Super-massive black holes do not emit any of their own light, hence the word "black" in their name. Though, many black holes pull in, or accrete, close material, and give off great bursts of X-rays. Together, these active black holes all over the sky can be supposed of a cosmic choir, singing in the language of X-rays. Their "song" is what scientists call the cosmic X-ray background.
The blue dot in this image of galaxies, known as the COSMOS field, shows galaxies that surround supermassive black holes producing high-energy X-ray. They were discovered by NASA's Nuclear Spectroscopic Array or known as NuSTAR, which spotted 32 such black holes in this COSMOS field and has detected hundreds across the whole sky so far. The other colored dots are galaxies that host black holes producing lower-energy X-rays and were discovered by NASA's Chandra X-ray Observatory. Chandra data show X-rays with energies between 0.5 to 7 kilo-electron volts while NuSTAR data show X-rays between 8 to 24 kilo-electron volts.
To date, NASA's Chandra mission has achieved to identify many of the different black holes donating to the X-ray background, but the ones that emit high-energy X-rays, those with the maximum-pitched "voices" have stayed elusive. NuSTAR, has, for the first time, begun to find large numbers of the black holes emitting the high-energy X-rays. More precisely, NuSTAR has made important progress in solving the high-energy X-ray background.
Fiona Harrison, Benjamin M. Rosen Professor of Physics and Astronomy at Caltech, the principal investigator of NuSTAR, and lead author of a new study explaining the discovery in an upcoming topic of The Astrophysical Journal says, "We have gone from resolving just 2% of the high-energy X-ray background to 35%. We can see the most unnoticed black holes, hidden in dense gas and dust."
The consequences will finally help astronomers study how the development patterns of supermassive black holes change with the passage of time, an important factor in the development of black holes and the galaxies that host them. For example, the supermassive black hole at the middle of our Milky Way galaxy is inactive now, but at some point in the past, it would have tapped the gas and bulked up in size.
As black holes develop, their strong gravity pulls matter toward them. The matter heats up to enormously high temperatures and particles get triggered to close to the speed of light. Together, these methods make the black hole surroundings spark with X-rays. A supermassive black hole with a plenty supply of fuel, or gas, will emit more high-energy X-rays. NuSTAR is the first telescope which has the capability of aiming these high-energy X-rays into sharp pictures.
Harrison says, "Before NuSTAR, the X-ray background in high-energies was just one distortion with no resolved bases. To untie what is going on, you have to find and count up the individual causes of the X-rays."
Co-author Daniel Stern, the project researcher for NuSTAR at JPL says, "We knew this cosmic choir had a tough high-pitched part, but we still don't know if it comes from a lot of lesser, quiet singers, or a few with loud voices. Now, cheers to NuSTAR, we are achieving a better understanding of the black holes and beginning to address these questions."
High-energy X-rays can expose what lies around the most covered supermassive black holes, which are else hard to see. In the same way that medical X-rays can travel through your skin to take images of bones, NuSTAR can see through the gas and dust around black holes, to get a deeper sight of what is going on inside. With NuSTAR's more complete picture of supermassive black hole populations, scientists can begin to problem together how these objects develop and change over time. When did they start and stop increasing? What is the scattering of the gas and dust that both feed and hide the black holes?
The scientists assume that over time, NuSTAR will be capable of resolving more of the high-energy X-ray background, and better translate the X-ray song of the universe's black holes.