LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

Aims. We present new high sensitivity observations of the radio relic in A 521 carried out using the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. Methods. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327−610 MHz. In our present analysis we use our new GMRT observations in addition to proprietary and archival data. We search for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. Results. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is supported by our detection of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is probably a shock front.

The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ∼2 years of observing, 405 cosmic-ray events in the energy range of 10 16 −10 18 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ∼1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.

We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster Abell 754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas temperature jump across the X-ray brightness edge previously seen in the imaging data. A 754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number M= 1.57 +0.16 −0.12 . In our new GMRT observation at 330 MHz, we find that the previously-known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74 MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of preexisting relativistic electrons is more plausible.

The intra-cluster medium contains cosmic rays and magnetic fields that are manifested through the large scale synchrotron sources, termed radio haloes, relics, and mini-haloes. The Extended Giant Metrewave Radio Telescope (GMRT) Radio Halo Survey (EGRHS) is an extension of the GMRT Radio Halo Survey (GRHS) designed to search for radio haloes using GMRT 610/235 MHz observations. The GRHS and EGRHS consists of 64 clusters in the redshift range 0.2−0.4 that have an X-ray luminosity larger than 5 × 10 44 erg s −1 in the 0.1−2.4 keV band and declination, δ > −31 • in the REFLEX and eBCS X-ray cluster catalogues. In this second paper in the series, GMRT 610/235 MHz data on the last batch of 11 galaxy clusters and the statistical analysis of the full sample are presented. A new mini-halo in RX J2129.6+0005 and candidate diffuse sources in Z5247, A2552, and Z1953 have been discovered. A unique feature of this survey are the upper limits on the detections of 1 Mpc sized radio haloes; 4 new are presented here, making a total of 31 in the survey. Of the sample, 58 clusters with adequately sensitive radio information were used to obtain the most accurate occurrence fractions so far. The occurrence fractions of radio haloes, mini-haloes and relics in our sample are ∼22%, ∼16% and ∼5%, respectively. The P 1.4 GHz − L X diagrams for the radio haloes and mini-haloes are presented. The morphological estimators -centroid shift (w), concentration parameter (c), and power ratios (P 3 /P 0 ) derived from the Chandra X-ray images -are used as proxies for the dynamical states of the GRHS and EGRHS clusters. The clusters with radio haloes and mini-haloes occupy distinct quadrants in the c− w, c− P 3 /P 0 and w− P 3 /P 0 planes, corresponding to the more and less morphological disturbance, respectively. The non-detections span both the quadrants.Key words. galaxies: clusters: general -radio continuum: galaxies IntroductionThe intra-cluster medium (ICM) is the diffuse matter that pervades the space between the galaxies in galaxy clusters. It is dominated by hot (∼10 7 −10 8 K) thermal plasma that emits thermal Bremsstrahlung that is detectable in soft X-ray bands and is also responsible for the thermal Sunyaev-Zel'dovich (SZ) effect. It has been found that the ICM also contains magnetic fields (0.1−1 μG) and relativistic electrons (Lorentz factors 1000) distributed over the entire cluster volume. The most direct evidence of this are the cluster-wide diffuse synchrotron sources detected in radio bands (see Feretti et al. 2012;Brunetti & Jones 2014, for reviews). They occur in a variety of morphologies and sizes and are classified into three main types, namely, radio haloes, radio relics, and mini-haloes.Radio haloes (RHs) are Mpc-sized sources found in massive, merging galaxy clusters. They typically trace the morphology of the X-ray surface brightness, show negligible polarisation, and Appendices are available in electronic form at http://www.aanda.org have synchrotron spectral indices 1 , α ≥ 1. The emitting relativistic electro...

Pulsars emit low-frequency radio waves through to high-energy gamma-rays that are generated anywhere from the surface out to the edges of the magnetosphere. Detecting correlated mode changes in the multi-wavelength emission is therefore key to understanding the physical relationship between these emission sites. Through simultaneous observations, we have detected synchronous switching in the radio and X-ray emission properties of PSR B0943+10. When the pulsar is in a sustained radio 'bright' mode, the X-rays show only an un-pulsed, non-thermal component. Conversely, when the pulsar is in a radio 'quiet' mode, the X-ray luminosity more than doubles and a 100%-pulsed thermal component is observed along with the non-thermal component. This indicates rapid, global changes to the conditions in the magnetosphere, which challenge all proposed pulsar emission theories. Main Text:Radio pulsars are powered by the energy released as the highly magnetized neutron star spins down. The radio pulses are generated in the pulsar magnetosphere, most probably close to the neutron star surface (1,2). Shortly after the discovery of pulsars, it was observed that the radio pulse behavior can discretely change on timescales as short as a rotation period. These changes in emission mode can manifest as switches between ordered and disordered states or variations in intensity and pulse shape, including the complete cessation of observable radio emission (3,4).Because the emitted radio luminosity is a negligible fraction of the available spin-down energy, usually substantially less than 10 -5 , this phenomenology was presumed to be related solely to microphysics of the radio emission mechanism itself. This perception has recently been challenged by the identification of a relationship between the spin properties of neutron stars and their radio emission modes. PSR B1931+24 was observed to cease emitting for tens of days, during which it spins down ~50% less rapidly (5). PSR J1841-0500 (6) and PSR J1832+0029 (7) exhibit similar behaviors. A number of other pulsars display smaller changes in spin-down rate, which correlate with variations in their average radio pulse shapes (8). The implication of these results is that mode changing is due to an inherent, perhaps universal pulsar process which causes a sudden change in the rate of angular momentum loss that is communicated along the open field lines of the magnetosphere. Whereas changes in spindown rate can only be detected on time-scales of a few days or longer, the recently identified link with the rapid switching observed in radio emission modes suggests a transformation of the global magnetospheric state in less than a rotation period. Despite the recent flurry of pulsar detections at high energies (9), the only causal relation between the radio pulses and emission at other wavelengths, likely emanating from different locations in the magnetosphere, has been made for optical emission and giant radio pulses from the Crab pulsar (10) PSR B0943+10 is a paragon of mode-changing pul...

