Four years ago, the supermassive black hole at the heart of the galaxy SDSS1335+0728 woke up with a loud bang and announced its presence with a burst of radiation. It was the first time astronomers observed the sudden activation of a supermassive black hole in real time.
“Imagine you have been observing a distant galaxy for years and it always seems quiet and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study on this object. “Suddenly its [core] begins to show dramatic changes in brightness, unlike any typical event we have seen so far.”
This is what happened to SDSS1335+0728, which is now officially classified as an active galactic nucleus (AGN) galaxy. It has experienced what is known as a “nuclear transient.” Essentially, this means that the galaxy now has a very bright, compact region. However, it wasn't always this bright, and astronomers want to understand what caused it to wake up.
This artist's impression shows two stages in the formation of a disk 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 continues to shine brighter today—the first time astronomers have observed a massive black hole become active. Credit: ES M. Kornmesser
Looking for transients in the right places
The unusual brightness variations were discovered by the Zwicky Transient Facility in California, which provides constant real-time information about things like temporary flares and brightenings in the hearts of galaxies like SDSS1335+0728. In addition, several other facilities also observed the variations, and brightness changes were found in archival data from several other observatories.
The sudden brightening could have many causes, including cannibalization of stars and gas clouds that get too close to supermassive black holes. How often they brighten and how a dormant galactic core turns into an active one are topics that astronomers are trying to understand with such surveys and observations. They are studying not only distant galaxies but also the activity in the vicinity of our own galaxy's supermassive black hole.
A galaxy and its supermassive black hole
Most galaxies have massive black holes in their cores. They usually hold at least a hundred thousand times the mass of the Sun (sometimes even more). Everything is trapped by gravity and nothing escapes, not even light. “These huge monsters are usually asleep and not directly visible,” said study co-author Claudio Ricci of Diego Portales University in Chile. “In the case of SDSS1335+0728, we were able to observe the massive black hole waking up. [which] suddenly began to feed on the gas present in its surroundings and became 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. There, material trapped by the black hole's strong gravitational pull swirls around like water flowing down a drain. All of this stuff – mostly gas, some dust – is laced with magnetic fields. The friction between the accretions of material heats it up. And that heating releases radiation. When there's enough of it, we see light being emitted. Intensely active regions emit X-rays, which indicate the level of activity.
The slice-and-dice activity of gravity
There's also something called a tidal stream, which happens when something like a star or a cloud of gas gets caught in the gravitational field. These things take time – on the order of years. When they happen, the black hole's gravitational pull eventually tears the star or cloud apart, also releasing radiation. In fact, a very slow tidal stream event may be happening at the heart of SDSS1335+0728. If true, it could be one of the longest and darkest events ever observed.
Regardless of what causes the brightening, some of the material will ultimately end up inside the black hole. The rest will be superheated in the accretion disk and reveal its fate through increased radiation.
Growth of black holes and a wake-up call
The supermassive black holes at the heart of galaxies grow by merging smaller ones into larger ones. We don't see these growth patterns in real time because they take place over millions of years. The merger scenario states that when galaxies merge, their central black holes (if they have them) also merge.
Simulation of the merger of supermassive black holes. Image credit: NASA Goddard Space Flight Center/Scott Noble
Eventually you get these gigantic monsters. They just sit there and nibble on passing gas clouds to gain extra mass. So they gain mass through acquisitions that happen over shorter periods of time. That's apparently what the object in SDSS1335+0728 is doing now. It's just not often that astronomers see one wake up and start nibbling in a short period of time.
So many questions remain about this star, especially about its formation history. Since mergers take a long time, it is difficult to say what happened to this star in the past. If it is a tidal event, astronomers would like to know how often something like this happens.
This artist's impression shows what astronomers call a “tidal disruption event” (TDE). This is when an object, such as a star, comes too close to a black hole and is destroyed by the tidal forces created by the black hole's strong gravitational pull. (Source: NASA/CXC/M.Weiss.)
Currently, there is no direct evidence for SDSS1335+0728 of previous outbursts that would indicate previous awakenings of the supermassive black hole. Astronomers will need to make many follow-up observations to understand what is really happening there and perhaps find evidence of other outbursts and activities related to the black hole, says 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 advanced instruments on ESO's Very Large Telescope should give astronomers a better picture of the processes taking place at this black hole. In addition, further time-domain sky surveys with the upcoming Vera C. Rubin telescope should be able to follow the core brightenings of this galaxy.
For more informations
Astronomers observe the awakening of a giant black hole in real time
SDSS1335+0728: The awakening of a ~10^6 M_sun black hole
arXiv preprint
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