  The fluctuation-dissipation relation (FDR) links thermal fluctuations and
dissipation at thermal equilibrium through temperature. Extending it beyond
equilibrium conditions in pursuit of broadening thermodynamics is often
feasible, albeit with system-dependent specific conditions. We demonstrate
experimentally that a generalized FDR holds for a harmonically trapped tracer
colliding with self-propelled walkers. The generalized FDR remains valid across
a large spectrum of active fluctuation frequencies, extending from underdamped
to critically damped dynamics, which we attribute to a single primary channel
for energy input and dissipation in our system.
