The very steep spectrum radio halo in Abell 697
  In this paper we present a detailed study of the giant radio halo in the
galaxy cluster Abell 697, with the aim to constrain its origin and connection
with the cluster dynamics. We performed high sensitivity GMRT observations at
325 MHz, which showed that the radio halo is much brighter and larger at this
frequency, compared to previous 610 MHz observations. In order to derive the
integrated spectrum in the frequency range 325 MHz--1.4 GHz, we re--analysed
archival VLA data at 1.4 GHz and made use of proprietary GMRT data at 610 MHz.
{Our multifrequency analysis shows that the total radio spectrum of the giant
radio halo in A\,697 is very steep, with $\alpha_{\rm~325 MHz}^{\rm~1.4 GHz}
\approx 1.7-1.8$. %\pm0.1$. Due to energy arguments, a hadronic origin of the
halo is disfavoured by such steep spectrum. Very steep spectrum halos in
merging clusters are predicted in the case that the emitting electrons are
accelerated by turbulence, observations with the upcoming low frequency arrays
will be able to test these expectations.}
