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The central regions of galaxy clusters are permeated by magnetic fields and filled with relativistic electrons1. When clusters merge, the magnetic fields are amplified and relativistic electrons are re-accelerated by turbulence in the intracluster medium2,3. These electrons reach energies of 1–10 GeV and, in the presence of magnetic fields, produce diffuse radio halos4that typically cover an area of  around 1 Mpc2. Here we report observations of four clusters whose radio halos are embedded in much more extended, diffuse radio emission, filling a volume 30 times larger than that of radio halos. The emissivity in these larger features is about 20 times lower than the emissivity in radio halos. We conclude that relativistic electrons and magnetic fields extend far beyond radio halos, and that the physical conditions in the outer regions of the clusters are quite different from those in the radio halos.

Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends—called positive and negative leaders1. Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band2–8that can be remotely sensed and imaged with high spatial and temporal resolution9–11. Positive leaders propagate more continuously and thus emit very little high-frequency radiation12. Radio emission from positive leaders has nevertheless been mapped13–15, and exhibits a pattern that is different from that of negative leaders11–13,16,17. Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders9,12,16,18–20, which may lead to current pulses that both reconnect positive leaders to negative leaders11,16,17,20–22and cause multiple cloud-to-ground lightning events1. The disconnection process is thought to be due to negative differential resistance18, but this does not explain why the disconnections form primarily on positive leaders22, or why the current in cloud-to-ground lightning never goes to zero23. Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures—which we call ‘needles’—that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times.

Context. Hierarchical galaxy formation models indicate that during their lifetime galaxy clusters undergo several mergers. An example of such a merging cluster is Abell 2256. Here we report on the discovery of three diffuse radio sources in the periphery of Abell 2256, using the Giant Metrewave Radio Telescope (GMRT).

Context. Hierarchal models of large-scale structure (LSS) formation predict that galaxy clusters grow via gravitational infall and mergers of smaller subclusters and galaxy groups. Diffuse radio emission, in the form of radio halosand relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic field strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at $z=0.27$, using Giant Metrewave Radio Telescope (GMRT) observations.

Context. The physical conditions in a collapsing cloud can be traced by observations of molecular lines. To correctly interpret these observations the abundance distributions of the observed species need to be derived. The chemistry in a collapsing molecular cloud is not in a steady state as the density and temperature evolve. We therefore need to follow chemical reactions, both in the gas phase and on dust grains, as well as gas-grain interactions, to predict the abundance distributions.

High-resolution radio measurements of air showers—cascades of secondary particles in the atmosphere initiated by cosmic rays—reveal that cosmic rays with energies of 1017–1017.5electronvolts have a mixed composition, with light elements (protons and helium nuclei) making up 80 per cent of their mass.

Objective: Contrast-enhanced computed tomography (CECT) has been introduced for the evaluation of cartilage integrity. Furthermore, CECT enables imaging of the structure and density of subchondral bone. In this laboratory study, we investigate the potential of microCECT to simultaneously image cartilage and subchondral bone for the evaluation of tissue healing.Design: Osteochondral lesions (Ø = 6 mm) were surgically created in equine intercarpal joints (n= 7). After spontaneous healing for 12 months, the horses were sacrificed and osteochondral plugs (Ø = 14 mm), including the repair cartilage and adjacent intact tissue, were harvested. The nonfibrillar and fibrillar moduli and the permeability of cartilage were determined using indentation testing. Contrast agent diffusion into the samples was imaged for 36 hours using high-resolution CT. Results from CECT, mechanical testing, and microscopic analyses were compared and correlated.Results: The contrast agent diffusion coefficient showed a significant (P< 0.05) difference between the repair and adjacent intact tissue. MicroCECT revealed altered (P< 0.05) bone volume fraction, mineral density, and microstructure of subchondral bone at the repair site. The contrast agent diffusion coefficient correlated with the moduli of the nonfibrillar matrix (R= −0.662, P= 0.010), collagen fibril parallelism index (R= −0.588, P= 0.035), and glycosaminoglycan content (R= −0.503, P= 0.067). The repair cartilage was mechanically and structurally different from adjacent intact tissue (P< 0.05).Conclusions: MicroCECT enabled simultaneous quantitative evaluation of subchondral bone and monitoring of cartilage repair, distinguishing quantitatively the repair site from the adjacent intact tissue. As the only technique able to simultaneously image cartilage and determine subchondral bone mineral density and microstructure, CECT has potential clinical value.

