With many hidden in a shroud of gas and dust, how do astronomers spot these shy black holes? A telescope that 'sees' X-rays has the answer, and it has recently uncovered the presence of not just one, but five previously concealed "monster black holes."
These observations were made by NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) satellite observatory. Its telescopic 'eye' was pointed towards nine different potential black hole candidates; clouds of dust and matter that could potentially be spots where black holes are hiding.
NuSTAR saw high-energy X-rays for five of these sites: This signal is suggestive of the presence of a black hole within the dust. Black holes don't technically produce these X-rays since nothing can escape the immense gravitational pull of a black hole. However, light can still escape their accretion disk, a swirling disk of dust and matter that circles the black hole like a tutu. This disk forms when matter is sucked from something in the black hole's surroundings, a neighboring star for example. The matter zooms around the accretion disk, getting faster as it spins, creating friction and heating up. Eventually, the matter gets so hot that it emits a powerful burst of X-rays, a signal NuSTAR can use to identify possible black holes.
Daniel Stern, a NuSTAR project scientist at NASA's Jet Propulsion Laboratory, explained how X-rays can be detected through the dust cloud: "High-energy X-rays are more penetrating than low-energy X-rays, so we can see deeper into the gas burying the black holes. NuSTAR allows us to see how big the hidden monsters are and is helping us learn why only some black holes appear obscured."
These findings also show off the X-ray viewing capabilities of NuSTAR, whose observations would have been impossible in earlier satellite observatories. George Lansbury, the lead author on the study published in The Astrophysical Journal, said, “Thanks to NuSTAR for the first time we have been able to clearly see these hidden monsters that are predicted to be there, but have previously been elusive because of their ‘buried’ state.
“Although we have only detected five of these hidden supermassive black holes, when we extrapolate our results across the whole Universe then the predicted numbers are huge and in agreement with what we would expect to see.”
These findings indicate that there are far more black holes in the universe than we can currently see. With technology like NuSTAR scanning the skies, it looks like we will be able to uncover the existence of celestial objects that were previously undetectable.
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