  The energy output in the gamma-ray burst (GRB) prompt emission and afterglow
phase is expected to photoionize the surrounding medium out to large radii.
Cooling of this gas produces line emission, particularly strong in the optical,
whose variability is a strong diagnostics of the gas density and geometry in
the close environment of the burst. We present the results of a spectral time
series analysis of the host galaxy of GRB 990712 observed up to ~6 years after
the burst. We analyze the emission line fluxes together with those of the
previous observations of the same GRB, in search for photoionization
signatures. We find that the emission line fluxes show no variation within the
uncertainities up to 6 years after the burst, and we use the measured line
intensities to set a limit on the density of the gas within a few parsecs of
the burst location. This is the first time that emission from cooling GRB
remnants is probed on years time scales.
