A long decay of X-ray flux and spectral evolution in the supersoft active galactic nucleus GSN 069.

GSN 069 is an optically identified very low-mass AGN which shows supersoft X-ray emission. The source is known to exhibit huge X-ray outburst, with flux increased by more than a factor of ~240 compared to the quiescence state. We report its long-term evolution in the X-ray flux and spectral variations over a time-scale of ~decade, using both new and archival X-ray observations from the XMM and Swift. The new Swift observations detected the source in its lowest level of X-ray activity since outburst, a factor of ~4 lower in the 0.2-2 keV flux than that obtained with the XMM observations nearly 8 years ago. Combining with the historical X-ray measurements, we find that the X-ray flux is decreasing slowly. There seemed to be spectral softening associated with the drop of X-ray flux. In addition, we find evidence for the presence of a weak, variable hard X-ray component, in addition to the dominant thermal blackbody emission reported before. The long decay of X-ray flux and spectral evolution, as well as the supersoft X-ray spectra, suggest that the source could be a tidal disruption event, though a highly variable AGN cannot be fully ruled out. Further continued X-ray monitoring would be required to test the TDE interpretation, through better determining the flux evolution in the decay phase.