Black holes have voracious appetites. As these objects, which are so dense that not even light can escape their gravitational pull, chow down on gas and stellar debris, they emit powerful bursts of X-rays, creating what is known as a cosmic X-ray background — a “song” of X-rays being emitted by millions of black holes, which fills the entire sky.
Although astrophysicists have long known about this “cosmic choir,” identification of individual singers has proven elusive.
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Now, data gathered by NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) — a space-based X-ray telescope — is finally helping scientists pinpoint the black holes emitting high-energy X-rays, thereby taking a significant step toward resolving the cosmic X-ray background.
“Before NuSTAR, the X-ray background in high energies was just one blur with no resolved sources,” lead author Fiona Harrison, the principal investigator of NuSTAR at Caltech in Pasadena, said in a statement.  “We've gone from resolving just two percent of the high-energy X-ray background to 35 percent. ... We can see the most obscured black holes, hidden in thick gas and dust.”
pia20865 The blue dots in this field of galaxies, known as the COSMOS field, show galaxies that contain supermassive black holes emitting high-energy X-rays. Photo: NASA/JPL-Caltech
Black holes emit X-rays when the gas and dust surrounding them gets heated and accelerated to nearly the speed of light. As a black hole grows, so does the amount of its high-energy X-ray emissions. Unsurprisingly, supermassive black holes — those found in the centers of most galaxies, including ours — give off more high-energy X-rays than lone black holes adrift in the dark void of space.
Scientists hope that the new observations will ultimately help them better understand how the feeding patterns of supermassive black holes change over time, providing a clearer picture of their evolution.
“We knew this cosmic choir had a strong high-pitched component, but we still don't know if it comes from a lot of smaller, quiet singers, or a few with loud voices,” co-author Daniel Stern, the project scientist for NuSTAR at NASA's Jet Propulsion Laboratory in Pasadena, California, said in the statement. “Now, thanks to NuSTAR, we're gaining a better understanding of the black holes and starting to address these questions.”