Context. A fraction of galaxy clusters host diffuse radio sources called radio halos, radio relics and mini-halos. These are associated with the relativistic electrons and magnetic fields present on ∼Mpc scales in the intra-cluster medium. Aims. We aim to carry out a systematic radio survey of all luminous galaxy clusters selected from the REFLEX and eBCS X-ray catalogues with the Giant Metrewave Radio Telescope, to understand the statistical properties of the diffuse radio emission in galaxy clusters. Methods. We present the sample and first results from the Extended GMRT Radio Halo Survey (EGRHS), which is an extension of the GMRT Radio Halo Survey (GRHS, Venturi et al. 2007. Analysis of radio data at 610/ 235/ 325 MHz on 12 galaxy clusters are presented. Results. We report the detection of a newly discovered mini-halo in the cluster RX J1532.9+3021 at 610 MHz. The presence of a small-scale relic (∼200 kpc) is suspected in the cluster Z348. We do not detect cluster-scale diffuse emission in 11 clusters. Robust upper limits on the detection of radio halo of size of 1 Mpc are determined. We also present upper limits on the detections of minihalos in a sub-sample of cool-core clusters. The upper limits for radio halos and mini-halos are plotted in the radio power-X-ray luminosity plane and the correlations are discussed. Diffuse extended emission that is not related to the target clusters, but detected as by-products in the sensitive images of two of the cluster fields (A689 and RX J0439.0+0715) is also reported. Conclusions. Based on the information about the presence of radio halos (or upper limits), available on 48 clusters out of the total sample of 67 clusters (EGRHS+GRHS), we find that 23±7% of the clusters host radio halos. The radio halo fraction rises to 31±11%, when only the clusters with X-ray luminosities >8 × 10 44 erg s −1 are considered. Mini-halos are found in ∼50% of cool-core clusters. A qualitative examination of the X-ray images of the clusters with no diffuse radio emission indicates that a majority of these clusters do not show extreme dynamical disturbances and supports the idea that mergers play an important role in generating radio halos and relics. The analysis of the full sample will be presented in a future work.

Aims. To gain insight into the origin of diffuse radio sources in galaxy clusters and their connection with cluster merger processes, we performed GMRT low frequency observations of the radio haloes, relics and new candidates belonging to the GMRT radio Halo cluster sample first observed at 610 MHz. Our main aim was to investigate their observational properties and integrated spectra at frequencies below 610 MHz. Methods. High sensitivity imaging was performed using the GMRT at 325 MHz and 240 MHz. The properties of the diffuse emission in each cluster were compared to our 610 MHz images and/or literature information available at other frequencies, in order to derive the integrated spectra over a wide frequency range. Results. Cluster radio haloes form a composite class in terms of spectral properties. Beyond the classical radio haloes, whose spectral index α is in the range ∼1.2 ÷ 1.3 (S ∝ ν −α ), we found sources with α ∼ 1.6 ÷ 1.9. This result supports the idea that the spectra of the radiating particles in radio haloes is not universal and that inefficient mechanisms of particle acceleration are responsible for their origin. We also found a variety of brightness distributions, i.e. both centrally peaked and clumpy haloes. Even though the thermal and relativistic plasma tend to occupy the same cluster volume, in some cases a positional shift between the radio and X-ray peaks of emission is evident. Our observations also revealed diffuse cluster sources that cannot be easily classified as either haloes or relics. New candidate relics were found in A 1300 and in A 1682, and in some clusters "bridges" of radio emission have been detected, connecting the relic and radio halo emission. Finally, by combining our new data with information in the literature, we derived the Log L X -Log P 325 MHz correlation for radio haloes, and investigated the possible correlation of the spectral index of radio haloes with the temperature of the intracluster medium.