090416

We propose an XMM observation of the A671 cluster. It is the lowest mass clusterfor which a radio halo has been observed in the LOFAR survey covering nearly 30%of the Northern sky at the low frequency of 150 MHz. Radio halos in low massclusters are a test-bed for turbulent re-acceleration which is currently thefavoured scenario to explain their formation. They should be rare in mergingsystems and with a very steep radio spectrum. With a modest amount of XMMexposure time (25 ks) we will be able to obtain a rich characterization of thedynamical state and thermal structure of the cluster given its high fluxproviding a powerful test of radio-halo formation. [truncated! Please see actual data for full text]