Astronomers watch a black hole wake up from ancient slumber

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

“Imagine you’ve observed a distant galaxy for years, and it always seemed quiet and inactive,” said Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study of this object. “Suddenly, it is [core] begins to show dramatic changes in brightness unlike any typical event we’ve seen before.”

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

Looking for passers in all the right places

The unusual brightness changes were detected by the Zwicky transient facility in California, which provides continuous, real-time alerts on things like flares and transient brightness in the hearts of galaxies like SDSS1335+0728. In addition, several other objects observed the changes, too, and brightness changes were found in archival data from several other observatories.

The sudden flares can be due to many things, including the cannibalization of stars and gas clouds that are too close to supermassive black holes. How often they shine and how a quiescent galactic core changes into an active core are topics astronomers are using such surveys and observations to understand. They are looking not only at distant galaxies, but also at activity in the vicinity of our galaxy’s supermassive black hole.

A galaxy and its supermassive black hole

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

A black hole itself does not emit any light. Instead, it absorbs everything, including light. However, the region around the black hole – called the accretion disk – is a very active place. It’s where material trapped by the black hole’s intense gravitational pull swirls around like water going down a drain. All of this matter—mostly gas, some dust—is brought in through magnetic fields. Friction between clumps of material heats it up. And, that act of heating gives off radiation. If there are enough, we see the light emitted. Intensely active regions emit x-rays, which indicate the level of activity.

Religion Activity and Gravity Dice

There’s also something called tidal disruption, which happens when something like a star or a gas cloud gets trapped in the gravitational field. These things take time—on the order of years to happen. When they do, the black hole’s gravitational pull eventually rips the star or cloud apart. This also emits 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 darkest ever seen.

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

Black hole growth and a wake-up call

Supermassive black holes in the hearts of galaxies grow from smaller to larger through mergers. We don’t see those growth patterns in real time, as they occur over millions of years. The merger scenario states that when galaxies merge, their central black holes (if they have any) do too.

Eventually you get these giant monsters. They just sit there and absorb the passing gas clouds to gain extra mass. This is how they gain mass through acquisitions, which occur in shorter periods of time. That’s apparently what he’s doing right now in SDSS1335+0728. It’s just not often that astronomers see one smart and start eating in a short period of time.

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

At the moment, for SDSS1335+0728, there is no immediate evidence of previous explosions that signal previous wakes of the supermassive black hole. Astronomers need to do a lot of follow-up observations to understand what’s really going on there and possibly find evidence for explosions and other activity related to the black hole, according to Sánchez Sáez. “Regardless of the nature of the variations, [this galaxy] provides valuable information about how black holes grow and evolve,” she said, noting that the advanced instruments on ESO’s Very Large Telescope should give astronomers a better idea of ​​the processes taking place in this black hole. In addition, further all-sky time-domain surveys with the upcoming Vera C. Rubin telescope should be able to trace the nuclear luminosities of this galaxy.

For more information

Astronomers see a massive black hole waking up in real time
SDSS1335+0728: The wake of a black hole ~ 10^6 M_sun
arXiv preprint