Key takeaways

  • Four years ago, the supermassive black hole at the center of galaxy SDSS1335+0728 emitted a sudden burst of radiation, marking the first real-time observation of such an event.
  • This phenomenon, called a “nuclear transient,” transformed the galaxy’s core into a highly luminous active galactic nucleus (AGN).
  • The brightness changes were detected by the Zwicky Transient Facility and corroborated by various observatories using archived data.
  • Potential causes for the black hole’s activation include the cannibalization of nearby stars or gas clouds, leading to the release of intense radiation.
  • The event provides valuable insights into the growth and evolution of supermassive black holes, with further observations expected to reveal more about the processes involved.

Four years ago, the supermassive black hole hiding in the center of galaxy SDSS1335+0728 came to life and emitted a burst of radiation. It is the first time that scientists have watched a supermassive black hole suddenly activate in real time.

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

This is what occurred with SDSS1335+0728, which is now officially designated as having an active galactic nucleus (AGN). It underwent what is known as a “nuclear transient.” Essentially, this implies the galaxy now has a very bright, compact sector. However, it wasn’t always as brilliant, and astronomers want to know what caused it to awaken.

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 unexpected brightness changes were observed by the Zwicky Transient Facility in California, which provides continual, real-time warnings regarding transient flaring and brightening in galaxies such as SDSS1335+0728. In addition, numerous additional facilities saw the fluctuations, and brightness changes were discovered in archived data from several more observatories.

The abrupt brightenings might be caused by a variety of factors, including the cannibalization of stars and gas clouds that come too close to supermassive black holes. Astronomers use surveys and observations to understand how frequently they brighten and how a quiet galactic nucleus evolves to an active one. They are looking not only at faraway galaxies, but also at activity around our own galaxy’s supermassive black hole.

A Galaxy and Its Supermassive Black Hole

Most galaxies have enormous black holes at their centers. They generally store at least a hundred thousand times the mass of the Sun (possibly more). Everything is confined by gravity, and nothing, even light, ever escapes. “These giant monsters are usually sleeping and not directly visible,” said Claudio Ricci, a research co-author 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 emits no light at all. Instead, it takes in everything, even light. However, the region around the black hole, known as the accretion disk, is quite active. It’s where material is trapped by the black hole’s tremendous gravitational attraction and swirls about like water down a drain. Magnetic fields go through all of that material, which is largely gas and some dust. The substance warms up due to friction between its accretions. And the act of heating emits radiation. If there is enough of it, we observe light being emitted. Intense active areas release x-rays, indicating their amount of activity.

Gravity’s Slice-and-dice Activity

Tidal disruption occurs when a star or a cloud of gas become stuck in the gravitational field. These things take years to occur. When this happens, the black hole’s gravitational force eventually destroys the star or cloud. That also emits radiation. A extremely slow-motion tidal disruption event may be taking place in the center of SDSS1335+0728. If so, it might be one of the longest and darkest ever seen.

Regardless of the cause of the brightening, some of the material will eventually wind up within the black hole. The rest of material becomes superheated in the accretion disk, signaling its fate with enhanced radiation.

Black Hole Growth and a Wake-up Call

Supermassive black holes in galaxies evolve from tiny to larger ones as they collide. We don’t notice those development patterns in real time since they happen across millions of years. According to the merger scenario, as galaxies merge, so do their core black holes (if any).

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

Eventually, you get these massive beasts. They simply sit there and nibble at passing gas clouds to build bulk. That is how they build bulk through acquisitions, which occur on shorter durations. This is evidently what the one in SDSS1335+0728 is now doing. Astronomers don’t typically get to observe one wake up and start chewing in such a short amount of time.

So, many doubts remain regarding this one, particularly about its creation history. Because mergers take so long, it’s difficult to say what happened to this one in the past. If this is a tidal disturbance, astronomers want to know how often it occurs.

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.

For SDSS1335+0728, there is currently no evidence of past outbursts signifying the supermassive black hole’s awakening. According to Sánchez Sáez, astronomers must conduct several follow-up observations to determine what is truly going on and to look for signs of further eruptions and activity related with the black hole. “Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve,” she said, adding that upgraded detectors at ESO’s Very Large Telescope should help astronomers better understand the activities at this black hole. Furthermore, future time-domain all-sky surveys with the Vera C. Rubin telescope should be able to follow this galaxy’s nuclear brightenings.

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