Cosmic first: supermassive black hole caught “turning on” in real-time
- Out in the Universe, galaxies everywhere contain supermassive black holes at their centers, where they occasionally feed on matter, creating quasars and active galactic nuclei.
- Because these “active periods” typically last for tens-to-hundreds of millions of years, we’ve never seen an active galaxy “turn on” or “turn off” in real-time before.
- But in 2018, galaxy 1ES 1927+654 suddenly brightened, and in the aftermath, a new jet was launched from its supermassive black hole. Astronomers are now working out just what happened.
At the center of practically every galaxy lies a supermassive black hole. The one in our Milky Way is about four million times the mass of our Sun, but others can be billions or even tens of billions of solar masses. Most of the time, these black holes are quiet: simply gravitating as matter orbits around them. But every once in a while, some object or collection of objects passes too close by the black hole, and the black hole begins to feed on that matter: heating it, accelerating it, and launching jets of particles and radiation. Overall, what gets emitted is so energetic, it can be detected from all across the Universe, with active galaxies and quasars among the most distant objects ever discovered.
These periods of activity can endure for long periods of time: tens of millions, hundreds of millions, or even more than a billion years (in at least one case), or far longer than humans have been observing the Universe. Up until recently, we had never witnessed a supermassive black hole in an active galaxy either “turn on” or “turn off” before, but a 2018 event changed everything. After an unexpected optical “brightening” in distant galaxy 1ES 1927+654, a remarkable set of new features appeared: consistent with a supermassive black hole activating and launching a jet after a long period of inactivity. Here’s what we saw, and what we think it might mean.
The star of the show is an otherwise fairly normal galaxy: 1ES 1927+654. Up until 2018, it was known for being a Seyfert galaxy: the most common type of active galaxy that’s very bright in X-rays in its central region. With a relatively impressive central supermassive black hole — estimated to be around a billion solar masses — it’s not too surprising that there could be an accretion disk of material around it, which heats up and emits the energetic radiation that helps us understand its full nature. But starting in 2018, an event happened that appears to have led to something remarkable: a galaxy observed to change its look on human timescales.
Because this galaxy is so close on the scale of the Universe, just 236 million light-years away, we can take detailed measurements of its light that would be impossible with most active galaxies, which are largely found much farther away, on average. Looking at this galaxy with optical (visible light) telescopes, it suddenly brightened in 2018: not by a little bit, but by a factor of around 100 times, or about the difference in the brightness you’d observe between a thin, crescent Venus and a thin, crescent Moon from the surface of the Earth.
That brightness didn’t last, however. After reaching its maximum brightness, it faintened once again, declining back down to its original brightness. Based on how bright the galaxy got and how quickly it got there, many astronomers inferred that this was an example of a tidal disruption event: where a large, massive object passes close to a small, dense object, like a neutron star or a black hole. From the perspective of a black hole, one “side” of an object that passes close by it is going to be closer to the black hole than the opposite “side,” leading to a greater gravitational force on the near side than the far side. If those forces are great enough, they can tear the original object apart: a gas cloud, a star, a planet, or even a stellar remnant itself.
Still, that brightening-and-faintening that was seen only describes what occurred in the optical; there are also other wavelengths of light that we can observe. In the X-ray part of the spectrum, the brightness:
- suddenly plummeted to practically zero in 2018,
- where it remained for a few weeks,
- then jumped back up to its original brightness, where it remained until 2021,
- then slowly declined in 2021,
- varied slightly but irregularly until 2023,
- and then gradually brightened from 2023 through the end of 2024,
which brings us to the present day. Meanwhile, over in the radio part of the spectrum, it was a radio-quiet object: unsurprising, since only about 5-10% of Seyfert galaxies have strong radio emissions. But then, in 2023/2024, it suddenly “turned on,” blasting large quantities of radio waves across the Universe.
The radio data was not only particularly surprising, but represented a “smoking gun” for the launching of a jet from this active galactic nucleus. The VLBA — the Very Long Baseline Array of 10 radio telescopes — found two “blobs” of radio-loud emissions. Very faint and quiet in February of 2023, those blobs:
- turned on,
- brightened,
- and separated in space,
from the remainder of 2023 through all of 2024. Importantly, the brightening increased the radio flux remarkably, by about a factor of 60, and has remained at that elevated value ever since.
When scientists ask the question of, “What is happening to cause this?” they have a class of objects that show remarkably similar features: gigahertz-peaked radio sources. The nature of these gigahertz-peaked radio sources are strongly suspected to be young radio jets that are “only” 1000 years old or fewer. The fact that the VLBA data shows two well-resolved “blobs” moving apart from one another suggests that what we’re seeing is:
- a new outflow,
- at relativistic (close to the speed of light) speeds,
- that emits bipolar jets,
- being newly launched.
With more than a year of data, the two “blobs” are now seen to be separated by about half-a-light-year, indicating that these jets are rapidly moving at around 30% the speed of light.
The combination of the optical data, indicating a tidal disruption event, with the radio data, indicating the launch of a galactic jet from around the supermassive black hole, allows us to draw a partial outline of what likely occurred. A massive object — perhaps only a normal star, perhaps an evolved (giant) star, perhaps a stellar remnant like a white dwarf or even a neutron star — came into close proximity to the supermassive black hole at the center of galaxy 1ES 1927+654. The differing gravitational forces from the black hole tore the object (or at least the outer layers of the object) apart, leading to the brightening consistent with a tidal disruption event.
