  Lossy bosonic channels play an important role in a number of quantum
information tasks, since they well approximate thermal dissipation in an
experiment. Here, we characterize their metrological power in the idler-free
and entanglement-assisted cases, using respectively single- and two-mode
Gaussian states as probes. In the problem of estimating the lossy parameter, we
study the energy-constrained quantum Fisher information (QFI) for generic
temperature and lossy parameter regimes, showing qualitative behaviours of the
optimal probes. We show semi-analytically that the two-mode squeezed-vacuum
state optimizes the QFI for any value of the lossy parameter and temperature.
We discuss the optimization of the {\it total} QFI, where the number of probes
is allowed to vary by keeping the total energy-constrained. In this context, we
elucidate the role of the "shadow-effect" for reaching a quantum advantage. We
also consider a photon-number normalization for the environment, widely used in
the analysis of quantum illumination and quantum reading protocols. With this
normalization, we prove that the large bandwidth TMSV state is the optimal
probe for any parameter value. Here, the quantum advantage is of at most a
factor of $2$, and is reached in the bright environment case for {\it any}
lossy parameter values. Finally, we discuss the implications of our results for
quantum illumination and quantum reading applications.
