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International audience; NenuFAR is a new, large low-frequency radio telescope, in construction and commissioning at the Nançay Radioastronomy Observatory, that starts to provide high sensitivity observations in the 10-85 MHz range. NenuFAR’s 1938 dual polarization antennas are connected to a suite of receivers allowing the instrument to operate, simultaneously if needed, in 4 distinct modes : as a standalone beamformer, a standalone imager, a waveform snapshots recorder, and a giant low-frequency station of the LOFAR array. We provide here an overview of the antennas, receivers, data products, operation and scientific context of the instrument.

We present wideband (1 − 6.5 GHz) polarimetric observations, obtained with the Karl G. Jansky Very Large Array, of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We used both rotation measure synthesis and QU-fitting to find a reasonable agreement of the results obtained with these methods, particularly when the Faraday distribution is simple and the depolarization is mild. The relic is highly polarized over its entire length (850 kpc), reaching a fractional polarization > 30% in some regions. We also observe a strong wavelength-dependent depolarization for some regions of the relic. The northern part of the relic shows a complex Faraday distribution, suggesting that this region is located in or behind the intracluster medium (ICM). Conversely, the southern part of the relic shows a rotation measure very close to the Galactic foreground, with a rather low Faraday dispersion, indicating very little magnetoionic material intervening along the line of sight. Based on a spatially resolved polarization analysis, we find that the scatter of Faraday depths is correlated with the depolarization, indicating that the tangled magnetic field in the ICM causes the depolarization. We conclude that the ICM magnetic field could be highly turbulent. At the position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of two components that are clearly separated in the Faraday space. The high Faraday dispersion component seems to be associated with the NAT, suggesting the NAT is embedded in the ICM while the southern part of the relic lies in front of it. If true, this implies that the relic and this radio galaxy are not necessarily physically connected and, thus, the relic may, in fact, not be powered by the shock re-acceleration of fossil electrons from the NAT. The magnetic field orientation follows the relic structure indicating a well-ordered magnetic field. We also detected polarized emission in the halo region; however, the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission that was previously considered as the part of the halo does, in fact, originate from the shock(s).

MS 0735.6+7421 is a galaxy cluster which hosts a central radio galaxy with a very steep spectrum, produced by one of the most powerful known jetted active galactic nuclei (AGN). The radio plasma, ejected at nearly light speed from the central AGN, have displaced the intra-cluster medium, leaving two pairs of cavities observable in the X-ray, associated to two different outbursts, and have distributed energy to the surrounding medium. In this work we have performed for the first time a detailed, high-resolution spectral study of the source at radio frequencies and investigated its duty cycle to be compared with previous X-ray estimates. We have used new observations at 144 MHz produced with the LOw Frequency ARray (LOFAR) together with archival data at higher frequencies. At LOFAR frequency, the source presents two large outer radio lobes, wider than at higher frequencies, and a smaller Intermediate lobe located south-west of the core. A new inspection of X-ray data, allowed us to identify an intermediate cavity, associated with that lobe, indicating the presence of a further phase of jet activity. The radio lobes have a steep spectrum even at LOFAR frequencies, reaching $\alpha_{144}^{610}=2.9$ in the outer lobes and $\alpha_{144}^{610}=2.1$ in the Intermediate lobe. Fitting the lobe spectra using a single injection model of particle ageing, we derived a total age of the source between 170 and 106 Myr, in agreement with the buoyancy and sound crossing time-scales derived from X-ray data. We then reconstructed the duty cycle of the source. There were three phases of jet activity, with the AGN being active for most of the time with only brief quiescent phases, ensuring the repeated heating of the central gas. Finally, energetic estimates revealed that a source of additional pressure support must be present to sustain the bubbles against the pressure of the external medium., Comment: 18 pages, 12 figures; accepted for publication in A&A

We present new LOFAR observations of the massive merging galaxy cluster MACS J0717.5+3745. The cluster hosts the most powerful radio halo known to date. These new observations, in combination with published uGMRT (300$-$850 MHz) and VLA (1$-$6.5 GHz) data, reveal that the halo is more extended than previously thought, with a largest linear size of $\sim2.2 \rm Mpc$. The halo shows a steep spectrum ($\alpha_{144\,\text{MHz}}^{1.5\,\text{GHz}}\sim-1.4$) and a steepening ($\alpha_{1.5 \text{GHz}}^{5.5 \text{GHz}}\sim-1.9$) above 1.5 GHz. We find a strong scattering in spectral index maps on scales of 50$-$100 kpc. We suggest that such a strong scattering may be a consequence of the regime where inverse Compton dominate the energy losses of electrons. The spectral index becomes steeper and shows an increased curvature in the outermost regions of the halo. We combined the radio data with \textit{Chandra} observations to investigate the connection between the thermal and non-thermal components of the intracluster medium (ICM). Despite a significant substructure in the halo emission, the radio brightness correlates strongly with the X-ray brightness at all observed frequencies. The radio-versus-X-ray brightness correlation slope steepens at a higher radio frequency (from $b_{144 \text{MHz}}=0.67\pm0.05$ to $b_{3.0 \text{GHz}}=0.98\pm0.09$) and the spectral index shows a significant anti correlation with the X-ray brightness. Both pieces of evidence further support a spectral steepening in the external regions. The compelling evidence for a steep spectral index, the existence of a spectral break above 1.5 GHz, and the dependence of radio and X-ray surface brightness correlation on frequency are interpreted in the context of turbulent reacceleration models. Under this scenario, our results allowed us to constrain that the turbulent kinetic pressure of the ICM is up to 10%., Comment: 16 pages, 12 figures, accepted for publication in A&A