Context. Hierarchal models of large-scale structure (LSS) formation predict that galaxy clusters grow via gravitational infall and mergers of smaller subclusters and galaxy groups. Diffuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic field strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl?2341.1+0000, a complex merging structure of galaxies located at z=0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims. The main aim of the observations is to study the nature of the diffuse radio emission in the galaxy cluster ZwCl?2341.1+0000. Methods. We carried out GMRT 610, 241, and 157?MHz continuum observations of ZwCl?2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results. The GMRT observations show a double peripheral radio relic in the cluster ZwCl?2341.1+0000. The spectral index is -0.49 ?0.18 for the northern relic and -0.76 ?0.17 for the southern relic. We have derived values of 0.48-0.93??Gauss for the equipartition magnetic field strength. The relics are probably associated with outward traveling merger shock waves.?

Context. Hierarchical models of structure formation predict that galaxy clusters grow via mergers of smaller clusters and galaxy groups, as well as through continuous accretion of gas. MACS?J0717.5+3745 is an X-ray luminous and complex merging cluster, located at a redshift of 0.5548. The cluster is suspected to host a bright radio relic, but up until now no detailed radio observations have been reported. Here we present Giant Metrewave Radio Telescope (GMRT) radio observations at 610?MHz of this cluster. Aims. The main aim of the observations is to study the diffuse radio emission within the galaxy cluster MACS?J0717.5+3745 related to the ongoing merger. Methods. We have carried out GMRT 610?MHz continuum observations of MACS?J0717.5+3745. These are complemented by Very Large Array (VLA) archival observations at 1.4, 4.9 and 8.5?GHz. Results. We have discovered a radio halo in the cluster MACS?J0717.5+3745 with a size of about 1.2?Mpc. The monochromatic radio power at 1400?MHz (P1.4) is 5 ? 1025?W?Hz-1, which makes it the most powerful radio halo known to date. A 700?kpc radio structure, which we classify as a radio relic, is located in between the merging substructures of the system. The global spectral index of radio emission within the cluster is found to be -1.24 ? 0.05 between 4.9?GHz and 610?MHz. We derive a value of 5.8??G for the equipartition magnetic field strength at the location of the radio halo. The location of the relic roughly coincides with regions of the intra-cluster medium (ICM) that have a significant enhancement in temperature as shown by Chandra. The major axis of the relic is also roughly perpendicular to the merger axis. This shows that the relic might be the result of a merger-related shock wave, where particles are accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively, the relic might trace an accretion shock of a large-scale galaxy filament to the south-east.

Context. Diffuse radio emission, in the form of radio halos and relics, traces regions in clusters with shocks or turbulence, probably produced by cluster mergers. The shocks and turbulence are important for the total energetics and detailed temperature distribution within the intracluster medium (ICM). Only a small fraction of clusters exhibit diffuse radio emission, whereas a large majority of well-studied clusters shows clear substructure in the ICM. Some models of diffuse radio emission in clusters indicate that virtually all clusters should contain diffuse radio sources with a steep spectrum. External accretion shocks associated with filamentary structures of galaxies could also accelerate electrons to relativistic energies and hence produce diffuse synchrotron emitting regions. The detection of radio emission from such filaments is important for our understanding of the origin of the Warm-Hot Intergalactic Medium (WHIM), and relativistic electrons and magnetic fields in the cosmic web. Here we report on Giant Metrewave Radio Telescope (GMRT) observations of a sample of steep spectrum sources from the 74?MHz?VLSS survey. These sources are diffuse on scales??15?, and not clearly associated with nearby (z ?0.1) galaxies. Aims. The main aim of the observations is to search for diffuse radio emission associated with galaxy clusters or the cosmic web. Methods. We have carried out GMRT?610?MHz continuum observations of unidentified diffuse steep spectrum sources. Results. We have constructed a sample of diffuse steep spectrum sources, selected from the 74?MHz VLSS survey. We identified eight diffuse radio sources probably all located in clusters. We found five radio relics, one cluster with a giant radio halo and a radio relic, and one radio mini-halo. The giant radio halo has the highest radio power (P1.4) known to date. By complementing our observations with measurements from the literature we find correlations between the physical size of relics and the spectral index, in the sense that smaller relics have steeper spectra. Furthermore, larger relics are mostly located in the outskirts of clusters while smaller relics are located closer to the cluster center.

