Constraining the environment of GRB 990712 through emission line fluxes

Aims. 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 that is particularly strong in the optical, whose variability is a strong diagnostics of the gas density and geometry in the close environment of the burst. Methods. 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 of photoionization signatures. Results. We find that the emission line fluxes show no variation within the uncertainties 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 time scales of years.