Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole
  galactic nucleus
  In the past two decades, high amplitude electromagnetic outbursts have been
detected from dormant galaxies and often attributed to the tidal disruption of
a star by the central black hole. X-ray emission from the Seyfert 2 galaxy GSN
069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010
July and implies an X-ray brightening of more than a factor of 240 over ROSAT
observations performed 16 years earlier. The emission has smoothly decayed over
time since 2010, possibly indicating a long-lived tidal disruption event. The
X-ray spectrum is ultra-soft and can be described by accretion disc emission
with luminosity proportional to the fourth power of the disc temperature during
long-term evolution. Here we report observations of X-ray quasi-periodic
eruptions from the nucleus of GSN 069 over the course of 54 days, 2018 December
onwards. During these eruptions, the X-ray count rate increases by up to two
orders of magnitude with event duration of just over 1 hour and recurrence time
of about 9 hours. These eruptions are associated with fast spectral transitions
between a cold and a warm phase in the accretion flow around a low-mass black
hole (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-like
feature in the X-ray spectra of luminous active nuclei. If the observed
properties are not unique to GSN 069, and assuming standard scaling of
timescales with black hole mass and accretion properties, typical active
galactic nuclei with more massive black holes can be expected to exhibit
high-amplitude optical to X-ray variability on timescales as short as months or
years.
