We all know that black holes are destructive monsters. Their tremendous gravitational pull sucks in anything that gets in their way. This is especially true for supermassive black holes at the hearts of galaxies. They can tear stars. And, once in a while, like once every 10,000 years, it happens. The star passes too close and the black hole’s gravity tears it apart.
When a star experiences a “tidal disturbance event” (TDE), it illuminates the core of the galaxy. Astronomers know about 100 of these TDEs in distant galaxies. Most of the light they detect from this catastrophic event comes 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 captured one occurring in the galaxy NGC 7392. The galaxy is about 137 million light-years away. of Earth, and the discovery at its heart is one of the first time astronomers have seen an infrared view of star-shredding by a black hole.
They named the event “WTP14adbjsh”. Because dust clouds obscured the view, there were no X-ray or ultraviolet views. However, the dust absorbed much of the radiation from the event, causing the clouds to emit infrared light.
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The discovery came about almost by accident as MIT postdoctoral fellow Christos Panagiotou and his colleagues were browsing the data from the NEOWISE mission. It has been scanning the sky in infrared wavelengths since 2010. The team discovered a bright flash that showed up in the data. Panagiotou wasn’t looking for tidal disturbance events, actually. The team was looking for transients, sources of light that appear and then disappear. Then they discovered this flash. “We could see there was nothing at the start,” Panagiotou recalled. “Then all of a sudden, in late 2014, the source got brighter and in 2015 hit high brightness, then started to go back to its previous quiescence.”
They eventually traced the flash back to NGC 7392 and began to wonder what kind of astrophysical process could be creating it. “For example, supernovae are sources that suddenly explode and light up, then come back down, on time scales similar to tidal disturbance events,” Panagiotou said. “But supernovae are not as bright and energetic as what we have observed.”
Eventually, the team figured out that the flash was caused by a TDE, i.e. a star torn apart by a supermassive black hole. It matched the data and, if it worked, was the closest astronomers had ever observed.
Proving a TDE
It’s one thing to claim that the transient flash of light was a star shredded by a black hole, but how do you prove it? First, the team needed to understand the black hole and its environment. So they studied the galaxy. Data from various sources showed that the galaxy had a supermassive black hole about 30 million times more massive than the Sun. It’s actually quite massive. “It’s almost 10 times larger than the black hole we have at our galactic center, so it’s quite massive, although black holes can be up to 10 billion solar masses,” Panagiotou said.
For a star to come close enough to meet the black hole, that means the galaxy has a population of stars and could create some near the black hole. Observations at different wavelengths showed that NGC 7392 is creating new stars. However, it is not as active as some galaxies and is busier than others. It is considered a “green” star-forming galaxy. This is because it produces a few stars, enough to provide something for the black hole to eat. It turns out that most of the TDEs happened in the rare “green” type of galaxy.
However, there is another factor to consider. Star-forming galaxies produce a lot of dust, especially at the core. Infrared light can pass through most of the dust, while X-ray, optical or ultraviolet light is blocked. Thus, this may be a factor in why astronomers have not detected more TDEs in star-forming galaxies when looking through conventional optical telescopes.
The future of TDE observation is infrared
This finding highlights the need for more infrared observations of galaxies to search for TDEs. “The fact that optical and X-ray surveys missed this bright TDE in our own backyard is very illuminating and demonstrates that these surveys only give us a partial count of the total TDE population,” Suvi Gezari said. , associate astronomer and chairman of the science staff at the Space Telescope Science Institute in Maryland, who was not involved in the study. “Using infrared surveys to pick up the dust echo from 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 searches for exoplanets) also captured a TDE in 2019. A ground-based survey called the All-Sky Automated Survey for Supernovae (ASAS-SN) alerted astronomers. They were able to obtain other observations, in particular from TESS to follow the progress of the event.
For more information
Astronomers detect closest example yet of a black hole devouring a star
A candidate for a tidal disturbance event obscured by luminous dust in a star-forming galaxy at 42 Mpc
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