AN OBJECTIVE SURVEY OF Mpc-SCALE RADIO EMISSION IN 0.03 < z < 0.3 BRIGHT X-RAY CLUSTERS

We have performed the first census of Mpc-scale radio emission to include control fields and quantifiable upper limits for bright X-ray clusters in the range 0.03 < z < 0.3. Through reprocessing radio images from the Westerbork Northern Sky Survey, we detect diffuse emission from ∼30% of the sample. We find a correlation similar to the well-studied relationship between radio halo and X-ray luminosities of the host cluster, but also find that large-scale radio galaxy (RG) detections follow a similar trend to that for radio halos. With this quantitative study, we thus confirm the upper envelope to the radio luminosities for X-ray-selected clusters, and the higher detection rates for diffuse radio emission (including halos, relics, and RGs) at X-ray luminosities above ∼1045 erg s-1. Beyond this, because many of the upper limits are only slightly below the detections, we can neither confirm nor refute the claims for a tight correlation between these radio halos and X-ray luminosities, i.e., whether the halo luminosity function of X-ray clusters is bimodal (having high/on and low/off states), or whether the radio luminosity can take on a wide range of values up to a maximum at each cluster X-ray luminosity. The resolution of this issue, also recently addressed in a Giant Metrewave Radio Telescope (GMRT) survey at 610 MHz, may provide a unique diagnostic for the timescales over which relativistic particles can be accelerated following cluster mergers. We discuss several important selection effects on radio versus X-ray luminosity correlations, including surface brightness thresholds and non-X-ray-selected diffuse radio sources. We also report several new detections of diffuse emission, including an Mpc-scale relic in RXJ1053.7+5450, a possible halo/relic combination in Abell 2061, a serendipitous diffuse X-ray source associated with poor clusters in the Abell 781 field, and confirmation of very weak diffuse emission patches outside Abell 2255.