We all know that black holes are destructive monsters. Her tremendous power of attraction sucks in everything that gets in her way. This is especially true for supermassive black holes at the heart of galaxies. You can rip stars. And every now and then—about every 10,000 years—that happens. The star passes too close, and the black hole’s gravity rips it apart.
When a star experiences a Tidal Disruption Event (TDE), it illuminates the core of the galaxy. Astronomers know about 100 of these TDEs in distant galaxies. Most of the light they see from this cataclysmic event arrives in the form of X-rays and optical light. But it turns out they can tune in to infrared signals from a TDE, and MIT scientists recently picked up one found in galaxy NGC 7392. The galaxy lies about 137 million light-years from Earth, and the discovery at its heart is one that astronomers have seen for the first time an infrared view of star fragments through a black hole.
They named the event WTP14adbjsh. With dust clouds obscuring the view, there were no X-ray or UV views. However, the dust absorbed much of the event’s radiation, causing the clouds to emit infrared light.
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Capture a view of a black hole shredding a star
The discovery came about almost by accident as MIT postdoc Christos Panagiotou and his colleagues combed through data from the NEOWISE mission. It has been scanning the sky in infrared wavelengths since 2010. The team spotted a bright flash appearing in the data. Panagiotou wasn’t actually looking for tidal disturbances. The team looked for transients – light sources that appear and then disappear. Then they discovered this flash. “We could see at first that there was nothing,” Panagiotou recalled. “Then suddenly, in late 2014, the source brightened and reached high luminosity in 2015 and then returned to its previous dormancy.”
Infrared sign of the closest tidal disturbance event (TDE) so far. In this event, a star moved too close to a central supermassive black hole. In 2015, a bright flare was spotted from galaxy NGC 7392 (top left). Observations of the same galaxy were taken in 2010-2011 (top right) before the TDE. Below left is the difference between the first two images showing the actual TDE detected. For comparison, the lower right image shows the same galaxy in the optical waveband. Courtesy of the researchers
They eventually tracked the flash to NGC 7392 and began asking what kind of astrophysical process might be producing it. “For example, supernovae are sources that explode and suddenly brighten and then come back down, on similar timescales to tidal disturbances,” Panagiotou said. “But supernovae are not as luminous and energetic as what we have observed.”
Eventually, the team found that the flash was due to a TDE — that is, a star being torn apart by a supermassive black hole. It fitted the data and, when it turned out, was the closest astronomers have ever observed.
Evidence of a TDE
It’s one thing to claim that the momentary flash of light was a star ripped apart by a black hole, but how can you prove that? First, the team needed to understand the black hole and its surroundings. So they studied the galaxy. Data from various sources showed that the galaxy had a supermassive black hole about 30 million times the mass of 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, even though black holes can reach up to 10 billion solar masses,” Panagiotou said.
That a star comes close enough to encounter the black hole means that the galaxy has a population of stars and some could form near the black hole. The observations at different wavelengths showed that NGC 7392 is busy making new stars. However, it is not as active as some galaxies and busier than others. It is considered a “green” star-forming galaxy. That’s because it produces a few stars, enough to feed the black hole. It turns out that most of the TDEs took place in the rare breed of “green” galaxies.
However, there is one more factor to consider. Star-forming galaxies produce a lot of dust, especially in the core. Infrared light can penetrate most of the dust while blocking X-rays, optical or ultraviolet light. This could be a factor in why astronomers haven’t spotted more TDEs in star-forming galaxies when looking with traditional optical telescopes.
The future of observing TDEs is infrared
This discovery indicates a need for more infrared observations of galaxies to find TDEs. “The fact that optical and X-ray surveys have missed this glowing TDE in our own backyard is very revealing and shows that these surveys only provide us with a partial census of the total population of TDEs,” said Suvi Gezari, associate astronomer and chair of the Scientific Employees at the Space Telescope Science Institute in Maryland who were not involved in the study. “Using infrared surveys to capture the dust echo from eclipsed TDEs…has already shown us that there is a population of TDEs in dusty, star-forming galaxies that we have previously missed.”
Interestingly, the TESS satellite (best known for its search for exoplanets) also acquired 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 TESS, to track the progress of the event.
For more informations
Astronomers discover the closest example yet of a black hole devouring a star
A candidate tidal disturbance event obscured by glowing dust in a star-forming galaxy at 42 Mpc
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