  In a recent paper we reported on the discovery of a radio halo with very
steep spectrum in the merging galaxy cluster Abell 521 through observations
with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep
spectrum of the halo is inconsistent with a secondary origin of the
relativistic electrons and supports a turbulent acceleration scenario. At that
time, due to the steep spectrum, the available observations at 1.4 GHz
(archival NRAO - Very Large Array - VLA CnB-configuration data) were not
adequate to accurately determine the flux density associated with the radio
halo. In this paper we report the detection at 1.4 GHz of the radio halo in
Abell 521 using deep VLA observations in the D-configuration. We use these new
data to confirm the steep-spectrum of the object. We consider Abell 521 the
prototype of a population of very-steep spectrum halos. This population is
predicted assuming that turbulence plays an important role in the acceleration
of relativistic particles in galaxy clusters, and we expect it will be unveiled
by future surveys at low frequencies with the LOFAR and LWA radio telescopes.