Context. Understanding the gas abundance distribution is essential when tracing star formation using molecular line observations. Variations in density and temperature conditions can cause gas to freeze-out onto dust grains, and this needs to be taken into account when modeling a collapsing molecular cloud.Aims. This study aims to provide a realistic estimate of the CO abundance distribution throughout the collapse of a molecular cloud. We derive abundance profiles and synthetic spectral lines that can be compared with observations.Methods. We use a two-dimensional hydrodynamical simulation of a collapsing cloud and subsequent formation of a protoplanetary disk as input to the chemical calculations. From the resulting abundances, synthetic spectra are calculated using a molecular excitation and radiation transfer code.Results. We compare three different methods to calculate the abundance of CO. Our models also consider cosmic ray desorption and the effects of an increased CO binding energy. The resulting abundance profiles are compared with observations from the literature and are found to agree well.Conclusions. The resulting abundance profiles agree well with analytic approximations, and the corresponding line fluxes match the observational data. Our method to calculate abundances in hydrodynamical simulations should help comparisons with observations, and can easily be generalized to include gas-phase reaction networks.

Nature 531, 70–73 (2016); doi:10.1038/nature16976 In this Letter, we omitted to cite preliminary results from the low-energy extension of the Pierre Auger Observatory, as presented at the International Cosmic Ray Conference 2015 (ref. 1). Figure 1 of this Corrigendum shows measurements of the average value of Xmax for the Low Frequency Array (LOFAR), and earlier experiments using different techniques, now including the data from the Pierre Auger Observatory1, specifically the contribution of A.

The synthesis of planar strip geometries for planar waveguides by a layered manufacturing technique, called fused deposition modelling (FDM™), is presented. Channels were deposited on silicon wafers by melting a thermoplastic polymer and extruding it through a small orifice. Sol-gel-derived, amorphous fluoride films were dip-coated into these channels and yielded straight, 1 mm strips on subsequent removal of the polymer.

In this article we present deep, high-resolution radio interferometric observations at 153 MHz to complement the extensively studied NOAO Boötes field. We provide a description of the observations, data reduction and source catalog construction. From our single pointing GMRT observation of  ~12 h we obtain a high-resolution (26″ × 22″) image of  ~11.3 square degrees, fully covering the Boötes field region and beyond. The image has a central noise level of  ~1.0 mJy beam-1, which rises to 2.0–2.5 mJy beam-1at the field edge, placing it amongst the deepest  ~150 MHz surveys to date. The catalog of 598 extracted sources is estimated to be  ~92 percent complete for  > 10 mJy sources, while the estimated contamination with false detections is  < 1 percent. The low rms position uncertainty of 1.24″ facilitates accurate matching against catalogs at optical, infrared and other wavelengths. Differential source counts are determined down to  ≲10 mJy. There is no evidence for flattening of the counts towards lower flux densities as observed in deep radio surveys at higher frequencies, suggesting that our catalog is dominated by the classical radio-loud AGN population that explains the counts at higher flux densities. Combination with available deep 1.4 GHz observations yields an accurate determination of spectral indices for 417 sources down to the lowest 153 MHz flux densities, of which 16 have ultra-steep spectra with spectral indices below  −1.3. We confirm that flattening of the median spectral index towards low flux densities also occurs at this frequency. The detection fraction of the radio sources in NIR KS-band is found to drop with radio spectral index, which is in agreement with the known correlation between spectral index and redshift for brighter radio sources.

Context.Radio halos and relics are diffuse radio sources found in galaxy clusters showing significant substructure at X-ray wavelengths. These sources provide important information about non-thermal processes taking place in the intracluster medium (ICM). Until now only a few dozen relics and halos are known, while models predict that a much larger number of these sources exist. In this paper we present the results of an extensive observing campaign to search for new diffuse radio sources in galaxy clusters.

Context.Some merging galaxy clusters host diffuse elongated radio sources, also called radio relics. It is proposed that these radio relics trace shock waves in the intracluster medium (ICM) created during a cluster merger event. Within the shock waves particles are accelerated to relativistic energies, and in the presence of a magnetic field synchrotron radiation will be emitted. Here we present giant metrewave radio telescope (GMRT) and Westerbork synthesis radio telescope (WSRT) observations of a new double relic in the galaxy cluster ZwCl 0008.8+5215.

Context.Galaxy clusters grow by a sequence of mergers with other clusters and galaxy groups. During these mergers, shocks and/or turbulence are created within the intracluster medium (ICM). In this process, particles could be accelerated to highly relativistic energies. The synchrotron radiation from these particles is observed in the form of radio relics and halos that are generally characterized by a steep radio spectral index. Shocks can also revive fossil radio plasma from a previous episode of AGN activity, creating a so-called radio “phoenix”. Here we present multi-frequency radio observations of diffuse steep-spectrum radio sources selected from the 74 MHz VLSS survey. Previous Giant Metrewave Radio Telescope (GMRT) observations showed that some of these sources had filamentary and elongated morphologies, which are expected for radio relics.