Constraints on the narrow-line region of the X-ray quasi-periodic eruption source GSN 069

The origins of quasi-periodic eruptions (QPEs) are poorly understood, although most theoretical explanations invoke an accretion disc around a supermassive black hole. The gas and stellar environments in the galactic nuclei of these sources are also poorly constrained. In this paper, we present an analysis of archival Hubble Space Telescope (HST) images to study the narrow-line [OIII] emission in the QPE source GSN 069. We find strong evidence for a compact nuclear [OIII] emission region of size less than or similar to 35 pc, overlaid on top of extended [OIII] emission up to 1 kpc away from the nucleus. The age of the accretion system is estimated to be between 10 and 100 yr. The [OIII] luminosity of the compact region was measured to be (2.1 +/- 0.3) x 10(40), erg, s(-1). Based on cloudy simulations, we constrain that the [OIII] emitting gas has a hydrogen number density in the range 2 x 10(3) < n(H) less than or similar to 10(8), cm(-3) and volume filling factor f(V) < 4 x 10(-3). We suggest that the dense gas in the nuclear region of GSN 069 originates from molecular clouds (with total mass greater than or similar to 3 x 10(3)M(circle dot)), which are freshly ionized by the soft X-ray photons from the accretion disc. We predict possible evolution of the compact narrow-line region on emission-line diagnostic diagrams, and hence future HST or integral-field unit observations can be used to further pin down the age of this puzzling system.