  Radio minihalos are diffuse synchrotron sources of unknown origin found in
the cool cores of some galaxy clusters. We use GMRT and VLA data to expand the
sample of minihalos by reporting three new minihalo detections (A 2667, A 907
and PSZ1 G139.61+24.20) and confirming minihalos in five clusters (MACS
J0159.8-0849, MACS J0329.6-0211, RXC J2129.6+0005, AS 780 and A 3444). With
these new detections and confirmations, the sample now stands at 23, the
largest sample to date. For consistency, we also reanalyze archival VLA 1.4 GHz
observations of 7 known minihalos. We revisit possible correlations between the
non-thermal emission and the thermal properties of their cluster hosts.
Consistently with our earlier findings from a smaller sample, we find no strong
relation between the minihalo radio luminosity and the total cluster mass.
Instead, we find a strong positive correlation between the minihalo radio power
and X-ray bolometric luminosity of the cool core (r<70 kpc). This supplements
our earlier result that most if not all cool cores in massive clusters contain
a minihalo. Comparison of radio and Chandra X-ray images indicates that the
minihalo emission is typically confined by concentric sloshing cold fronts in
the cores of most of our clusters, supporting the hypothesis that minihalos
arise from electron reacceleration by turbulence caused by core gas sloshing.
Taken together, our findings suggest that the origin of minihalos should be
closely related to the properties of thermal plasma in cluster cool cores.
