Astronomers detect the closest example of a black hole devouring a star

About once every 10,000 years, the center of a galaxy lights up as its supermassive black hole tears apart a passing star. This “tidal disruption event” literally happens in a flash, as the central black hole pulls in stellar material and emits massive amounts of radiation in the process.

Astronomers know about 100 tidal disruption events (TDEs) in distant galaxies, based on the burst of light arriving at telescopes on Earth and in space. Most of this light comes from X-rays and optical radiation.

MIT astronomers, tuning in beyond the conventional X-ray and UV/optical bands, have discovered a new tidal disruption event, shining brightly in the infrared. It is one of the first times that scientists have directly identified a TDE at infrared wavelengths.

Furthermore, the new outburst appears to be the closest tidal disruption event observed to date: the flare was found in NGC 7392, a galaxy that lies about 137 million light-years from Earth, which corresponds to a region in our cosmic backyard which is a quarter the size of the next closest TDE.

This new flare, labeled WTP14adbjsh, did not stand out in standard optical and radiographic data. Scientists suspect these traditional detections missed nearby TDE, not because it didn’t emit X-rays and UV light, but because that light was obscured by a huge amount of dust that absorbed the radiation and released heat in the form of infrared. power.

The researchers determined that WTP14adbjsh occurred in a young star-forming galaxy, in contrast to most TDEs that have been found in quieter galaxies. Scientists expected that star-forming galaxies should host TDEs, since the stars they produce would provide plenty of fuel for a galaxy’s central black hole to devour. But observations of TDEs in star-forming galaxies were rare until now.

The new study suggests that traditional X-ray and optical detections may have missed TDEs in star-forming galaxies because these galaxies naturally produce more dust that could obscure any light from their cores. Searching in the infrared band could reveal many more TDEs previously hidden in galaxies actively forming stars.

“Finding this close TDE means that, statistically, there must be a large population of these events that traditional methods were blind to,” says Christos Panagiotou, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “So, we should try to find them in the infrared if we want a complete picture of black holes and their host galaxies.”

A document detailing the team’s discovery appears today in Letters from the astrophysicist diary. Panagiotou’s MIT co-authors are Kishalay De, Megan Masterson, Erin Kara, Michael Calzadilla, Anna-Christina Eilers, Danielle Frostig, Nathan Lourie, and Rob Simcoe, along with Viraj Karambelkar, Mansi Kasliwal, Robert Stein, and Jeffry Zolkower of Caltech, and Aaron Meisner of the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory.

A flash of possibilities

Panagiotou did not intend to look for tidal disruption events. He and his colleagues were looking for signs of general transient sources in the observational data, using a search tool developed by De. The team used De’s method to search for potential transient events in archival data taken by NASA’s NEOWISE mission, a space telescope that has been making regular scans of the entire sky since 2010, at infrared wavelengths.

The team discovered a bright flash that appeared in the sky in late 2014.

“We could see that there was nothing at first,” Panagiotou recalls. “Then suddenly, in late 2014, the source got brighter and in 2015 it reached high brightness, then started to go back to its former quiescence.”

They traced the flash to a galaxy 42 megaparsecs from Earth. The question then was: What triggered it? To answer this question, the team looked at the brightness and timing of the flash, comparing the actual observations with models of various astrophysical processes that could produce a similar flash.

“For example, supernovae are sources that explode and brighten suddenly, then flash back, on timescales similar to tidal disruption events,” notes Panagiotou. “But supernovae aren’t as bright and energetic as the ones we’ve observed.”

By working through several possibilities of what the explosion might be, the scientists were finally able to rule out all but one: The flash was most likely a TDE and the closest observed so far.

“It’s a very clean light curve and it really follows what we expect the temporal evolution of a TDE should be,” says Panagiotou.

Red or green

From there, the researchers took a closer look at the galaxy where the TDE arose. They gathered data from multiple ground- and space-based telescopes that observed the part of the sky where the galaxy resides, across various wavelengths, including the infrared, optical, and X-ray bands. With this accumulated data, the team estimated that the supermassive black hole at the center of the galaxy was about 30 million times more massive than the sun.

“This is nearly 10 times the size of the black hole we have at our galactic center, so it’s quite massive, even though black holes can reach up to 10 billion solar masses,” Panagiotou says.

The team also found that the galaxy itself is actively producing new stars. Star-forming galaxies are a class of “blue” galaxies, in contrast to the quieter “red” galaxies that have stopped producing new stars. Blue star-forming galaxies are the most common type of galaxy in the universe.

“Green” galaxies fall between red and blue, as they occasionally produce a few stars. Green is the least common galaxy type, but curiously, most TDEs detected to date have been traced to these rarer galaxies. Scientists had struggled to explain these detections, as theory predicts that blue star-forming galaxies should exhibit TDEs, as they would present more stars for black holes to destroy.

But star-forming galaxies also produce a lot of dust from the interactions between and between stars near a galaxy’s core. This dust is detectable at infrared wavelengths, but can obscure any X-ray or UV radiation that would otherwise be detected by optical telescopes. This may explain why astronomers have not detected TDEs in star-forming galaxies using conventional optical methods.

“The fact that optical and X-ray surveys have missed this bright TDE in our backyard is very illuminating and demonstrates that these surveys are providing us with only a partial census of the total TDE population,” says Suvi Gezari, associate astronomer and staff chairman. scientist at the Space Telescope Science Institute in Maryland, which was not involved in the study. “Using infrared soundings to capture the dust echo of obscured TDEs has already shown us that there is a population of TDEs in dusty and star-forming galaxies that we lacked.”