Astronomers See a Black Hole Wake Up from its Ancient Slumber

Four years ago, the supermassive black hole hidden in the heart of galaxy SDSS1335+0728 roared awake and announced its presence with a blast of radiation. It marks the first time astronomers witnessed a sudden activation of a supermassive black hole in real time.

“Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive,” said Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study of this object. “Suddenly, its [core] starts showing dramatic changes in brightness, unlike any typical events we’ve seen before.”

This is what happened to SDSS1335+0728, which is now officially classified as having an active galactic nucleus (AGN). It experienced what’s called a “nuclear transient.” Essentially, that means the galaxy now has a very bright compact region. However, it wasn’t always that bright and astronomers want to understand what caused it to wake up.

This artist’s impression shows two stages in the formation of a disc of gas and dust around the massive black hole at the center of the galaxy SDSS1335+0728. The core of this galaxy lit up in 2019 and keeps brightening today — the first time astronomers observed a massive black hole become active as it happened. Credit: ESO/M. Kornmesser

Looking for Transients in all the Right Places

The unusual brightness variations were detected by the Zwicky Transient Facility in California, which gives constant, real-time alerts about such things as transient flaring and brightening in the hearts of galaxies like SDSS1335+0728. In addition, several other facilities observed the variations, too, and brightness changes were found in archival data from several other observatories.

The sudden brightenings could be due to many things, including the cannibalization of stars and clouds of gas that stray too near supermassive black holes. How often they brighten and how a quiescent galaxy nucleus changes to an active one are topics that astronomers are using such surveys and observations to understand. They’re looking not just at distant galaxies, but activity within the neighborhood of our own galaxy’s supermassive black hole, too.

A Galaxy and Its Supermassive Black Hole

Most galaxies have stupendously massive black holes at their hearts. They typically sequester away at least a hundred thousand times the mass of the Sun (sometimes more). It’s all trapped by gravity and nothing ever escapes, not even light. “These giant monsters usually are sleeping and not directly visible,” said study co-author Claudio Ricci, from Chile’s Diego Portales University. “In the case of SDSS1335+0728, we were able to observe the awakening of the massive black hole, [which] suddenly started to feast on gas available in its surroundings, becoming very bright.”

A black hole itself doesn’t emit any light at all. Instead, it sucks everything in, including light. However, the region around the black hole—called the accretion disk—is a pretty active place. It’s where material trapped by the intense gravitational pull of the black hole swirls around like water going down a drain. All that stuff—mostly gas, some dust—is threaded through with magnetic fields. Friction between accretions of the material heats it up. And, that act of heating gives off radiation. If there’s enough of it, we see light being given off. Intense active regions emit x-rays, which indicate the level of activity.

Gravity’s Slice-and-dice Activity

There’s also something called tidal disruption, which happens when something like a star or a cloud of gas gets trapped in the gravitational field. These things take time—on the order of years to occur. When they happen, the gravitational pull of the black hole eventually rips the star or cloud apart. That also gives off radiation. In fact, a very slow-motion tidal disruption event may be occurring at the heart of SDSS1335+0728. If so, it could be one of the longest and dimmest ones ever seen.

Regardless of what’s causing the brightening, the ultimate fate of some of the material is to end up inside the black hole. The rest of it gets superheated in the accretion disk and signals its fate through increased radiation.

Black Hole Growth and a Wake-up Call

The supermassive black holes in the hearts of galaxies grow from smaller ones to larger ones through mergers. We don’t see those growth patterns in real time, since they occur over millions of years. The merger scenario says that when galaxies come together, their central black holes (if they have them) do, too.

Simulation of merging supermassive black holes. Credit: NASA’s Goddard Space Flight Center/Scott Noble

Eventually you get these gargantuan monsters. They just sit there and nibble away at passing gas clouds to gain additional mass. That’s how they gain mass through acquisitions, which occur over shorter timescales. This is apparently what the one in SDSS1335+0728 is doing now. It’s just not often that astronomers get to see one wake up and start munching away in a short period of time.

So, a lot of questions remain about this one, mostly about its formation history. Since the mergers take a long time, it’s hard to know what’s happened to this one in the past. If this is a tidal disruption event, astronomers want to know how often such things happen.

This artist’s illustration depicts what astronomers call a “tidal disruption event,” or TDE, when an object such as a star wanders too close to a black hole and is destroyed by tidal forces generated from the black hole’s intense gravitational forces. (Credit: NASA/CXC/M.Weiss.)

At the moment, for SDSS1335+0728, there’s no immediate evidence of prior outbursts signaling prior awakenings of the supermassive black hole. Astronomers need to do a lot of follow-up observations to understand what’s really happening there, and perhaps find evidence for other eruptions and activity associated with the black hole, according to Sánchez Sáez. “Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve,” she said, noting that advanced instruments at ESO’s Very Large Telescope should give astronomers a better idea of the processes occurring at this black hole. In addition, further time-domain all-sky surveys with the upcoming Vera C. Rubin telescope should be able to track this galaxy’s nuclear brightenings.

For More Information

Astronomers See a Massive Black Hole Awaken in Real Time
SDSS1335+0728: The awakening of a ~10^6 M_sun Black Hole
arXiv preprint

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