We present deep and high fidelity images of the merging galaxy cluster Abell 2256 at low frequencies, using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics, with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6 arcsec resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of $-1.07\pm0.02$ between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re-)acceleration of electrons that are responsible for the observed radio emission., Comment: 27 pages, 14 Figures, accepted for publication in ApJ

Radio synchrotron emission from the bridges of low-density gas connecting galaxy clusters and groups is a challenge for particle acceleration processes. In this work, we analyze the Coma radio bridge using new LOw Frequency ARray (LOFAR) observations at 144 MHz. LOFAR detects the bridge and its substructures with unprecedented sensitivity and resolution. We found that the radio emission peaks on the NGC 4839 group. Toward the halo, in front of the NGC 4839 group, the radio brightness decreases and streams of radio emission connect the NGC 4839 group to the radio relic. Using X-ray observations, we found that thermal and non-thermal plasma are moderately correlated with a sublinear scaling. We use archival radio data at 326 MHz to constrain the spectral index in the bridge, and quantify the distribution of particles and magnetic field at different frequencies. We found that the spectrum is steeper than −1.4 ± 0.2, and that the emission is clumpier at 326 MHz than at 144 MHz. Using cosmological simulations and a simplified approach to compute particle acceleration, we derive under which conditions turbulent acceleration of mildly relativistic electrons generate the radio emission in the bridge. Assuming that the initial energy ratio of the seed electrons is 3 · 10−4 with respect to the thermal gas, we are able to reproduce the observed luminosity. Our results suggest that the seed electrons released by radio galaxies in the bridge and the turbulence generated by the motion of gas and galaxies are essential to producing the radio emission.

Diffuse cluster radio sources, in the form of radio halos and relics, reveal the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). These cosmic rays are thought to be (re)accelerated through the ICM turbulence and shock waves generated by cluster merger events. Here we characterize the presence of diffuse radio emission in known galaxy clusters in the HETDEX Spring Field, covering 424 deg2. For this, we developed a method to extract individual targets from LOFAR observations processed with the LoTSS DDF-pipeline software. This procedure enables improved calibration as well as the joint imaging and deconvolution of multiple pointings of selected targets. The calibration strategy can also be used for LOFAR low-band antenna and international-baseline observations. The fraction of Planck PSZ2 catalog clusters with any diffuse radio emission apparently associated with the ICM is 73 ± 17%. We detect a total of ten radio halos and twelve candidate halos in the HETDEX Spring Field. Of these ten radio halos, four are new discoveries, two of which are located in PSZ2 clusters. Five clusters host radio relics, two of which are new discoveries. The fraction of radio halos in Planck PSZ2 clusters is 31 ± 11%, or 62 ± 15% when including the candidate radio halos. Based on these numbers, we expect that there will be at least 183 ± 65 radio halos found in the LoTSS survey in PSZ2 clusters, in agreement with past predictions. The integrated flux densities for the radio halos were computed by fitting exponential models to the radio images. From these flux densities, we determine the cluster mass (M500) and Compton Y parameter (Y500) 150 MHz radio power (P150 MHz) scaling relations for Planck PSZ2-detected radio halos. Using bivariate correlated errors and intrinsic scatter orthogonal regression, we find slopes of 6.13 ± 1.11 and 3.32 ± 0.65 for the M500–P150 MHz and M500–P150 MHz relations, respectively. These values are consistent with the results of previous works.

Radio relics trace shock fronts generated in the intracluster medium (ICM) during cluster mergers. The particle acceleration mechanism at the shock fronts is not yet completely understood. We observed the Toothbrush relic with the Effelsberg and Sardinia Radio Telescope at 14.25 GHz and 18.6 GHz, respectively. Unlike previously claimed, the integrated spectrum of the relic closely follows a power law over almost three orders of magnitude in frequency, with a spectral index of $\alpha_{\rm 58\,MHz}^{\rm 18.6\,GHz}=-1.16\pm0.03$. Our finding is consistent with a power-law injection spectrum, as predicted by diffusive shock acceleration theory. The result suggests that there is only little magnetic field strength evolution downstream to the shock. From the lack of spectral steepening, we find that either the Sunyaev-Zeldovich decrement produced by the pressure jump is less extended than $\sim$ 600\,kpc along the line of sight or, conversely, that the relic is located far behind in the cluster. For the first time, we detect linearly polarized emission from the "brush" at 18.6 GHz. Compared to 8.3 GHz, the degree of polarization across the brush increases at 18.6 GHz, suggesting a strong Faraday depolarization towards lower frequencies. The observed depolarization is consistent with an intervening magnetized screen that arise from the dense ICM containing turbulent magnetic fields. The depolarization, corresponding to a standard deviation of the Rotation Measures as high as $\sigma_{\rm RM}= 212\pm23\rm \,rad\,m^{-2}$, suggests that the brush is located in or behind the ICM. Our findings indicate that the Toothbrush can be consistently explained by the standard scenario for relic formation, Comment: Accepted for publication in A&A Letters

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,\mu$Jy beam$^{-1}$ and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and the positional accuracy is within 0.2$''$. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further $\sim$20 articles that highlight the scientific potential of LoTSS., Comment: 16 figures, 1 table and 22 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal