'Extragalactic Old Faithful': Scientists Discover Galaxy That Erupts Regularly

A team of researchers discovered a galaxy with a rather odd behavior: it emits flares at regular intervals. What could be causing the galaxy's flares?

Galaxies that have "unusually active" centers are classified by astronomers as active galaxies. NASA says these centers can even produce more energy than all of the stars in the host galaxy combined.

In November of 2014, the All-Sky Automated Survey for Supernovae (ASAS-SN) spotted an outburst event, called ASASSN-14ko, in active galaxy ESO 253-3, which is 570 million light-years away from the Earth. At the time, scientists thought it was a supernova explosion, a one-time outburst.

'Predictable' repeating outburst

It eventually became clear that the event wasn't a supernova explosion. In 2020, Anna Payne of the University of Hawaii at Mānoa (UHM) discovered that there were actually several such events in ESO 253-3.

According to the ASAS-SN data, the center of ESO 253-3 actually emits a flare every 114 days. Further analysis of the events from NASA and other observatories also found that the events were happening in visible, X-ray and ultraviolet wavelengths, the UHM statement explained.

Based on the data, the researchers were able to predict when the flare would happen again, which came true when they spotted flare events on May 17, Sept. 7 and Dec. 20, 2020. In total, there were 20 observed outbursts from ESO 253-3, with the next ones expected to happen in April and August this year. Scientists said the galaxy's behavior is rather like that of the Old Faithful geyser at Yellowstone National Park.

"These are the most predictable and frequent recurring multiwavelength flares we've seen from a galaxy's core, and they give us a unique opportunity to study this extragalactic Old Faithful in detail," Payne said in a statement from NASA about the team's findings, which were presented at the virtual meeting of the American Astronomical Society on Tuesday.

What's behind the 'extragalactic old faithful'?

NASA says there are several possibilities, including an interaction between two supermassive black holes at the center of the galaxy or perhaps a star with an inclined orbit passing the galaxy's black hole. But the team believes that what's known as a "tidal disruption event" (TDE) may be the most plausible explanation.

TDEs happen when a star comes a little too close to a black hole and it essentially gets ripped apart into a stream of gas. Scientists have not previously seen repeating TDEs.

In the case of ESO 243-3, it's possible that a giant star is passing close enough to the galaxy's black hole to trigger a TDE but far enough to not get completely destroyed, thereby only producing a partial TDE, the researchers said. With each pass, the TDE strips away an amount of gas equal to three times the mass of Jupiter, the UHM statement said.

A video shared by NASA even shows how the partial TDEs may be causing ESO 253-3's Old Faithful-like activity.

Of course, the star will eventually run out of mass to give. But for now, ESO 253-3 still provides scientists with an excellent opportunity to observe and prepare for these rather special events.

"Unlike singular TDEs, ASASSN-14ko is extremely forgiving. It allows us to plan for future outbursts, knowing when they will occur, which enables us to get data on these fascinating events that is simply not possible for their one-and-done cousins," Payne said in the UHM statement. "ASASSN-14ko promises to be the gift that keeps on giving."