SIMBAD references

We present a detailed study of the giant radio halo in the galaxy cluster Abell 697 to constrain its origin and connection with the cluster dynamics. We performed high sensitivity GMRT observations at 325MHz, which showed that the radio halo is much brighter and larger at this frequency than in previous 610MHz observations. To derive the integrated spectrum in the frequency range 325MHz-1.4GHz, we reanalysed archival VLA data at 1.4GHz and used proprietary GMRT data at 610MHz. Our multifrequency analysis shows that the total radio spectrum of the giant radio halo in A697 is very steep, with α_325MHz^1.4GHz≃1.7-1.8. Owing to energy arguments, a hadronic origin of the halo is disfavoured by this steep spectrum. Very steep spectrum halos in merging clusters are predicted in the case that the emitting electrons are accelerated by turbulence. Observations with upcoming low frequency arrays will be able to test these expectations.