For the first time, researchers believe they have observed a black hole activating “in real time.”

In late 2019, the previously unobserved galaxy SDSS1335+0728 suddenly began to shine brighter than ever before. A team of astronomers analyzed this phenomenon and linked it to the sudden awakening of the massive black hole, which has a mass of about one million solar masses, at its center. The study has just been published in the journal Astronomy and astrophysics.

“Imagine you have been observing a distant galaxy for years, and it always appears quiet and inactive,” says lead author Paola Sánchez Saez, an astronomer at the European Southern Observatory (ESO) in Germany.

“Suddenly, its core begins to show radical changes in brightness, in a completely different way than was observed before,” the researcher says. That's what happened to SDSS1335+0728 when it increased dramatically in brightness in December 2019. That's why it's now classified as an “active galactic nucleus” (AGN, a bright, compact region fed by a massive black hole).

Some phenomena, such as supernova explosions or tidal disruption events (when a star gets too close to a black hole and disintegrates), can cause galaxies to suddenly light up. But these variations in brightness usually last a few tens, or at most, a few hundred days.

Today, more than four years after it was first seen “on fire,” SDSS1335+0728 continues to increase in brightness. Furthermore, the variations detected in the galaxy, located 300 million light-years away in the constellation Virgo, are unlike anything observed before, suggesting a different explanation.

How did they notice the black hole?

The team sought to understand these brightness variations using a combination of archival data and new observations from several space-based and ground-based observatories, including the X-shooter instrument mounted on the European Southern Observatory's Very Large Telescope (VLT) in the Atacama Desert.

When comparing data taken before and after December 2019They discovered that SDSS1335+0728 now emits a greater amount of light At ultraviolet, optical and infrared wavelengths. The galaxy also began emitting X-rays in February 2024. “This behavior is unprecedented,” says Sanchez Saez, also affiliated with the Millennium Institute of Astrophysics (MAS) in Chile.

“The most obvious option to explain this phenomenon is that we see how the galactic core is starting to show activity,” says co-author Lorena Hernandez García, of MAS and the University of Valparaiso in Chile. “If so, that is This will be the first time we have seen a massive black hole activate in real time“.

Most galaxies, including the Milky Way, have a massive black hole at their center (more than a hundred thousand times the mass of our Sun). “These giant monsters They are usually asleep and invisible explains co-author Claudio Ricci, of the Universidad Diego Portales, also in Chile. “In the case of SDSS1335+0728, we were able to observe the awakening of the supermassive black hole, which suddenly began feeding on the gas available in its surroundings, and became extremely bright.”

“This process has not been observed before“, confirms Hernandez García. Previous studies have indicated that dormant galaxies become active after several years, but this is the first time that the process itself, i.e. the wake-up of a black hole, has been observed in real time.

“This is something that could also happen to our massive black hole Sgr A* at the center of our galaxy,” adds Ritchie, who is also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University in China. , but it is not clear how likely this is to happen.

Rule out other possibilities

Follow-up observations are still needed to rule out alternative explanations. Another possibility is that we are witnessing an unusually slow tidal disturbance event, or even a new phenomenon. If this is indeed a tidal disturbance event, it would be the longest and weakest event ever observed.

“Regardless of the nature of the differences, this galaxy provides valuable information about how black holes grow and evolve,” says Sanchez Saez. “We hope that instruments like MUSE on the VLT or those on the Extremely Large Telescope (ELT) will be a key to understanding why the galaxy is so bright,” the researcher concludes.

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