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

ABSTRACT 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 O iii emission in the QPE source GSN 069. We find strong evidence for a compact nuclear O iii emission region of size ≲ 35 pc, overlaid on top of extended O iii 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 O iii luminosity of the compact region was measured to be (2. 1 0. 3) 10^40\, erg\, s^-1. Based on cloudy simulations, we constrain that the O iii emitting gas has a hydrogen number density in the range 2 10^3 n ₇ 10^8\, cm^-3 and volume filling factor fV 4 × 10−3. We suggest that the dense gas in the nuclear region of GSN 069 originates from molecular clouds (with total mass 3 10^3\, M), 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.