Faraday Rotation Observations of Magnetic Fields in galaxy Clusters

The presence of magnetic fields in the intracluster medium in clusters of galaxies has been revealed through several different observational techniques. These fields may be dynamically important in clusters as they will provide additional pressure support to the intracluster medium as well as inhibit transport mechanisms such as thermal conduction. Here, we review the current observational state of Faraday rotation measure studies of the cluster fields. The fields are generally found to be a few to 10 microG in non-cooling core clusters and ordered on scales of 10-20 kpc. Studies of sources at large impact parameters show that the magnetic fields extend from cluster cores to radii of at least 500 kpc. In central regions of cooling core systems the field strengths are often somewhat higher (10-40 microG) and appear to be ordered on smaller scales of a few to 10 kpc. We also review some of the recent work on interpreting Faraday rotation measure observations through theory and numerical simulations. These techniques allow us to build up a much more detailed view of the strength and topology of the fields.Comment: 6 pages, including 2 colour figures. To appear in a dedicated issue of the Journal of the Korean Astronomical Society (JKAS). Proceedings of the "International conference on Cosmic Rays and Magnetic Fields in Large Scale Structure", Busan, Korea, 200

Occurrence of radio minihalos in a mass-limited sample of galaxy clusters

We investigate the occurrence of radio minihalos --- diffuse radio sources of unknown origin observed in the cores of some galaxy clusters --- in a statistical sample of 58 clusters drawn from the Planck Sunyaev-Zel'dovich cluster catalog using a mass cut ($M_{500}>6\times 10^{14} M_{\odot}$). We supplement our statistical sample with a similarly-sized non-statistical sample mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray and radio data, which includes lower-mass clusters. Where necessary (for 9 clusters), we reanalyzed the Very Large Array archival radio data to determine if a mihinalo is present. Our total sample includes all 28 currently known and recently discovered radio minihalos, including 6 candidates. We classify clusters as cool-core or non-cool core according to the value of the specific entropy floor in the cluster center, rederived or newly derived from the Chandra X-ray density and temperature profiles where necessary (for 27 clusters). Contrary to the common wisdom that minihalos are rare, we find that almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of massive clusters exhibit minihalos. The supplementary sample shows that the occurrence of minihalos may be lower in lower-mass cool-core clusters. No minihalos are found in non-cool-cores or "warm cores". These findings will help test theories of the origin of minihalos and provide information on the physical processes and energetics of the cluster cores.Comment: 34 pages, accepted for publication in ApJ. Added a section "Definition of a minihalo" and an appendix "Radio size and average surface brigthtness of minihalos and halos

HESS J1943+213: a non-classical high-frequency-peaked BL Lac object

HESS J1943+213 is an unidentified TeV source that is likely a high-frequency-peaked BL Lac (HBL) object but also compatible with a pulsar wind nebula (PWN) nature. Each of these enormously different astronomical interpretations is supported by some of the observed unusual characteristics. In order to finally classify and understand this object we took a three-pronged approach, through time-domain, high angular resolution, and multi-frequency radio studies. First, our deep time-domain observations with the Arecibo telescope failed to uncover the putative pulsar powering the proposed PWN. We conclude with ~70% certainty that HESS J1943+213 does not host a pulsar. Second, long-baseline interferometry of the source with e-MERLIN at 1.5- and 5- GHz, shows only a core, a point source at ~ 1 - 100 milli-arcsecond resolution. Its 2013 flux density is about one-third lower than detected in 2011 observations with similar resolution. This radio variability of the core strengthens the HBL object hypothesis. More evidence against the PWN scenario comes, third, from the radio spectrum we compiled. The extended structure follows a power-law behavior with spectral index alpha = -0.54 +- 0.04 while the core component is flat spectrum (alpha = -0.03 +- 0.03). In contrast, the radio synchrotron emission of PWNe predicts a single power-law distribution. Overall we rule out the PWN hypothesis and conclude the source is a BL Lac object. The consistently high fraction (70%) of the flux density from the extended structure then leads us to conclude that HESS J1943+213 must be a non-classical HBL object.Comment: 8 pages, 4 figures, ApJ submitte

The Curious Case of Abell 2256

Abell 2256 is a rich, nearby (z=0.0594) galaxy cluster that has significant evidence of merger activity. We present new radio and X-ray observations of this system. The low-frequency radio images trace the diffuse synchrotron emission of the Mpc-scale radio halo and relics as well as a number of recently discovered, more compact, steep spectrum sources. The spectral index across the relics steepens from the north-west toward the south-east. Analysis of the spectral index gradients between low and and high-frequencies shows spectral differences away from the north-west relic edge such that the low-frequency index is significantly flatter than the high frequency spectral index near the cluster core. This trend would be consistent with an outgoing merger shock as the origin of the relic emission. New X-ray data from XMM-Newton reveal interesting structures in the intracluster medium pressure, entropy and temperature maps. The pressure maps show an overall low pressure core co-incident with the radio halo emission, while the temperature maps reveal multiple regions of cool emission within the central regions of Abell 2256. The two cold fronts in Abell 2256 both appear to have motion in similar directions.Comment: 4 pages, 3 figures, proceedings of contributed talk at "Non-Thermal Phenomena in Colliding Galaxy Clusters", held in Nice, 15-18 November 2010. To be published in Mem. S.A.I

New Detections of Radio Minihalos in Cool Cores of Galaxy Clusters

Cool cores of some galaxy clusters exhibit faint radio minihalos. Their origin is unclear, and their study has been limited by their small number. We undertook a systematic search for minihalos in a large sample of X-ray luminous clusters with high-quality radio data. In this article, we report four new minihalos (A 478, ZwCl 3146,RXJ 1532.9+3021, and A 2204) and five candidates found in the reanalyzed archival Very Large Array observations.The radio luminosities of our minihalos and candidates are in the range of 102325 W Hz1 at 1.4 GHz, which is consistent with these types of radio sources. Their sizes (40160 kpc in radius) are somewhat smaller than those of previously known minihalos. We combine our new detections with previously known minihalos, obtaining a total sample of 21 objects, and briefly compare the cluster radio properties to the average X-ray temperature and the total masses estimated from Planck.We find that nearly all clusters hosting minihalos are hot and massive. Beyond that, there is no clear correlation between the minihalo radio power and cluster temperature or mass (in contrast with the giant radio halos found in cluster mergers, whose radio luminosity correlates with the cluster mass). Chandra X-ray images indicate gas sloshing in the cool cores of most of our clusters, with minihalos contained within the sloshing regions in many of them. This supports the hypothesis that radio-emitting electrons are reaccelerated by sloshing. Advection of relativistic electrons by the sloshing gas may also play a role in the formation of the less extended minihalos