This illustration shows SS 433, a black hole or neutron star, pulling material away from its companion star. The stellar matter formed a disk around SS 433, and some of the material was thrown into space in the form of two thin (pink) jets traveling in opposite directions from SS 433. Source: DESY / Science Communications Lab
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It’s hard to miss a beam of flashlights pointed straight at you. But when viewed from the side, this light looks much darker. The same is true for some cosmic objects: like flashlights, they shine mainly in one direction and look very different depending on whether the beam is directed away from Earth (and the nearest space telescope) or straight there.
New data from NASA’s NuSTAR space observatory shows that this phenomenon applies to some of the most famous X-ray emitters in the local universe: ultraluminous X-ray sources, or ULX. Most cosmic objects, including stars, emit very little X-ray light, especially in the high energy range NuSTAR sees. On the other hand, ULX is like an X-ray beacon through the darkness. To be considered ULX, a source must have an X-ray luminosity that is approximately one million times brighter than the total sunlight output (at all wavelengths). ULX is so bright that it can be seen millions of light years away in other galaxies.
The new study shows that the object known as SS 433, which is in the Milky Way and only about 20,000 light-years from Earth, is ULX, although it appears to be about 1,000 times darker than the assumed minimum threshold.
According to the study, this drawback is a perspective gimmick: the high-energy X-rays from SS 433 are initially enclosed in two gas cones extending outward from opposite sides of the central object. These cones resemble mirror bowls surrounding a flashlight bulb: they combine X-ray light from SS 433 into narrow beams until they appear and are recognized by NuSTAR. However, because the cone is not pointing directly at the earth, NuSTAR cannot see the object’s full brightness.
If ULX that is relatively close to Earth can hide its true brightness due to its orientation, then it is likely that there are more ULX – especially in other galaxies – that are disguised in the same way. That means the ULX population as a whole should be much larger than what scientists currently observe.
Cone of darkness
About 500 ULX have been found in other galaxies, and their distance from Earth often makes it nearly impossible to tell what type of object is producing X-rays. The x-rays likely come from large amounts of gas heated to extreme temperatures when attracted to the gravity of a very dense object. This object could be a neutron star (the remnant of a collapsed star) or a small black hole no more than 30 times the mass of our sun. The gas forms a disk around the object, such as water orbiting a drain. The friction on the disc increases its temperature, causing it to radiate and sometimes get so hot that the X-ray system explodes. The faster the material falls on the central object, the brighter the X-ray.
Astronomers suspect that the object at the heart of SS 433 is a black hole that has a mass about 10 times our sun. What is certain is that it cannibalizes a large nearby star, with its gravity sucking in material at high speeds: in one year, SS 433 steals its neighbor equivalent to roughly 30 Earth masses, making it the most voracious black hole or neutron star known to our galaxy. .
“This thing has long been known to eat at phenomenal rates,” Middleton said. “This is what distinguishes ULX from other objects and may be the main reason for the large amount of X-rays we see from these objects.”
The object in SS 433 has eyes bigger than its belly: it steals more material than it can use. Some of the excess material is blown off the disc and forms two halves on opposite sides of the disc. Inside each of them is a conical cavity that opens into space. It is a cone that fuses high-energy X-ray light into a beam. Anyone looking down on one of the cones will see a clear ULX. Although the cones are made entirely of gas, they are so thick and massive that they act like lead cladding in the X-ray space, preventing X-rays from passing through them laterally.
The SS 433 cosmic object contains a bright X-ray light source surrounded by two hot gas clefts. Gas binds light in rays that point in the opposite direction from its source. SS 433 tilts periodically, causing X-rays to point toward the earth. Photo credit: NASA / JPL-Caltech
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Scientists have suggested that some ULX may not be visible because of this. SS 433 offers a unique opportunity to test this idea because it wobbles around its axis like a peak – a process astronomers call precession.
Most of the time, both SS 433 cones point away from Earth. But because of the way SS 433 precesses, the cone periodically tilts slightly toward Earth, allowing scientists to see some of the X-ray light emanating from the top of the cone. In the new study, scientists looked at how the X-rays observed by NuSTAR changed as SS 433 moved. They showed that if the cone continued to tilt toward Earth so scientists could see it directly, they would see enough X-ray light to officially designate SS 433 as ULX.
Black holes feeding at extreme speeds have shaped the history of our universe. Supermassive black holes, which are millions to billions of times the mass of the sun, can have a major impact on their host galaxies if they eat. Early in the history of the universe, some of these massive black holes probably devoured as fast as SS 433 and released massive amounts of radiation that altered the local environment. Outflows (like the cones in SS 433) distribute material that can eventually form stars and other objects.
Illustration of the NuSTAR spacecraft, which has a 10 meter long mast separating the optical module (right) from the detector in the focal plane (left). This separation is necessary for the X-ray detection method. Photo credit: NASA/JPL-Caltech
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But because these fast-eating giants live in galaxies so far away (those at the heart of the Milky Way don’t eat much at the moment), they remain difficult to study. With SS 433, scientists have found a miniature example of this process, closer to home and easier to study, and NuSTAR has provided new insight into the activity taking place there.
“When we first launched NuSTAR, no one expected ULX to be such a rich research area for us,” said Fiona Harrison, senior researcher at NuSTAR and professor of physics at Caltech in Pasadena, California. “But NuSTAR is unique in that it can see almost the entire wavelength range of X-rays emitted by these objects, and that gives us insight into the extreme processes that must drive them.”
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory, a division of Caltech, for the agency’s Science Mission Directorate in Washington. NuSTAR was developed in collaboration with the Technical University of Denmark and the Italian Space Agency (ASI). The spacecraft was built by the Orbital Sciences Corporation in Dulles, Virginia (now part of Northrop Grumman). The NuSTAR Mission Operations Center is located at the University of California, Berkeley, and the official data archive is located at the NASA High Energy Astrophysics Science Archive Research Center. ASI provides mission earth stations and mirror archives.
NASA satellite discovers mystery that disappeared in an instant
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www.nasa.gov/mission_pages/nustar/main/index.html
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Quote: Seeing some cosmic x-ray emitters can be a matter of perspective (2021, 9 July), accessed 10 July 2021 from https://phys.org/news/2021-07-cosmic-x-ray-emitters-perspective.html
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