153 MHz GMRT follow-up of steep-spectrum diffuse emission in galaxy
  clusters
  In this paper we present new high sensitivity 153 MHz Giant Meterwave Radio
Telescope follow-up observations of the diffuse steep spectrum cluster radio
sources in the galaxy clusters Abell 521, Abell 697, Abell 1682. Abell 521
hosts a relic, and together with Abell 697 it also hosts a giant very steep
spectrum radio halo. Abell 1682 is a more complex system with candidate steep
spectrum diffuse emission. We imaged the diffuse radio emission in these
clusters at 153 MHz, and provided flux density measurements of all the sources
at this frequency. Our new flux density measurements, coupled with the existing
data at higher frequencies, allow us to study the total spectrum of the halos
and relic over at least one order of magnitude in frequency. Our images confirm
the presence of a very steep "diffuse component" in Abell 1682. We found that
the spectrum of the relic in Abell 521 can be fitted by a single power-law with
$\alpha=1.45\pm0.02$ from 153 MHz to 5 GHz. Moreover, we confirm that the halos
in Abell 521 and Abell 697 have a very steep spectrum, with $\alpha=1.8-1.9$
and $\alpha=1.52\pm0.05$ respectively. Even with the inclusion of the 153 MHz
flux density information it is impossible to discriminate between power-law and
curved spectra, as derived from homogeneous turbulent re-acceleration. The
latter are favored on the basis of simple energetic arguments, and we expect
that LOFAR will finally unveil the shape of the spectra of radio halos below
100 MHz, thus providing clues on their origin.
