Representational Image: This illustration shows a glowing stream of material from a star, torn to shreds as it was being devoured by a supermassive black hole.
(NASA/JPL-Caltech)
Our Universe is a happening place, but many unfathomable events happen very, very far. The lights from such events take millions of years to reach Earth. Even then, catching a glimpse of these events is super rare.
However, with the help of incredibly powerful modern telescopes and space instruments, astronomers are always on the lookout for such spectacular events. In one such stellar but eerie flash, scientists have now spotted a supermassive black hole ripping a star to shreds and devouring it like spaghetti at the centre of the galaxy NGC 7392.
Capturing the closest-known black-hole dinner
Interestingly, scientists used an unconventional approach to capture this event named WTP14adbjsh. It is the first such event seen as a bright infrared flare in place of the usual optical, ultraviolet, or X-radiation.
Remarkably, the event took place just 137 million light years away from Earth, making it the closest-known example of a tidal disruption event (TDE). TDE refers to a star being pulled apart by the enormous gravitational pull of a black hole.
As distant as that sounds, only around 100 events with such proximity have been observed so far, and this one is about four times closer than the previous record holder. But there’s a more profound mystery associated with this particular black hole!
Spotting nearby cosmic monsters
You see, black holes are named as they are for a reason. Their gravitational pull is so strong that even light can’t escape their clutches, which makes them hard to spot, especially if they aren’t great eaters.
But active black holes are messy eaters and generate tremendous amounts of light as they feed themselves. So our telescopes and space instruments can usually spot them in X-ray or optical light.
However, the TDE in question stayed stealthily hidden!
As the radiation from the event reached the Earth now — 137 million light years later — it did not leave any trace on the X-ray and optical range of any telescopes. It was only when the scientists revisited almost a decade-old archival data collected by the NEOWISE spacecraft, an infrared space telescope, that they discovered this rare event.
“We could see there was nothing at first. Then suddenly, in late 2014, the source got brighter and by 2015 reached a high luminosity, then started going back to its previous quiescence,” says astrophysicist and lead author Christos Panagiotou.
These findings highlight that there could be more TDEs or other, even more mysterious happenings in our cosmic backyard that we are missing simply because we aren’t looking at the right place.
NGC 7392 is a blue galaxy — churning out many new stars and creating a lot of dust in the process. This dust could make it difficult to view the supermassive black holes in optical and ultraviolet light. And one would expect black holes to eat most stars in blue galaxies that constantly churn out new stars. But, surprisingly, most of the TDEs we have detected so far are found in the so-called green galaxies, which don’t create as many stars.
Therefore, scientists are now hopeful that using infrared light could enable us to peer through all that dust and witness the stellar events in some of the most active galaxies of our Universe.
“Finding this nearby TDE means that, statistically, there must be a large population of these events that traditional methods were blind to. So, we should try to find these in the infrared if we want a complete picture of black holes and their host galaxies,” says Panagiotou.
This study was published in The Astrophysical Journal Letters and can be accessed here.
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