Astronomers Watch Black Hole Awaken From Ancient Slumber

Four years ago, the supermassive black hole hidden at the heart of the galaxy SDSS1335+0728 woke up and announced its presence with a burst of radiation. This is the first time that astronomers have observed the sudden activation of a supermassive black hole in real time.

“Imagine that you've been observing a distant galaxy for years, and it always seemed quiet and inactive,” says Paula Sanchez Saez, an astronomer at ESO in Germany and lead author of the study on the object. “Suddenly it [ядро] begins to show sudden changes in brightness that are unlike the typical events we have seen before.”

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

Finding transients in the right places

The unusual brightness variations were detected by the Zwicky Transient Search Center in California, which routinely provides real-time alerts for events such as transient flares and brightenings in the hearts of galaxies like SDSS1335+0728. The variations were also observed by several other objects, and the brightness changes were found in archived data from several other observatories.

The sudden brightening can be caused by many things, including the cannibalization of stars and gas clouds that are too close to supermassive black holes. How often they brighten and how a quiescent galactic core turns into an active one are topics astronomers are trying to understand with surveys and observations like these. They study not only distant galaxies, but also activity in the vicinity of our own galaxy’s supermassive black hole.

The Galaxy and Its Supermassive Black Hole

At the center of most galaxies is a staggeringly massive black hole. Typically, these have a mass at least 100,000 times that of the Sun (and sometimes more). The whole thing is trapped by gravity, with nothing able to escape, not even light. “These giant monsters are usually dormant and not directly visible,” says study co-author Claudio Ricci of the Universidad Diego Portales in Chile. “In the case of SDSS1335+0728, we were able to observe the awakening of a massive black hole, [которая] suddenly began to feed on the gas present in its vicinity and became very bright.”

The black hole itself doesn't emit any light. Instead, it sucks everything into itself, including light. But the region around the black hole, called the accretion disk, is a pretty active place. In it, material trapped by the black hole's strong gravitational pull swirls around like water in a drain. All of this material — mostly gas, some dust — is threaded through with magnetic fields. The friction between the clumps of material heats it up. And when it heats up, it emits radiation. If there's enough of it, we can see the light coming from the disk. Intense active regions emit X-rays, which indicate the level of activity.

Destructive activity of gravity

There is also something called a tidal disruption event, which occurs when something like a star or a gas cloud gets trapped in a gravitational field. This takes time, on the order of years. When this happens, the gravitational pull of the black hole eventually tears the star or gas cloud apart. This also produces radiation. In fact, a very slow tidal disruption event may be occurring at the center of SDSS1335+0728. If so, it could be one of the longest and faintest tidal disruptions ever observed.

Regardless of what causes the bright glow, the ultimate fate of some of the material is to end up inside the black hole. The rest overheats in the accretion disk and signals its fate with increased radiation.

Black Hole Growth and Alarm Clock

Supermassive black holes at the hearts of galaxies grow from smaller to larger through mergers. We don't see these growth processes in real time, as they happen over millions of years. The merger scenario says that when galaxies come together, their central black holes (if they have one) also merge.

The end result is these gargantuan monsters. They just sit there and “gnaw” at passing clouds of gas to gain additional mass. This is how they gain mass through absorptions that occur on shorter time scales. Apparently, this is what the object in SDSS1335+0728 is doing now. It's just not often that astronomers get to see one of them wake up and start “eating” in a short period of time.

So there are still many questions about it, mostly about its formation history. Because mergers take a long time, it's hard to tell what happened to it in the past. If it's a tidal disruption event, astronomers want to know how often that happens.

At this point, there is no direct evidence for SDSS1335+0728 that the previous outbursts signaled the awakening of the supermassive black hole. Astronomers will need to conduct many follow-up observations to understand what is really going on and perhaps find evidence of other eruptions and activity associated with the black hole, Sanchez Saez said. “Regardless of the nature of the oscillations, [эта галактика] “provides valuable information about how black holes grow and evolve,” she said, noting that advanced instruments on ESO's Very Large Telescope should give astronomers a better understanding of the processes occurring in this black hole. In addition, further all-sky time-lapse surveys on the upcoming Vera Rubin Telescope will allow us to track the galaxy's nuclear amplifications.