Writing in Nature, Miniutti et al.1 report a stunning phenomenon unlike anything observed before in a galaxy: almost-periodic eruptions that increase the X-ray emission from the host galaxy by a factor of up to 100, and which last only about an hour. Even more remarkably, these galactic hiccups recur on timescales of hours — raising questions about the mechanism that could produce such rapidly repeating events.
The galaxy in question is called GSN 069, and has a past record of surprising behaviour. It was initially classified as a field galaxy2 (a galaxy that does not belong to a larger group or cluster of galaxies) in 1989, and at first was not observed to produce any unusual emission. But then, in 2010, its X-ray emission brightened3 by a factor of at least 240.
This event identified GSN 069 as an ‘active’ galaxy, in which the efficient inflow of plasma onto a central supermassive black hole causes enormous amounts of energy to be dissipated as the material approaches the black hole’s event horizon (the boundary beyond which nothing can escape the black hole’s gravitational field). Some of the dissipated energy leads to the strong emission of electromagnetic radiation. Because the inflow is turbulent, the emission from the central regions of active galaxies is variable. The variability is usually stochastic, with continuous irregular increases and decreases in brightness at all timescales — which, in the case of GSN 069, were superimposed on an overall dimming trend1.
However, in late 2018, Miniutti et al. observed variations in the emission from GSN 069 that were nothing like the pattern described above. They detected two X-ray eruptions by chance on 24 December 2018, during a half-day observation period using the XMM-Newton space telescope. The X-ray emission unexpectedly brightened by a factor of more than 30 over the course of about 30 minutes, returned to its previous state in about another 30 minutes, and then erupted again after a further 8 hours. The authors recorded five more eruptions during 16 and 17 January 2019, using the same instrument (Fig. 1), and three more during 14 and 15 February, using the space-based Chandra X-ray Observatory. These eruptions were extremely regular, separated in time by about 9 hours, and each lasted just over an hour.
What could the explanation be? The radiation spectra produced by many active galaxies have been interpreted as coming from a complex flow of material onto a black hole4,5 — that is, a flow in which most of the material comes from a relatively cold (about 100,000 kelvin) disk of matter, but some of which comes from a corona of less-dense material that surrounds the disk and is about ten times warmer. In the case of the eruptions observed by Miniutti and co-workers, most of the energy must be released from a region close to a black hole that is compact enough to undergo coordinated changes of state.
Some hints about the eruption source in GSN 069 come from details of Miniutti and co-workers’ analysis of how the amplitude of the outbursts depends on the range of energies of the studied photons. The biggest variations in emission (up to 100-fold increases of amplitude) were observed at frequencies that correspond to the highest-energy photons, whereas little variation was observed for low-energy frequencies. From their spectral analysis, the authors conclude that the quasi-periodic eruptions of GSN 069 are probably associated with the transient formation and disappearance of a warm corona around a disk (Fig. 1).
In most active galaxies, however, such a corona would not form transiently — it would either be permanent or would not form at all5. In rare cases, known as changing-look active galaxies6, coronas have disappeared, but the disappearance process was not followed over time7. The physics that underlies corona formation and eruptions is not well established, but might involve overheating associated with the presence of strong radiation in the disk.
The quasi-periodic eruptions of GSN 069 also bear some similarity to the ‘heartbeat’ states8 that occasionally occur in micro-quasars — binary systems consisting of a Sun-like star and a black hole about ten times the mass of our Sun. However, the duration and recurrence of heartbeat states are on timescales of seconds.
Could the regular events that underpin the eruptions in GSN 069 occur in other active galaxies? Perhaps, but they might be more difficult to observe. The black hole at the centre of GSN 069 has a relatively small mass (about 400,000 times that of our Sun) whereas bright active galaxies can contain central black holes with a mass greater than 1 billion solar masses9. This means that the timescales of the eruptions in GSN 069 might be relatively short, which makes it possible for them to be seen by humans.
It should be noted that the periodic eruptions of GSN 069 only began once the initial X-ray outburst recorded in 2010 had dimmed to moderate levels — none were seen immediately after the outburst, or in observations made in 2014. Miniutti et al. report that the amplitude of the eruptions was decreasing over time, which means that the eruptions probably ended around late June this year, when the source finally became quite dim.
The mechanism that drives the quasi-periodic eruptions of GSN 069 must now be determined, and an explanation found for why this previously dormant galaxy rapidly became active. Further information about this source is likely to be reported in the future. More broadly, Miniutti and colleagues’ observations might provide clues about the processes that underpin similar eruptions that occur much more slowly in other active galaxies. Astronomers have occasionally caught sight of such events, such as the dimming and brightening of changing-look galaxies that occurs on a timescale of years10,11 — which is still very sudden, in the context of galactic processes. More changing-look galaxies are being sought12, and their observation might add to our knowledge of these fascinating regular eruptions.
Nature 573, 354-355 (2019)