A supermassive black hole has been detected devouring a star in a galaxy just 137 million light-years from Earth — the closest such phenomena seen to date. The flash of light released as the unlucky star was pulled into the black hole was first detected by NASA’s NEOWISE space telescope back in 2014.
Unusually, the phenomena was detected in the infrared part of the spectrum, and from a young, dusty, star-forming galaxy. Researchers believe that these two facts are connected — with the dust absorbing the X-ray and visible light from the star’s remains and re-emitting it in the infrared. The discovery, the team said, may point to a way for astronomers to detect similar events in active galaxies in the future.
Black holes are regions of where the fabric of spacetime is so deformed by concentrated mass that, beyond their “event horizon”, nothing — not even light — can escape their gravity.
Typically, black holes weigh in at around 5–10 times the mass of our Sun, being constrained by the size of the star that collapsed to form them in the first place.
However, astronomers have also found that most galaxies also contain “supermassive” black holes at their very centre.
These staggering concentrations of matter that can contain millions or even billions of times the mass of the Sun.
If a star should be unlucky enough to wander too close to its host galaxy’s supermassive black hole, it can be ripped apart by gravitational forces in what astrophysicists call a “tidal disruption event”.
As the star’s matter is pulled around and into the hole, it releases a bright flash of electromagnetic radiation.
To date, astronomers have detected the light from around 100 of these tidal disruption events in distant galaxies .
Most of these flashes — picked up by ground- and space-based telescopes — were seen in the X-ray and visible part of the spectrum.
In their new study, however, astrophysicist Dr Christos Panagiotou of the Massachusetts Institute of Technology and his colleagues detected a tidal disruption event at infrared wavelengths for the first time ever.
The serendipitous discovery was made when scanning for general, relatively short-lived astronomical events in observations made by NEOWISE.
The flare from the star’s destruction — dubbed “WTP14adbjsh” — reached the solar system near the end of 2014.
Dr Panagiotou explained: “We could see there was nothing at first. Then suddenly, in late 2014, the source got brighter.”
By 2015, he added, the emission “reached a high luminosity, then started going back to its previous quiescence.”
The light, the team determined, had come from NGC 7392, a barred spiral galaxy located some 137 million light-years from Earth, in the direction of the constellation of Aquarius.
This makes the tidal disruption event the closest to Earth on record — being four times nearer to us than its next-closest counterpart.
WTP14adbjsh was also unusual for having come from a relatively young, star-forming galaxy, quite unlike most tidal disruption events, which have typically been detected occurring in less busy galaxies.
It is no surprise, however, that active galaxies would host tidal disruption events — as the stars they produce would provide their central supermassive black holes with plenty of fodder to snack on.
Analysis by the researchers using observations from various telescopes has indicated that the supermassive black hole at the heart of NGC 7392 is around 30 million times more massive than the Sun.
Dr Panagiotou said: “This is almost 10 times larger than the black hole we have at our galactic centre, so it’s quite massive — though black holes can get up to 10 billion solar masses.
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While WTP14adbjsh was visible in infrared observations of NGC 7392, it did not stand out in either X-ray or optical data, and thus was invisible to the kinds of surveys that normally detect such tidal disruption events.
However, the researchers noted, they do not believe that WTP14adbjsh did not originally come in the X-ray or visible parts of the spectrum.
Instead, they believe that these emissions were obscured by the larger amounts of dust typically found in active, star-forming galaxies — dust which absorbed this radiation and re-emitted it in the form of infrared energy.
Accordingly, looking for more transient light sources in the infrared band could help astronomers find more, previously hidden tidal disruption events in star-forming galaxies.
Dr Panagiotou concluded: “Finding this nearby tidal disruption event 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 infrared if we want a complete picture of black holes and their host galaxies.
“Not only have we missed a large amount of these events, but they were probably happening in a different environment.
“It shows that infrared-detected tidal disruption events may be the key to obtaining a complete understanding of these celestial sources.”
The full findings of the study were published in Astrophysical Journal Letters.
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