No abstract

Magnetic fields in galaxy clusters have been measured using a variety of techniques, including: studies of synchrotron relic and halo radio sources within clusters, studies of inverse Compton X-ray emission from clusters, surveys of Faraday rotation measures of polarized radio sources both within and behind clusters, and studies of Cluster Cold Fronts in X-ray images. These measurements imply that most cluster atmospheres are substantially magnetized, with typical field strengths of order 1 microGauss with high areal filling factors out to Mpc radii. There is likely, however, to be considerable variation in field strengths and topologies both within and between clusters, especially when comparing dynamically relaxed clusters to those that have recently undergone a merger. In some locations, such as the cores of cooling flow clusters, the magnetic fields reach levels of 10 - 40 microGauss, and may be dynamically important. In all clusters the magnetic fields have a significant effect on energy transport in the intracluster medium. We also review current theories on the origin of cluster magnetic fields.Comment: 36 pages, 9 figures, for Annual Reviews of Astronomy & Astrophysics 200

We report subarcsec-resolution X-ray imaging of the core of the Perseus cluster around the galaxy NGC 1275 with the Chandra X-ray Observatory. The ROSAT-discovered holes associated with the radio lobes have X-ray bright rims which are cooler than the surrounding gas and not due to shocks. The holes themselves may contain some hotter gas. We map strong photoelectric absorption across the Northern lobe and rim due to a small infalling irregular galaxy, known as the high velocity system. Two outer holes, one of which was previously known, are identified with recently found spurs of low-frequency radio emission. The spiral appearance of the X-ray cooler gas and the outer optical parts of NGC 1275 may be due to angular momentum in the cooling flow.Comment: 5 pages, 7 figures (6 colour), accepted by MNRAS, high resolution version at http://www-xray.ast.cam.ac.uk/papers/per_chandra.ps.g

Important insight into the nature of ␥-ray bursts (GRBs) has been gained in recent months mainly due to the immediate, precise localization of the bursts 1-3 and the discovery of relatively longlived X-ray afterglows 1,4 by the satellite BeppoSAX 5 . These advances have enabled deep searches which have led to the discovery of optical transients 6,7 coincident with fading X-ray sources. Optical spectroscopy of the latest burst (GRB970508; ref. 8) has clearly demonstrated that it lies at a cosmological distance, thus resolving a long-standing controversy about the distance scale to GRBs. Here we report a variable radio source within the error box of GRB970508 and coincident with the optical transient. We suggest that this is the much-sought-after radio counterpart of a GRB. If the observed fluctuations in the radio emission ('twinkling') are a result of a strong scattering by the irregularities in the ionized Galactic interstellar gas, then the source must have an angular size of about 3 microarcseconds in the first few weeks.

We present the first catalog of active galactic nuclei (AGN) detected by the LAT, corresponding to 11 months of data collected in scientific operation mode. The First LAT AGN Catalog (1LAC) includes 671 γ-ray sources located at high Galactic latitudes (|b| > 10 •) that are detected with a test statistic (T S) greater than 25 and associated statistically with AGNs. Some LAT sources are associated with multiple AGNs, and consequently, the catalog includes 709 AGNs, comprising 300 BL Lacertae objects (BL Lacs), 296 flat-spectrum radio quasars (FSRQs), 41 AGNs of other types, and 72 AGNs of unknown type. We also classify the blazars based on their spectral energy distributions (SEDs) as archival radio, optical, and X-ray data permit. In addition to the formal 1LAC sample, we provide AGN associations for 51 low-latitude LAT sources and AGN "affiliations" (unquantified counterpart candidates) for 104 highlatitude LAT sources without AGN associations. The overlap of the 1LAC with existing γ-ray AGN catalogs (LBAS, EGRET, AGILE, Swift, INTEGRAL, TeVCat) is briefly discussed. Various properties-such as γ-ray fluxes and photon power law spectral indices, redshifts, γ-ray luminosities, variability, and archival radio luminosities-and their correlations are presented and discussed for the different blazar classes. We compare the 1LAC results with predictions regarding the γ-ray AGN populations, and we comment on the power of the sample to address the question of the blazar sequence.