We report the discovery of a giant radio halo in a new , hot , X-ray luminous galaxy cluster recently found by Planck , PLCKG171.9-40.7 .
The radio halo was found using Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz , and in the 1.4 GHz data from a NRAO Very Large Array Sky Survey pointing that we have reanalyzed .
The diffuse radio emission is coincident with the cluster X-ray emission , and has an extent of \sim 1 Mpc and a radio power of \sim 5 \times 10 ^ { 24 } W Hz ^ { -1 } at 1.4 GHz .
Its integrated radio spectrum has a slope of \alpha \approx 1.8 between 235 MHz and 1.4 GHz , steeper than that of a typical giant halo .
The analysis of the archival XMM-Newton X–ray data shows that the cluster is hot ( \sim 10 keV ) and disturbed , consistent with X-ray selected clusters hosting radio halos .
This is the first giant radio halo discovered in one of the new clusters found by Planck .