Astronomers watch a supermassive black hole awaken in real time

By Ashley Strickland, CNN

(CNN) — Astronomers are witnessing a never-before-seen spectacle in the cosmos: the awakening of a supermassive black hole at the center of a distant galaxy.

In late 2019, a team of astronomers took notice of an otherwise unremarkable galaxy named SDSS1335+0728, 300 million light-years away in the Virgo constellation. A sudden spike in the galaxy’s brightness had been automatically detected by the Zwicky Transient Facility’s telescope at the Palomar Observatory in California.

With its extreme wide-field view, the camera scans the entire northern sky every two days, capturing data on celestial objects such as near-Earth asteroids as well as distant, bright supernovas.

An interdisciplinary team of astronomers and engineers followed up on Zwicky’s observation by using information from space- and ground-based telescopes to see how the galaxy’s luminosity changed over time.

To their surprise, the researchers realized they were witnessing a unique moment as a cosmic monster awakened. Their study findings have been accepted for publication in the journal Astronomy & Astrophysics.

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

The team classified the galaxy as having an active galactic nucleus, or a bright, compact region that is powered by a supermassive black hole.

A number of celestial scenarios can cause a galaxy to suddenly brighten, such as supernova explosions or when stars draw too near to black holes and become torn apart during a phenomenon called tidal disruption events.

But such events only last dozens or hundreds of days — and SDSS1335+0728 continues to grow in brightness more than four years after researchers first observed it spiking in luminosity like the flick of a cosmic light switch.

And the brightness variations in the galaxy don’t resemble anything astronomers have seen before, which only puzzled them further.

An unprecedented cosmic event

To find answers, the team consulted archival data from NASA’s Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer, the Two Micron All Sky Survey, the Sloan Digital Sky Survey and other observatories.

The researchers compared the data with follow-up observations taken by the European Southern Observatory’s Very Large Telescope, or VLT, in Chile, the Southern Astrophysical Research Telescope in Chile, the W. M. Keck Observatory in Hawaii and NASA’s space-based Neil Gehrels Swift and Chandra X-ray observatories.

Together, the datasets presented a broad portrait of the galaxy both before and after the December 2019 observation, revealing that the galaxy shifted to emit much more ultraviolet, visible and infrared light in recent years, and X-rays beginning in February — which is unprecedented behavior, Sánchez Sáez said.

Given that the galaxy is 300 million light-years away, the events that astronomers are seeing happened in the past — but the light from these events is just now reaching Earth after traveling across space for millions of years. One light-year is the distance light travels in one year, which is 5.88 trillion miles (9.46 trillion kilometers).

“The most tangible option to explain this phenomenon is that we are seeing how the (core) of the galaxy is beginning to show (…) activity,” said study coauthor Lorena Hernández García, astronomer at the Millennium Institute of Astrophysics and the University of Valparaíso, both in Chile, in a statement. “If so, this would be the first time that we see the activation of a massive black hole in real time.”

Sleeping celestial giants

Supermassive black holes are classified as having masses more than 100,000 times that of our sun. They can be found at the center of most galaxies, including the Milky Way.

“These giant monsters usually are sleeping and not directly visible,” said study coauthor Claudio Ricci, associate professor at Diego Portales University in Chile, in a statement. “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.”

Previous research has pointed to inactive galaxies that appeared to become active after several years, which is usually triggered by black hole activity, but the process of a black hole awakening has never been directly observed before, until now, Hernández García said.

The same scenario may play out with Sagittarius A*, the supermassive black hole at the center of the Milky Way, but astronomers aren’t sure how likely it is to occur, Ricci said.

Astronomers can’t rule out that their observation could be an unusually slow tidal disruption event, or an unknown new celestial phenomenon.

“Regardless of the nature of the variations, (this galaxy) provides valuable information on how black holes grow and evolve,” Sánchez Sáez said. “We expect that instruments like (MUSE on the VLT or those on the upcoming Extremely Large Telescope) will be key in understanding (why the galaxy is brightening).”

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