Abstract. A significant fraction of galaxy clusters exhibits cluster-wide radio halos. We give a simple prediction of the local and higher redshift radio halo luminosity function (RHLF) on the basis of (i) an observed and a theoretical X-ray cluster luminosity function (XCLF) (ii) the observed radio-X-ray luminosity correlation (RXLC) of galaxy clusters with radio halos (iii) an assumed fraction of f rh ≈ 1 3 galaxy clusters to have radio halos as supported by observations. We then find 300-700 radio halos with S 1.4 GHz > 1 mJy, and 10 5 -10 6 radio halos with S 1.4 GHz > 1 µJy should be visible on the sky. 14% of the S 1.4 GHz > 1 mJy and 56% of the S 1.4 GHz > 1 µJy halos are located at z > 0.3. Subsequently, we give more realistic predictions taking into account (iv) a refined estimate of the radio halo fraction as a function of redshift and cluster mass, and (v) a decrease in intrinsic radio halo luminosity with redshift due to increased inverse Compton electron energy losses on the Cosmic Microwave Background (CMB). We find that this reduces the radio halo counts from the simple prediction by only 30 % totally, but the high redshift (z > 0.3) counts are more strongly reduced by 50-70%. These calculations show that the new generation of sensitive radio telescopes, including LOFAR, ATA, EVLA, SKA and the already-operating GMRT should be able to detect large numbers of radio halos and will provide unique information for studies of galaxy cluster merger rates and associated non-thermal processes.

Giant radio relics are the arc-shaped diffuse radio emission regions observed in the outskirts of some merging galaxy clusters. They are believed to trace shock-waves in the intra-cluster medium. Recent observations demonstrated that some prominent radio relics exhibit a steepening above 2 GHz in their radio spectrum. This challenges standard theoretical models because shock acceleration is expected to accelerate electrons to very high energies with a power-law distribution in momentum. In this work we attempt to reconcile these data with the shock-acceleration scenario. We propose that the spectral steepening may be caused by the highest energy electrons emitting preferentially in lower magnetic fields than the bulk of synchrotron bright electrons in relics. Here, we focus on a model with an increasing magnetic field behind the shock front, which quickly saturates and then declines. We derive the time-evolution of cosmic-ray electron spectra in time variable magnetic fields and an expanding medium. We then apply the formalism on the large radio relic in the cluster CIZA J2242.8+5301 (the Sausage relic). We show that under favourable circumstances of magnetic field amplification downstream, our model can explain the observed radio spectrum, the brightness profile and the spectral index profile of the relic. A possible interpretation for the required amplification of the magnetic field downstream is a dynamo acting behind the shock with an injection scale of magnetic turbulence of about 10 kpc. Our models require injection efficiencies of CRe -which are in tension with simple diffusive shock acceleration from the thermal pool. We show that this problem can likely be alleviated considering pre-existing CRe.

International audienceWhen an ultra-high energy neutrino or cosmic-ray strikes the Lunar surface a radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to detect these pulses. In this work we propose an efficient trigger implementation for LOFAR optimized for the observation of short radio pulses

Context. Diffuse cluster-scale synchrotron radio emission is discovered in an increasing number of galaxy clusters in the form of radio halos, probing the presence of relativistic electrons and magnetic fields in the intra-cluster medium (ICM). The favoured scenario to explain their origin is that they trace turbulent regions generated during cluster-cluster mergers where particles are re-accelerated. In this framework, radio halos are expected to probe cluster dynamics and are predicted to be more frequent in massive systems where more energy becomes available to the re-acceleration of relativistic electrons. For these reasons, statistical studies of galaxy cluster samples have the power to derive fundamental information on the radio halo populations and on their connection with cluster dynamics, and hence to constrain theoretical models. Furthermore, low-frequency cluster surveys have the potential to shed light on the existence of radio halos with very steep radio-spectra, which are a key prediction of turbulent models that should be generated in less energetic merger events and thus be more common in the Universe. Aims. The main question we will address in this paper is whether we can explain the observed properties of the RH population in this sample within the framework of current models. Methods. We study the occurrence and properties of radio halos from clusters of the second catalog of Planck Sunyaev Zel'dovich detected sources that lie within the 5634 deg 2 covered by the second Data Release of the LOFAR Two-meter Sky Survey. We derive their integral number, flux density and redshift distributions. We compare these observations with expectations of theoretical models. We also study the connection between radio halos and cluster mergers by using cluster morphological parameters derived through Chandra and/or XMM-Newton data. Results. We find that the number of observed radio halos, their radio flux density and redshift distributions are in line with what is expected in the framework of the re-acceleration scenario. In line with model expectations, the fraction of clusters with radio halos increases with the cluster mass, confirming the leading role of the gravitational process of cluster formation in the generation of radio halos. These models predict a large fraction of radio halos with very steep spectrum in the DR2 Planck sample, this will be tested in future studies, yet a comparison of the occurrence of halos in GMRT and LOFAR samples indeed shows a larger occurrence of halos at lower frequencies suggesting the presence of a population of halos with very steep spectrum that is preferentially detected by LOFAR. Using morphological information we confirm that radio halos are preferentially located in merging systems and that the fraction of newly LOFAR discovered radio halos is larger in less disturbed systems.

The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multi-wavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (∼1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120-168 MHz and the Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size of about 1.8 Mpc and a flux density S144 MHz = (1.52 ± 0.31) Jy. The spectral index distribution of the source is patchy with an average spectral index α ∼ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index α ∼ 2.1 has been obtained between 1.410 GHz and 1.782 GHz. A previously unseen patch of steep spectrum emission is clearly detected at 144 MHz in the south of the cluster. Overall, our findings suggest that we are observing an overlapping of different structures, powered by the turbulence associated with the primary and a possible secondary merger.