Recent studies have proposed that the bathymetric fabric of the seafloor formed at mid-ocean ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel ...changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footpring of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.
Context.
The discovery of pulsations in several ultraluminous X-ray sources (ULXs) has demonstrated that a fraction of them are powered by super-Eddington accretion onto neutron stars (NSs). This has ...raised questions regarding the NS to black hole (BH) ratio within the ULX population and the physical mechanism that allows ULXs to reach luminosities well in excess of their Eddington luminosity. Is this latter the presence of strong magnetic fields or rather the presence of strong outflows that collimate the emission towards the observer?
Aims.
In order to distinguish between these scenarios, namely, supercritically accreting BHs, weakly magnetised NSs, or strongly magnetised NSs, we study the long-term X-ray spectral evolution of a sample of 17 ULXs with good long-term coverage, 6 of which are known to host NSs. At the same time, this study serves as a baseline to identify potential new NS-ULX candidates.
Methods.
We combine archival data from
Chandra
,
XMM-Newton
, and
NuSTAR
observatories in order to sample a wide range of spectral states for each source. We track the evolution of each source in a hardness–luminosity diagram in order to identify spectral changes, and show that these can be used to constrain the accretion flow geometry, and in some cases the nature of the accretor.
Results.
We find NS-ULXs to be among the hardest sources in our sample with highly variable high-energy emission. On this basis, we identify M 81 X-6 as a strong NS-ULX candidate, whose variability is shown to be akin to that of NGC 1313 X-2. For most softer sources with an unknown accretor, we identify the presence of three markedly different spectral states, which we interpret by invoking changes in the mass-accretion rate and obscuration by the supercritical wind/funnel structure. Finally, we report on a lack of variability at high energies (≳10 keV) in NGC 1313 X-1 and Holmberg IX X-1, which we argue may offer a means to differentiate BH-ULXs from NS-ULXs.
Conclusions.
We support a scenario in which the hardest sources in our sample might be powered by strongly magnetised NSs, meaning that the high-energy emission is dominated by the hard direct emission from the accretion column. Instead, softer sources may be explained by weakly magnetised NSs or BHs, in which the presence of outflows naturally explains their softer spectra through Compton down-scattering, their spectral transitions, and the dilution of the pulsed-emission should some of these sources contain NSs.
Large earthquakes are the product of elastic stress that has accumulated over decades to centuries along segments of active faults. Assuming an elastic crust, one can roughly estimate the location ...and rate of accumulation of elastic stress. However, this general framework does not account for inelastic, irrecoverable deformation, which results in large‐scale topography. We do not know over which part of the earthquake cycle such deformation occurs. Using InSAR and GNSS measurements, we report on a potential correlation between long‐term, inelastic vertical rate and short‐term, interseismic vertical rate in northern Chile. Approximately 4% to 8% of the geodetically derived interseismic vertical rates translate into permanent deformation, suggesting that topography of the forearc builds up during the interseismic period. This observation provides a quantitative basis for an improved understanding of the interplay between short‐term and long‐term dynamics along convergent plate boundaries.
Key Points
We propose an approach to quantify the ratio between elastic and inelastic deformation in subduction forearcs
Two distinct correlations can be observed between interseismic and Quaternary uplift rates
We propose that 4% to 8% of interseismic uplift rates translate into persistent deformation in northern Chile
Our understanding of bacterial pathogenesis is dominated by the cell biology of the host-pathogen interaction. However, the majority of metabolites that are used in prokaryotic and eukaryotic ...physiology and signalling are chemically similar or identical. Therefore, the metabolic crosstalk between pathogens and host cells may be as important as the interactions between bacterial effector proteins and their host targets. In this Review we focus on host-pathogen interactions at the metabolic level: chemical signalling events that enable pathogens to sense anatomical location and the local physiology of the host; microbial metabolic pathways that are dedicated to circumvent host immune mechanisms; and a few metabolites as central points of competition between the host and bacterial pathogens.
Context.
Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have ...marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic black hole binaries, their evidence in the super-Eddington regime has so far remained elusive. However, recent circumstantial evidence hinted at the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X–1 and NGC 5204 X–1.
Aims.
We aim to build on our previous work and exploit the long-term high-cadence monitoring of
Swift
-XRT in order to provide robust evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions.
Methods.
We studied the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb–Scargle periodograms and Gaussian process modelling to look for periodic variability. We also applied a physically motivated model to the combined
Chandra
,
XMM-Newton
,
NuSTAR
, and
Swift
-XRT data of each of the source spectral states.
Results.
We robustly show that both sources follow a clear and recurrent evolutionary pattern in the HID that can be characterised by the hard ultra-luminous (HUL) and soft ultra-luminous (SUL) spectral regimes, and a third state with characteristics similar to the super-soft ultra-luminous (SSUL) state. The transitions between the soft states seem consistent with aperiodic variability, as revealed by a timing analysis of the light curve of Holmberg II X–1; albeit, further investigation is warranted. The light curve of NGC 5204 X–1 shows a stable periodicity on a longer baseline of ∼200 days, possibly associated with the duration of the evolutionary cycle.
Conclusions.
The similarities between both sources provide strong evidence of both systems hosting the same type of accretor and/or accretion flow geometry. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the super-critical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the rapid and aperiodic variability responsible for the SUL–SSUL spectral changes. The nature of the longer periodicity of NGC 5204 X–1 remains unclear, and robust determination of the orbital period of these sources could shed light on the nature of the periodic modulation found. Based on the similarities between the two sources, a long periodicity should be detectable in Holmberg II X–1 with future monitoring.
Context.
The presence of large ionised gaseous nebulae found around some ultraluminous X-ray sources (ULXs) provides the means to assess the mechanical and radiative feedback of the central source, ...and hence constrain the efficiency and impact on the surroundings of the super-Eddington regime powering most of these sources. NGC 1313 X–1 is an archetypal ULX, reported to be surrounded by abnormally high O
I
λ
6300/H
α
> 0.1 ratios, and for which high-resolution spectroscopy in X-rays has hinted at the presence of powerful outflows.
Aims.
We report observations taken with the integral field unit Multi-Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope of NGC 1313 X–1 in order to confirm the presence of a nebula inflated by the winds, investigate its main sources of ionisation and estimate the mechanical output of the source.
Methods.
We investigated the morphology, kinematics, and sources of ionisation of the bubble through the study of the main nebular lines. We compared the main line ratios with spatially resolved Baldwin–Phillips–Terlevich diagrams and with the prediction from radiative shock libraries, which allows us to differentiate regions excited by shocks from those excited by extreme ultraviolet and X-ray radiation.
Results.
We detect a bubble of 452 × 266 pc in size, roughly centred around the ULX, which shows clear evidence of shock ionisation in the outer edges. We estimate shock velocities to be in the ≈160 − 180 km s
−1
range based on the line ratios. This suggests that an average and continuous outflow power of ∼(2 − 4.5)×10
40
erg s
−1
over a timescale of (4.5 − 7.8)×10
5
yr is required to inflate the bubble. In the interior of the bubble and closer to the ULX we detect an extended (∼140 pc) X-ray ionised region. Additionally, we detect two supernova remnants coincidentally close to the ULX bubble of which we also report age and explosion energy estimates.
Conclusions.
The elongated morphology and the kinematics of the bubble strongly suggest that the bubble is being inflated by winds and/or jets emanating from the central source, supporting the presence of winds found through X-ray spectroscopy. The estimated mechanical power is comparable to or higher than the X-ray luminosity of the source, which provides additional evidence in support of NGC 1313 X–1 harbouring a super-Eddington accretor.
Huybers et al present new bathymetric spectra from an intermediate-spreading ridge as evidence for a primary contribution of sea level cycles to the morphology of the seafloor. Although we ...acknowledge the possibility that sea level-modulated magmatic constructions may be superimposed on a first-order tectonic fabric, we emphasize the difficulty of deciphering these different contributions in the frequency domain alone.
It is well established that slip on a frictionally weak low‐angle normal fault (LANF) can be more favorable than breaking a steep fault in strong crust. Very few studies, however, have considered the ...specific effect of crust and fault cohesion on LANF viability. We do so using Limit Analysis, a methodology for predicting the optimal orientation of faults with varying strength subjected to a specific set of boundary conditions. Accounting for crustal cohesion in our models reduces the lowest admissible LANF dip and even allows slip on high‐friction LANFs if the contrast between crust and fault cohesion is large. Fault cohesion, however, increases the lowest admissible LANF dip and introduces a locking depth above which LANF slip is not mechanically feasible. This is consistent with observations of steep splay faults rooting onto LANFs in a variety of settings. We further demonstrate that locking depth can help constrain LANF cohesion, friction, and fluid pressure on the Alto Tiberina (Italy) and western Corinth (Greece) LANFs. Specifically, assuming a measured fault friction of 0.2–0.3, we find that the shallow locking depth of the Alto Tiberina fault requires either (1) moderate fluid overpressure (57% of lithostatic) with cohesion of 8–12 MPa or (2) strong overpressure (77% of lithostatic) with cohesion of 13–20 MPa along the fault. By contrast, the larger locking depth characterizing the western Corinth LANF can reflect greater fault cohesion.
Key Points
We use Limit Analysis to investigate the feasibility of slip on partially active low‐angle normal faults (LANFs) in cohesive upper crust
Crustal cohesion enables slip on high‐friction LANFs, and fault cohesion enables slip on partially active LANFs with steep splays
Different locking depths on the Alto Tiberina (Italy) and western Corinth (Greece) LANFs reflect differences in fault cohesion
The effect of grain boundaries (GBs) on the diffusion of interstitial and substitutional atoms in α-iron was studied experimentally and theoretically. Autoradiography and the sectioning method, ...applied to different grain size polycrystals, prove that a higher GB area leads to a reduction of the migration of the 14C isotope. An opposite effect is observed for the 60Co isotope. Theoretical calculations of hydrogen and carbon migration in selected GBs having different disorientation angles show that, for both interstitial elements, the activation enthalpy for migration is higher in the GBs than in the bulk. Both experimental and theoretical results lead to the conclusion that GBs in α-iron act as traps for interstitial atoms and retard their diffusion. To put these results into perspective, hydrogen-induced intergranular cracking is discussed in relation to the specificity of hydrogen diffusivity along the GBs.
1. Measles eradication is the ultimate goal but it is premature to set a date for its accomplishment. Existing regional elimination goals should be vigorously pursued to enable setting a global ...target by 2020.
2. The basic strategic approaches articulated in the Global Measles and Rubella Strategic Plan 2012–2020 are valid to achieve the goals but have not been fully implemented (or not appropriately adapted to local situations).
3. The report recommends a shift from primary reliance on supplementary immunization activities (SIAs) to assure two doses of measles-containing vaccine (MCV) are delivered to the target population to primary reliance on ongoing services to assure administration of two doses of MCV. Regular high quality SIAs will still be necessary while ongoing services are being strengthened.
4. The report recommends a shift from primary reliance on coverage to measure progress to incorporating disease incidence as a major indicator.
5. The report recommends that the measles/rubella vaccination program be considered an indicator for the quality of the overall immunization program and that measles/rubella incidence and measles and rubella vaccination coverage be considered as primary indicators of immunization program performance.
6. Polio transition presents both risks and opportunities: risks should be minimized and opportunities maximized.
7. A school entry immunization check could contribute significantly to strengthening overall immunization services with assurance that recommended doses of measles and rubella vaccines as well as other vaccines have been delivered and providing those vaccines at that time if the child is un- or under-vaccinated.
8. Program decisions should increasingly be based on good quality data and appropriate analysis.
9. The incorporation of rubella vaccination into the immunization program needs to be accelerated – it should be accorded equivalent emphasis as measles.
10. Outbreak investigation and response are critical but the most important thing is to prevent outbreaks.
PDF
