Repeating tidal disruptions in GSN 069: Long-term evolution and constraints on quasi-periodic eruptions' models

GSN 069 is the first galactic nucleus where quasi-periodic eruptions (QPEs) have been identified. These are high-amplitude, soft X-ray bursts recurring every ~9 hr, lasting ~1 and during which count rate increases by up to two orders of magnitude with respect an otherwise stable quiescent level. The spectral properties long-term evolution in few years consistent a long-lived tidal disruption event (TDE). Here we derive QPEs over past 12 yr studying timing data from 11 XMM-Newton, one Chandra, 34 Swift observations on timescales ranging minutes years. transient phenomenon lifetime > 1.05 yr. QPE intensity recurrence time oscillate allow for alternating strong-weak long-short times be defined. In QPEs, level exhibits oscillation period equal average separation between consecutive QPEs. thermal emission very compact region that heats quickly subsequently cools down via while expanding factor ~3 radius. characterised repeating TDEs apart. We detect precursor flare prior second TDE may associated circularisation phase disc formation. A similar tentatively detected just before TDE. Future promise origin relation this will constrained, consequences other sources [abridged]