this small deficit being due to a fraction of AGNs residing in quiescent (i.e. low SSFR) galaxies. We estimate that 79 ± 10 per cent of moderate-luminosity AGNs are hosted in main-sequence galaxies, 15 ± 7 per cent in quiescent galaxies and <10 per cent in strongly starbursting galaxies. We derive the fractions of all main-sequence galaxies at z < 2 that are experiencing a period of moderate nuclear activity, noting that it is strongly dependent on galaxy stellar mass (M
stars), rising from just a few per cent at M
stars∼ 1010 M⊙ to ≳20 per cent at M
stars≥ 1011 M⊙. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate-luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, which suggests that high-redshift disc instabilities could be an important AGN feeding mechanism.
Abstract
We report results from continued timing observations of PSR J0740+6620, a high-mass, 2.8 ms radio pulsar in orbit with a likely ultracool white dwarf companion. Our data set consists of ...combined pulse arrival-time measurements made with the 100 m Green Bank Telescope and the Canadian Hydrogen Intensity Mapping Experiment telescope. We explore the significance of timing-based phenomena arising from general relativistic dynamics and variations in pulse dispersion. When using various statistical methods, we find that combining ∼1.5 yr of additional, high-cadence timing data with previous measurements confirms and improves on previous estimates of relativistic effects within the PSR J0740+6620 system, with the pulsar mass
m
p
=
2.08
−
0.07
+
0.07
M
⊙
(68.3% credibility) determined by the relativistic Shapiro time delay. For the first time, we measure secular variation in the orbital period and argue that this effect arises from apparent acceleration due to significant transverse motion. After incorporating contributions from Galactic differential rotation and off-plane acceleration in the Galactic potential, we obtain a model-dependent distance of
d
=
1.14
−
0.15
+
0.17
kpc (68.3% credibility). This improved distance confirms the ultracool nature of the white dwarf companion determined from recent optical observations. We discuss the prospects for future observations with next-generation facilities, which will likely improve the precision on
m
p
for J0740+6620 by an order of magnitude within the next few years.
Abstract
A nanohertz-frequency stochastic gravitational-wave background can potentially be detected through the precise timing of an array of millisecond pulsars. This background produces ...low-frequency noise in the pulse arrival times that would have a characteristic spectrum common to all pulsars and a well-defined spatial correlation. Recently the North American Nanohertz Observatory for Gravitational Waves collaboration (NANOGrav) found evidence for the common-spectrum component in their 12.5 yr data set. Here we report on a search for the background using the second data release of the Parkes Pulsar Timing Array. If we are forced to choose between the two NANOGrav models—one with a common-spectrum process and one without—we find strong support for the common-spectrum process. However, in this paper, we consider the possibility that the analysis suffers from model misspecification. In particular, we present simulated data sets that contain noise with distinctive spectra but show strong evidence for a common-spectrum process under the standard assumptions. The Parkes data show no significant evidence for, or against, the spatially correlated Hellings–Downs signature of the gravitational-wave background. Assuming we did observe the process underlying the spatially uncorrelated component of the background, we infer its amplitude to be
A
=
2.2
−
0.3
+
0.4
×
10
−
15
in units of gravitational-wave strain at a frequency of 1 yr
−1
. Extensions and combinations of existing and new data sets will improve the prospects of identifying spatial correlations that are necessary to claim a detection of the gravitational-wave background.
Elucidation of the genetic factors underlying chronic liver disease may reveal new therapeutic targets.
We used exome sequence data and electronic health records from 46,544 participants in the ...DiscovEHR human genetics study to identify genetic variants associated with serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Variants that were replicated in three additional cohorts (12,527 persons) were evaluated for association with clinical diagnoses of chronic liver disease in DiscovEHR study participants and two independent cohorts (total of 37,173 persons) and with histopathological severity of liver disease in 2391 human liver samples.
A splice variant (rs72613567:TA) in HSD17B13, encoding the hepatic lipid droplet protein hydroxysteroid 17-beta dehydrogenase 13, was associated with reduced levels of ALT (P=4.2×10
) and AST (P=6.2×10
). Among DiscovEHR study participants, this variant was associated with a reduced risk of alcoholic liver disease (by 42% 95% confidence interval {CI}, 20 to 58 among heterozygotes and by 53% 95% CI, 3 to 77 among homozygotes), nonalcoholic liver disease (by 17% 95% CI, 8 to 25 among heterozygotes and by 30% 95% CI, 13 to 43 among homozygotes), alcoholic cirrhosis (by 42% 95% CI, 14 to 61 among heterozygotes and by 73% 95% CI, 15 to 91 among homozygotes), and nonalcoholic cirrhosis (by 26% 95% CI, 7 to 40 among heterozygotes and by 49% 95% CI, 15 to 69 among homozygotes). Associations were confirmed in two independent cohorts. The rs72613567:TA variant was associated with a reduced risk of nonalcoholic steatohepatitis, but not steatosis, in human liver samples. The rs72613567:TA variant mitigated liver injury associated with the risk-increasing PNPLA3 p.I148M allele and resulted in an unstable and truncated protein with reduced enzymatic activity.
A loss-of-function variant in HSD17B13 was associated with a reduced risk of chronic liver disease and of progression from steatosis to steatohepatitis. (Funded by Regeneron Pharmaceuticals and others.).
Although large dams have been constructed and continue to be constructed on many rivers, the lack of long-term gauging data often makes it difficult to document either reservoir sedimentation or the ...dams' downstream impacts. More than 50years of water and sediment data from 20 gauging stations within the Yangtze River's basin provide us a unique opportunity to delineate the impacts from the Three Gorges Dam (TGD), the world's largest dam. During the first decade after TGD completion in 2003, 1.8Gt of sediments were trapped in the Three Gorges Reservoir (TGR). The TGR's sediment retention rate increased from ~65% during the first three years of operation to ~85% by 2008–2012, when the TGD was in normal operation; in the low-discharge drought years of 2006 and 2011, reservoir retention exceeded 90%. Sedimentation in the TGR has been discontinuous, the most prominent depocenters being at the broad section near the up-river entrance to the reservoir and just upstream of the dam, where sediment thickness locally exceeds 60m. Median size of the sediments trapped in the TGR is 11μm, whereas sediments discharged from the TGR are finer than 5μm. As a result of sediment retention in the TGR, the river downstream has been eroded at a rate of 65Mt/yr. Riverbed sediments have coarsened considerably in the first several hundred kilometers downstream of TGD. Sediment discharge into the Yangtze estuary, as measured at the Datong downstream gauging station, decreased by 130Mt/yr relative to the normal water years of 2001–2002, nearly 90% of which can be attributed to the TGD. With planned construction of large upstream Cascade Reservoirs, the amount of sediment entering the TGR will decline dramatically, thus reducing sedimentation in the TGR and thereby extending its lifespan; by the end of the 21st century, the TGR should have retained more than 80% of its original storage capacity. Sediment outflow from the TGR will likely be less than 15Mt/yr, compared to 50Mt/yr at present. Even with downstream channel erosion, the long-term average sediment discharge into the Yangtze estuary in future decades most likely will decrease to ca. 110Mt/yr, only 20% of its level in the 1960s, and further delta erosion is expected.
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.
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