We all know that black holes are destructive monsters. Their tremendous gravitational pull sucks in anything that gets in the way. This is particularly true for supermassive black holes in the hearts of galaxies. They can tear apart stars. And, every so often—like once every, 10,000 years, that happens. The star passes too close and the black hole’s gravity shreds it.
When a star experiences a “tidal disruption event” (TDE), it lights up the core of the galaxy. Astronomers know of about 100 of these TDEs in distant galaxies. Most of the light they detect from that catastrophic event arrives in the form of X-rays and optical light. But, it turns out they can tune into infrared signals from a TDE, and MIT scientists recently captured one occurring in the galaxy NGC 7392. The galaxy lies about 137 million light-years from Earth, and the discovery at its heart is one of the first times astronomers have seen an infrared view of star-shredding by a black hole.
They named the event “WTP14adbjsh.” Because dust clouds hid the view, there were no X-ray or ultraviolet views. However, the dust absorbed a great deal of radiation from the event, and that caused the clouds to emit infrared light.
Capturing a View of a Black Hole Shredding a Star
The discovery came about almost by accident as MIT postdoc Christos Panagiotou and his colleagues were combing through data from the NEOWISE mission. It has been making scans of the sky in infrared wavelengths since 2010. The team discovered a bright flash that appeared in the data. Panagiotou wasn’t looking for tidal disruption events, actually. The team was searching for transients—sources of light that appear and then disappear. Then, they discovered this flash. “We could see there was nothing at first,” Panagiotou recalled. “Then suddenly, in late 2014, the source got brighter and by 2015 reached a high luminosity, then started going back to its previous quiescence.”
They eventually traced the flash to NGC 7392 and began asking what kind of astrophysical process could create it. “For instance, supernovae are sources that explode and brighten suddenly, then come back down, on similar timescales to tidal disruption events,” Panagiotou said. “But supernovae are not as luminous and energetic as what we observed.”
Eventually, the team figured out that the flash was due to a TDE—that is, a star being ripped apart by a supermassive black hole. It fit the data and, if it panned out, was the closest one astronomers have ever observed.
Proving a TDE
It’s one thing to claim that the transient flash of light was a star getting shredded by a black hole, but how to prove it? First, the team had to understand the black hole and its environment. So, they studied the galaxy. Data from a variety of sources showed the galaxy had a supermassive black hole about 30 million times as massive as the Sun. That’s actually pretty massive. “This is almost 10 times larger than the black hole we have at our galactic center, so it’s quite massive, though black holes can get up to 10 billion solar masses,” Panagiotou said.
For a star to get close enough to encounter the black hole means the galaxy has a population of stars and could be creating some in the vicinity of the black hole. The observations at different wavelengths showed that NGC 7392 is busily creating new stars. However, it’s not as active as some galaxies and is busier than others. It’s considered a “green” star-forming galaxy. That’s because it produces a few stars, enough to provide something for the black hole to eat. It turns out that most TDEs happened in the rare type of “green” galaxy.
However, there’s one other factor to consider. Star-forming galaxies produce a lot of dust, particularly at the core. Infrared light can get through most of the dust, whereas X-ray, optical, or ultraviolet light gets blocked. So, this may be a factor in why astronomers haven’t detected more TDEs in star-forming galaxies when they look using conventional optical telescopes.
The Future of Observing TDEs is Infrared
This discovery points to a need for more infrared observations of galaxies to search out TDEs. “The fact that optical and X-ray surveys missed this luminous TDE in our own backyard is very illuminating and demonstrates that these surveys are only giving us a partial census of the total population of TDEs,” said Suvi Gezari, associate astronomer and chair of the science staff at the Space Telescope Science Institute in Maryland, who was not involved in the study. “Using infrared surveys to catch the dust echo of obscured TDEs … has already shown us that there is a population of TDEs in dusty, star-forming galaxies that we have been missing.”
Interestingly, the TESS satellite (best known for its exoplanet searches) also caught a TDE in 2019. A ground-based survey called the All-Sky Automated Survey for Supernovae (ASAS-SN) alerted astronomers. They were able to get other observations, including from TESS to follow the progress of the event.