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We explore the observational appearance of the photosphere of anultrarelativistic flow with internal dissipation of energy as predicted by themagnetic reconnection model. Previous study of the radiative transfer in thephotospheric region has shown that gradual dissipation of energy results in ahot photosphere. There, inverse Compton scattering of the thermal radiationadvected with the flow leads to powerful photospheric emission with spectralproperties close to those of the observed prompt GRB emission. Here, we buildon that study by calculating the spectra for a large range of thecharacteristics of the flow. An accurate fitting formula is given that providesthe photospheric spectral energy distribution in the ~10 keV to ~10 MeV energyrange (in the central engine frame) as a function of the basic physicalparameters of the flow. It facilitates the direct comparison of the modelpredictions with observations, including the variability properties of thelightcurves. We verify that the model naturally accounts for the observedclustering in peak energies of the E*f(E) spectrum. In this model, the Amatirelation indicates a tendency for the most luminous bursts to have more energyper baryon. If this tendency also holds for individual GRB pulses, the modelpredicts the observed narrowing of the width of pulses with increasing photonenergy.Comment: 10 pages, 6 figures, minor changes, A&A, in pres

Spectral and timing properties of a dissipative $\vec\gamma$-ray burst photosphere