D. Dallacasa

The Coma cluster magnetic field from Faraday rotation measures

Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical simulations of the magnetic field. Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B0 and radial power-law slope η ,w hereη indicates how the field scales with respect to the gas density profile. Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ∼2 kpc and maximum scale ∼34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B0 = 3.9 μG; η = 0.4) and (B0 = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B0 = 4.7 μG; η = 0.5. Values of B0 > 7 μ Ga nd 1.0 are incompatible with RM data at 99% confidence level.

Magnetic fields and Faraday rotation in clusters of galaxies

We present a numerical approach to investigate the relationship between magnetic fields and Faraday rotation effects in clusters of galaxies. We can infer the structure and strength of intra-cluster magnetic fields by comparing our simulations with the observed polarization properties of extended cluster radio sources such as radio galaxies and halos. We find the observations require a magnetic field which fluctuates over a wide range of spatial scales (at least one order of magnitude). If several polarized radio sources are located at different projected positions in a galaxy cluster, as is the case for A119, detailed Faraday rotation images allow us to constrain both the magnetic field strength and the slope of the power spectrum. Our results show that the standard analytic expressions applied in the literature overestimate the cluster magnetic field strengths by a factor of ∼2. We investigate the possible effects of our models on beam depolarization of radio sources whose radiation traverses the magnetized intracluster medium. Finally, we point out that radio halos may provide important information about the spatial power spectrum of the magnetic field fluctuations on large scales. In particular, different values of the index of the power spectrum produce very different total intensity and polarization brightness distributions.

A low-frequency radio halo associated with a cluster of galaxies

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 1015 solar masses of hot (108 K) gas, galaxies and dark matter in a typical volume of 10 Mpc3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant ‘radio haloes’, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster centre. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

A complete sample of GHz-peaked-spectrum radio sources and its radio properties

We dene a complete sample of thirty-three GHz-Peaked-Spectrum (GPS) radio sources based on their spectral properties. We present measurements of the radio spectra and polarization of the complete sample and a list of additional GPS sources which fail one or more criteria to be included in the complete sample. The majority of the data have been obtained from quasi-simultaneous multi-frequency observations at the Very Large Array (VLA) during 3 observing sessions. Low frequency data from the Westerbork Synthesis Radio Telescope (WSRT) and from the literature have been com- bined with the VLA data in order to better dene the spectral shape. The objects presented here show a rather wide range of spectral indices at high and low frequencies, including a few cases where the spectral index below the turnover is close to the theoretical value of 2.5 typical of self-absorbed incoherent synchrotron emission. Faint and diuse ex- tended emission is found in about 10% of the sources. In the majority of the GPS sources, the fractional po- larization is found to be very low, consistent with the residual instrumental polarization of 0.3% 1 .

GMRT Radio Halo Survey in galaxy clusters at z = 0.2 - 0.4. II.The eBCS clusters and analysis of the complete sample

Aims. We present the results of the GMRT cluster radio halo survey. The main purposes of our observational project are to measure which fraction of massive galaxy clusters in the redshift range z=0.2–0.4 hosts a radio halo, and to constrain the expectations of the particle re–acceleration model for the origin of the non–thermal radio emission. Methods. We selected a complete sample of 50 clusters in the X–ray band from the REFLEX (27) and the eBCS (23) catalogues. In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for all clusters still lacking high sensitivity radio information, i.e. 16 eBCS and 7 REFLEX clusters, thus completing the radio information for the whole sample. The typical sensitivity in our images is in the range 1� � 35 100 µJy b −1 . Results. We found a radio halo in A697, a diffuse peripheral source of unclear nature in A781, a core–halo source in Z7160, a candidate radio halo in A1682 and “suspect” central emission in Z2661. Including the literature information, a total of 10 clusters in the sample host a radio halo. A very important result of our work is that 25 out of the 34 clusters observed with the GMRT do not host extended central emission at the sensitivity level of our observations, and for 20 of them firm upper limits to the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey shows that radio halos are not common, and our findings on the fraction of giant radio halos in massive clusters are consistent with the statistical expectations based on the re–acceleration model. Our results favour primary to secondary electron models.

COSMIC RAYS AND RADIO HALOS IN GALAXY CLUSTERS : NEW CONSTRAINTS FROM RADIO OBSERVATIONS

Clusters of galaxies are sites of acceleration of charged particles and sources of nonthermal radiation. We report on new constraints on the population of cosmic rays in the intracluster medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray-luminous galaxy clusters in the redshift interval 0.2-0.4. The bulk of the observed galaxy clusters does not show any hint of megaparsec-scale synchrotron radio emission at the cluster center (radio halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of radio halos. Our measurements allow us to also derive a limit to the content of cosmic-ray protons in the ICM. Assuming spectral indices of these protons ? = 2.1-2.4 and ?G level magnetic fields, as from rotation measures, these limits are 1 order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.

Shock acceleration as origin of the radio relic in A 521

Aims. We present new high sensitivity observations of the radio relic in A521 carried out with 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. To this aim we used the new GMRT observations and other proprietary as well as archival data. We also searched 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 the 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 further supported by our finding of an X–ray surface brightness edge coincident with the outer border of the radio relic. This edge is likely a shock front.

Extended emission around GPS radio sources

Extended radio emission detected around a sample of GHz Peaked Spectrum (GPS) radio sources is discussed. Evidence for extended emission which is related to the GPS source is found in 6 objects out of 33. Three objects are associated with quasars with core-jet pc-scale morphology, and three are identified with galaxies with symmetric (CSO) radio morphology. We conclude that the core-jet GPS quasars are likely to be beamed objects with a continuous supply of energy from the core to the kpc scale. It is also possible that low surface brightness extended rad io emission is present in other GPS quasars but the emission is below our detection limit due to the high redshifts of the objects. On the other hand, the CSO/galaxies with extended large scale emission may be rejuvenated sources where the extended emission is the relic of previous activity. In general, the prese nce of large scale emission associated with GPS galaxies is uncommon, suggesting that in the context of the recurrent activity model, the time scale between subsequent bursts is in general longer than the radiative lifetime of the radio emission from the e arlier activity (� 10 8 yrs).

A2255: The first detection of filamentary polarized emission in a radio halo

A deep radio observation of the A2255 cluster of galaxies has been carried out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the excellent (u,v) coverage and sensitivity achieved by our observation, the low brightness diffuse extended sources in the cluster (radio halo and relic) have been imaged with unprecedented resolution and dynamic range. We find that the radio halo has filamentary structures that are strongly polarized. The fractional linear polarization reaches levels of {approx} 20-40% and the magnetic fields appear ordered on scales of {approx}400 kpc. This is the first successful attempt to detect polarized emission from a radio halo and provides strong evidence that in this cluster the magnetic field is ordered on large scales.

Multi{frequency VLA observations of a new sample of CSS/GPS radio sources

In this paper we present a new sample of 87 Compact Steep Spectrum radio sources (CSS) with flux density0.8 Jy at 0.4 GHz. This sample has been selected from the B3-VLA sample with the aid of new VLA observations at 4.9 and 8.5 GHz which allowed to clean an earlier selection based on VLA 1.5 GHz data. Redshifts, either spectroscopic or photometric, are known for 62% of the sources. About 75% of the sources are resolved or slightly resolved in the new observations. The range of measured linear sizes is from 20 h 1 kpc (selection upper limit) down to0.5 h 1 kpc, which corresponds to our resolution limit. The Largest Linear Size (LLS) distribution is well represented by the power law dN=d(LLS) / LLS 0:6 , consistent with earlier results. The majority of the radio sources smaller than 0.5 h 1 kpc shows a marked spectral flattening at low frequencies. The four frequency spectra computed in the range 0.4{8.5 GHz display a signicant steepening at high frequencies in the largest majority of cases. A considerable fraction of source components are polarized, with median values of their fractional polarization6% and4% at 8.5 and 4.9 GHz respectively.