  We present the experimental observation of the fluctuation-dissipation
theorem (FDT) violation in an assembly of interacting magnetic nanoparticles in
the low temperature superspin glass phase. The magnetic noise is measured with
a two-dimension electron gas Hall probe and compared to the out of phase ac
susceptibility of the same ferrofluid. For "intermediate" aging times of the
order of 1 h, the ratio of the effective temperature $T_{\rm eff}$ to the bath
temperature T grows from 1 to 6.5 when T is lowered from $T_g$ to 0.3 $T_g$,
regardless of the noise frequency. These values are comparable to those
measured in an atomic spin glass as well as those calculated for a Heisenberg
spin glass.
