We report new observations of Abell 2256 with the Karl G. Jansky Very Large Array (VLA) at frequencies between 1 and 8 GHz. These observations take advantage of the 2:1 bandwidths available for a single observation to study the spectral index, polari
zation and Rotation Measure as well as using the associated higher sensitivity to image total intensity features down to ~0.5 resolution. We find the Large Relic, which dominates the cluster, is made up of a complex of filaments which show correlated distributions in intensity, spectral index, and fractional polarization. The Rotation Measure varies across the face of the Large Relic but is not well correlated with the other properties of the source. The shape of individual filaments suggests that the Large Relic is at least 25 kpc thick. We detect a low surface brightness arc connecting the Large Relic to the Halo and other radio structures suggesting a physical connection between these features. The center of the F-complex is dominated by a very steep-spectrum, polarized, ring-like structure, F2, without an obvious optical identification, but the entire F-complex has interesting morphological similarities to the radio structure of NGC1265. Source C, the Long Tail, is unresolved in width near the galaxy core and is </~100pc in diameter there. This morphology suggests either that C is a one-sided jet or that the bending of the tails takes place very near the core, consistent with the parent galaxy having undergone extreme stripping. Overall it seems that many of the unusual phenomena can be understood in the context of Abell 2256 being near the pericenter of a slightly off-axis merger between a cluster and a smaller group. Given the lack of evidence for a strong shock associated with the Large Relic, other models should be considered, such as reconnection between two large-scale magnetic domains.
Characterizing the ejecta in young supernova remnants is a requisite step towards a better understanding of stellar evolution. In Cassiopeia A the density and total mass remaining in the unshocked ejecta are important parameters for modeling its expl
osion and subsequent evolution. Low frequency (<100 MHz) radio observations of sufficient angular resolution offer a unique probe of unshocked ejecta revealed via free-free absorption against the synchrotron emitting shell. We have used the Very Large Array plus Pie Town Link extension to probe this cool, ionized absorber at 9 arcseconds and 18.5 arcseconds resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 electrons per cubic centimeters and a total mass of 0.39 Solar masses with uncertainties of a factor of about 2. This is a significant improvement over the 100 electrons per cubic centimeter upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our estimates are sensitive to a number of factors including temperature and geometry. However using reasonable values for each, our unshocked mass estimate agrees with predictions from dynamical models. We also consider the presence, or absence, of cold iron- and carbon-rich ejecta and how these affect our calculations. Finally we reconcile the intrinsic absorption from unshocked ejecta with the turnover in Cas As integrated spectrum documented decades ago at much lower frequencies. These and other recent observations below 100 MHz confirm that spatially resolved thermal absorption, when extended to lower frequencies and higher resolution, will offer a powerful new tool for low frequency astrophysics.
We present results on 12 X-ray bright clusters observed at 1.4 GHz with the Green Bank Telescope. After subtraction of point sources, we reach a median (best) 1-sigma noise level of 0.01 (0.006) microJy/sq. arcsec, and find a significant excess of di
ffuse, low surface brightness emission in 11 of 12 clusters. We present initial 1.4 GHz Very Large Array results on Abell 2319. We find: (a) four new detections tentatively classified as two halos (A2065, A2069) and two relics (A2067, A2073); (b) the first detection of the radio halo in A2061 at 1.4 GHz, making it a possible ultra-steep spectrum halo (alpha ~ 1.8); (c) a ~2 Mpc radio halo in the sloshing, minor-merger cluster A2142; (d) a >2x increase of the giant radio halo extent and luminosity in A2319; (e) a ~7x increase to the integrated radio flux and >4x increase to the observed extent of the peripheral, polarized radio relic in A1367 to ~600 kpc; (f) significant excess emission of ambiguous nature in three clusters. Our radio halo detections agree with the well-known X-ray/radio luminosity correlation, but are larger and fainter than expected. The volume averaged synchrotron emissivities are 1-2 orders of magnitude below the previous characteristic values. Some of the halo-like detections may represent previously unseen, very low surface brightness emission or blends of shock structures and sub-Mpc scale turbulent regions. Four of the five tentative halos contain one or more X-ray cold fronts, suggesting a possible connection between gas sloshing and particle acceleration on large scales. We see evidence for a possible inter-cluster filament between A2061 and A2067. For our faintest detections, we note the possibility of residual contamination from faint radio galaxies. We also quantify the sensitivity of the NVSS to extended emission as a function of angular size.[abridged]
We present a 3-dimensional analysis of the supernova remnant Cassiopeia A using high resolution spectra from the Spitzer Space Telescope. We observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that t
he reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km/s. We determine that the velocity width of the nucleosynthetic layers is approximately 1000 km/s over 4000 square arcsecond regions, although the velocity width of a layer along any individual line of sight is <250 km/s. Si and O, which come from different nucleosynthetic layers in the progenitor star, are observed to be coincident in velocity space in some directions, but segregated by up to approximately 500 km/s in other directions. We compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale, corrugated velocity structures that are likely caused during the supernova explosion itself, rather than hundreds of years later by dynamical instabilities at the remnants reverse shock.
