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Emission from the photosphere in gamma-ray burst (GRB) jets can besubstantially affected by subphotospheric energy dissipation, which istypically caused by radiation-mediated shocks (RMSs). We study theobservational characteristics of such emission, in particular the spectralsignatures. Relevant shock initial conditions are estimated using a simpleinternal collision framework, which then serve as inputs for an RMS model thatgenerates synthetic photospheric spectra. Within this framework, we find thatif the free fireball acceleration starts at $r_0 \sim 10^{10}~$cm, in agreementwith hydrodynamical simulations, then the typical spectrum consists of a broad,soft power-law segment with a cutoff at high energies and a hardening inX-rays. The synthetic spectra are generally well fitted with a standard cutoffpower-law (CPL) function, as the hardening in X-rays is commonly outside theobservable energy range of current detectors. The CPL-fits yield values for thelow-energy index, $\alpha$, and the peak energy, $E_{\rm peak}$, that arecentered around $\sim -0.8$ and $\sim 220~$keV, respectively, similar totypical observed values. We also identify a non-negligible parameter region forwhat we call ``optically shallow shocks'': shocks that do not accumulate enoughscatterings to reach a steady-state spectrum before decoupling and therebyproduce more complex spectra. These occur for optical depths $\tau \lesssim 55\, u_u^{-2}$, where $u_u = \gamma_u\beta_u$ is the dimensionless specificmomentum of the upstream as measured in the shock rest frame.

Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission