Probing the Off-State of Cluster Giant Radio Halos
  We derive the best characterization to date of the properties of radio
quiescent massive galaxy clusters through a statistical analysis of their
average synchrotron emissivity. We stacked 105 radio images of clusters from
the 843 MHz SUMSS survey, all with X-ray luminosities greater than 1.0E+44
erg/s and redshifts z < 0.2, after removing point-source contamination and
rescaling to a common physical size. Each stacked cluster individually shows no
significant large-scale diffuse radio emission at current sensitivity levels.
Stacking of sub-samples leads to the following results: (i) clusters with L_{X}
> 3.0E+44 erg/s show a 6-sigma detection of Mpc-scale diffuse emission with a
1.4 GHz luminosity of 2.4\pm0.4 x 1.0E+23 W/Hz. This is 1.5-2 times lower than
the upper limits for radio quiescent clusters from the GMRT Radio Halo Survey
(Venturi et al. 2008), and is the first independent confirmation of radio halo
bi-modality; (ii) clusters with low X-ray concentrations have a mean radio
luminosity (2.6\pm0.6 x 1.0E+23 W/Hz) that is at least twice that of high X-ray
concentration clusters, and (iii) both of these detections are likely close to
the low-level "off-state" of GRHs in most or all luminous X-ray clusters, and
not due to contributions from a much smaller subset of "on-state" GRHs
following the radio/X-ray luminosity correlation. Upcoming deep radio surveys
will conclusively distinguish between these two options. We briefly discuss
possible origins for the "off-state" emission and its implications for magnetic
fields in most or all luminous X-ray clusters.
