Black Holes are NICER Than You Think!
Weâre learning more every day about black holes thanks to one of the instruments aboard the International Space Station! Our Neutron star Interior Composition Explorer (NICER) instrument is keeping an eye on some of the most mysterious cosmic phenomena.
Weâre going to talk about some of the amazing new things NICER is showing us about black holes. But first, letâs talk about black holes â how do they work, and where do they come from? There are two important types of black holes weâll talk about here: stellar and supermassive. Stellar mass black holes are three to dozens of times as massive as our Sun while supermassive black holes can be billions of times as massive!
Stellar black holes begin with a bang â literally! They are one of the possible objects left over after a large star dies in a supernova explosion. Scientists think there are as many as a billion stellar mass black holes in our Milky Way galaxy alone!
Supermassive black holes have remained rather mysterious in comparison. Data suggest that supermassive black holes could be created when multiple black holes merge and make a bigger one. Or that these black holes formed during the early stages of galaxy formation, born when massive clouds of gas collapsed billions of years ago. There is very strong evidence that a supermassive black hole lies at the center of all large galaxies, as in our Milky Way.
Imagine an object 10 times more massive than the Sun squeezed into a sphere approximately the diameter of New York City â or cramming a billion trillion people into a car! These two examples give a sense of how incredibly compact and dense black holes can be.
Because so much stuff is squished into such a relatively small volume, a black holeâs gravity is strong enough that nothing â not even light â can escape from it. But if light canât escape a dark fate when it encounters a black hole, how can we âseeâ black holes?
Scientists canât observe black holes directly, because light canât escape to bring us information about whatâs going on inside them. Instead, they detect the presence of black holes indirectly â by looking for their effects on the cosmic objects around them. We see stars orbiting something massive but invisible to our telescopes, or even disappearing entirely!
When a star approaches a black holeâs event horizon â the point of no return â itâs torn apart. A technical term for this is âspaghettificationâ â weâre not kidding! Cosmic objects that go through the process of spaghettification become vertically stretched and horizontally compressed into thin, long shapes like noodles.
Scientists can also look for accretion disks when searching for black holes. These disks are relatively flat sheets of gas and dust that surround a cosmic object such as a star or black hole. The material in the disk swirls around and around, until it falls into the black hole. And because of the friction created by the constant movement, the material becomes super hot and emits light, including X-rays. Â
At last â light! Different wavelengths of light coming from accretion disks are something we can see with our instruments. This reveals important information about black holes, even though we canât see them directly.
So what has NICER helped us learn about black holes? One of the objects this instrument has studied during its time aboard the International Space Station is the ever-so-forgettably-named black hole GRS 1915+105, which lies nearly 36,000 light-years â or 200 million billion miles â away, in the direction of the constellation Aquila.
Scientists have found disk winds â fast streams of gas created by heat or pressure â near this black hole. Disk winds are pretty peculiar, and we still have a lot of questions about them. Where do they come from? And do they change the shape of the accretion disk?
Itâs been difficult to answer these questions, but NICER is more sensitive than previous missions designed to return similar science data. Plus NICER often looks at GRS 1915+105 so it can see changes over time.
NICERâs observations of GRS 1915+105 have provided astronomers a prime example of disk wind patterns, allowing scientists to construct models that can help us better understand how accretion disks and their outflows around black holes work.
NICER has also collected data on a stellar mass black hole with another long name â MAXI J1535-571 (we can call it J1535 for short) â adding to information provided by NuSTAR, Chandra, and MAXI. Even though these are all X-ray detectors, their observations tell us something slightly different about J1535, complementing each otherâs data!
This rapidly spinning black hole is part of a binary system, slurping material off its partner, a star. A thin halo of hot gas above the disk illuminates the accretion disk and causes it to glow in X-ray light, which reveals still more information about the shape, temperature, and even the chemical content of the disk. And it turns out that J1535âs disk may be warped!
Image courtesy of NRAO/AUI and Artist: John Kagaya (Hoshi No Techou)
This isnât the first time we have seen evidence for a warped disk, but J1535âs disk can help us learn more about stellar black holes in binary systems, such as how they feed off their companions and how the accretion disks around black holes are structured.
NICER primarily studies neutron stars â itâs in the name! These are lighter-weight relatives of black holes that can be formed when stars explode. But NICER is also changing what we know about many types of X-ray sources. Thanks to NICERâs efforts, we are one step closer to a complete picture of black holes. And hey, thatâs pretty nice!
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