Discovery of Megaparsec-Scale, Low Surface Brightness Nonthermal
  Emission in Merging Galaxy Clusters using the Green Bank Telescope
  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 diffuse, 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]