Then, that material from the destroyed object (or the destroyed portion of the object) was funneled toward the black hole’s event horizon, where it heated up, became ionized, and created strong magnetic fields. Those electromagnetic fields, now interacting with the charged (ionized) particles accreting around the black hole, don’t just accelerate and bend these charged particles, but funnel them into jets: which slam into the surrounding matter. These features, after a sufficient amount of time had elapsed, led to the emission of radio waves, whose features can be seen clearly from hundreds of millions (or more) of light-years away.
That picture sounds consistent and compelling, but there’s more to the story, thanks to the X-ray data that was collected as well. In X-ray light, this object was always “loud” (or bright) in some sense, as it was known to be an active (Seyfert) galaxy even before the 2018 tidal disruption event occurred. However, after that event, the X-ray flux plummeted: dropping almost all the way to zero.
In active galactic nuclei systems, X-rays are thought to arise when optical and ultraviolet photons, produced in the accretion disk around the black hole, strike a corona of heated electrons located perpendicular to the disk itself. For the X-ray flux to plummet and practically disappear, which it did for about an entire month, something must have happened to — at least, temporarily — destroy that active galaxy’s X-ray corona.
Credit: ESO/Aurore Simmonet (Sonoma State U.)
Tidal disruption events can cause the disrupted matter to be splayed out omnidirectionally, and could easily be the culprit behind the sudden drop in X-ray flux: by destroying the conditions for producing those X-rays. Although we don’t have a full picture of how this occurred, the leading idea is that the electron corona gets disrupted, and must reassemble itself (likely out of new electrons) before the X-rays return.
So what happened here? What got disrupted, what remains, and how did it create the features we see?
Thanks to X-ray observations that come jointly from NICER and XMM-Newton, we can get some extra clues from how those X-ray properties have changed over time. Initially, after the X-rays came back (after disappearing for a month in 2018), they saw a strange emission line: one that suggested that there was reflected emission off of material around the black hole, also known as a light-echo. After re-brightening, the X-ray emissions increased, becoming around 10 times brighter than the pre-outburst baseline. After declining (in 2021) and varying irregularly (in 2021 and 2022), the X-ray flux started gradually increasing again, rising in 2022 and 2023, before leveling off in 2024 and becoming near-constant.
But one of the X-ray features that shows up starting in 2022 and continues thereafter (including at present) was a surprise: oscillations in the X-ray flux on short timescales. In 2022, the flux would brighten and fainten by about 10%, roughly every 18 minutes. At the last time it was measured, in mid-to-late 2024, the flux still brightens and faintens periodically, and roughly by the same factor, but now the variation has accelerated to occur only every 7 minutes.
Although these observations might sound like they only deepen the mystery, they may point the way to a potential solution: there could be an object within the accretion disk, orbiting close to the black hole, in the aftermath of the tidal disruption event. That object can’t be a star or a brown dwarf, as it would swiftly be destroyed entirely by tidal disruption if it were orbiting this black hole in just 7 minutes; it couldn’t persist. It also can’t be a black hole, because conservation of angular momentum would cause that black hole orbit to decay, adding to and growing the mass of the supermassive black hole at 1ES 1927+654’s center.
Credit: NASA’s Goddard Space Flight Center
But the object could potentially be either a neutron star or a white dwarf; so long as mass can be transferred, or siphoned, from the orbiting compact object to the supermassive black hole, it can remain in a close, quasi-stable orbit for long periods of time. If there was initially:
- a massive star with a dense core that got tidally disrupted,
- a binary system of a star-neutron star or a star-white dwarf, where the star gets disrupted,
- a massive white dwarf whose outer layers got disrupted,
- a white dwarf binary where one (and only one) member got disrupted,
- or a dying giant star whose outer layers got disrupted, but whose contracting core still remains,
the optical tidal disruption, the radio turn-on in 2023, and the X-ray flux features, including these oscillations, can all be explained.
The amount of mass that was necessary to create these observed features isn’t very great: less than one solar mass of disrupted material might still be sufficient to explain everything. Although it was initially the study of white dwarf binaries that motivated the exploration of this scenario, there are many potential options that remain viable. It’s also plausible that there is no “surviving object” causing these periodic features in X-ray flux; it could be the case that complex plasma oscillations are causing the regular rises-and-dips in the X-ray emissions we observe. It will take more time, and continued observations, to see how the system continues to evolve, which will further constrain the possible options that can account for all we observe.
We have to keep in mind that explanations which conventionally “don’t make sense” are still in play, as the conditions just a few gravitational radii outside a supermassive black hole’s event horizon are some of the most extreme in the Universe. Conventionally, it’s very difficult to imagine how a large, massive object could leave something like a white dwarf or neutron star behind after only 4-5 years of evolution, but with tidal disruption and a large initial mass, these truly are extreme conditions, and perhaps an extreme scenario is just what the Universe cooked up to deliver this system to us!
Regardless, for the very first time, we’ve observed an active galaxy “turn on” and emit-and-launch a radio jet in real time: as we were watching it. It’s possible that what many are assuming is a white dwarf is indeed an incredibly extreme object: something with just a tenth of a solar mass, or about 80% lighter than the lowest-mass white dwarfs known. Something remarkable happened, and our first glimpse of a galaxy “activating” is our reward for looking. Now, the hard work continues, as astronomers toil to uncover exactly what happened, and how.
