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In the past two decades, high amplitude electromagnetic outbursts have beendetected from dormant galaxies and often attributed to the tidal disruption ofa star by the central black hole. X-ray emission from the Seyfert 2 galaxy GSN069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010July and implies an X-ray brightening of more than a factor of 240 over ROSATobservations performed 16 years earlier. The emission has smoothly decayed overtime since 2010, possibly indicating a long-lived tidal disruption event. TheX-ray spectrum is ultra-soft and can be described by accretion disc emissionwith luminosity proportional to the fourth power of the disc temperature duringlong-term evolution. Here we report observations of X-ray quasi-periodiceruptions from the nucleus of GSN 069 over the course of 54 days, 2018 Decemberonwards. During these eruptions, the X-ray count rate increases by up to twoorders of magnitude with event duration of just over 1 hour and recurrence timeof about 9 hours. These eruptions are associated with fast spectral transitionsbetween a cold and a warm phase in the accretion flow around a low-mass blackhole (of approximately 4x10$^5$ solar masses) with peak X-ray luminosity of ~5x10$^{42}$ ergs per second. The warm phase has a temperature of about 120 eV,reminiscent of the typical soft X-ray excess, an almost universal thermal-likefeature in the X-ray spectra of luminous active nuclei. If the observedproperties are not unique to GSN 069, and assuming standard scaling oftimescales with black hole mass and accretion properties, typical activegalactic nuclei with more massive black holes can be expected to exhibithigh-amplitude optical to X-ray variability on timescales as short as months oryears.

Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus