According to foreign media reports, ultra-optical X-ray sources (ULX) are easy to detect when viewed directly head-on, but if they are slightly farther away from Earth, they may be hidden. It's hard to miss the beam of light pointing straight at your flashlight. But from the side, the light appears significantly dimmer. The same is true for some cosmic celestial bodies. Like flashlights, they radiate primarily in one direction, and they look very different depending on whether the beam is pointing away from Earth (and nearby space telescopes) or directly at Earth. New data from NASA's NuSTAR space telescope suggest that this phenomenon is true for some of the most prominent X-ray emitters in the local universe: superluminal X-ray sources.
Most cosmic objects, including stars, emit very few X-rays, especially in the high-energy range observed by NuSTAR. In contrast, ultra-optical X-ray sources cut through the darkness like X-ray lighthouses. To be considered an ultra-low frequency X-ray source, its X-ray brightness must be about a million times brighter than the sun's total light output (at all wavelengths). Ultra-light X-ray sources are so bright that they can be seen in other galaxies millions of light-years away.
The new study suggests that the object known as SS 433, located in the Milky Way, is only about 20,000 light-years from Earth and is a superluminous X-ray source, although its brightness is considered to be about 1/1/1000 of the minimum threshold for a superluminal X-ray source.
According to the study, this bleakness is a technique of perspective. High-energy X-rays from SS 433 were initially confined to two gas cones, extending outward from opposite sides of the central object. They aggregate X-rays from SS 433 into a narrow beam of light until it "escapes" and is detected by NuSTAR. But because these cones don't point directly at Earth, NuSTAR can't see the full brightness of the object.
Matt Middleton, professor of astrophysics at the University of Southampton in the UK and lead author of the study, said: "We have long suspected that some ultra-optical X-ray sources emit light in narrow columns of light, rather than in all directions like bare bulbs. In our study, we confirmed this hypothesis, showing that SS 433 would qualify as a superluminous X-ray source for a face-to-face observer. ”
If a superluminous X-ray source relatively close to Earth is able to hide its true brightness because of its orientation, there may be more superluminal X-ray sources —especially in other galaxies—"masked" in a similar way. This means that the total number of ultra-optical X-ray sources should be much more than scientists are currently observing.
Scientists have found about 500 superluminous X-ray sources in other galaxies, and their distance from Earth means it's often nearly impossible to tell what type of object produces X-ray emission. X-rays may come from large amounts of gas pulled in by the gravitational pull of a very dense celestial body and heated to extreme temperatures. This object could be a neutron star (the remnants of a collapsed star) or a small black hole, a black hole with a mass no more than 30 times that of our sun. The gas forms a disk around the celestial body, like water circling a drainage port. Friction in the disk causes the temperature to rise, causing it to radiate, sometimes so hot that the system erupts with X-rays. The faster the material falls onto the central object, the brighter the X-rays become.
Astronomers suspect that the object at the center of SS 433 is a black hole with a mass about 10 times that of our Sun. To be sure, it is devouring a large nearby star, and its gravitational pull sucks away the material at an extremely fast rate. During the year, SS 433 "stole" the equivalent of 30 times the mass of Earth from its "neighbors," making it the most "greedy" black hole or neutron star known in our galaxy.
Middleton said: "It has long been known that this celestial body is 'eating' at an alarming rate. That's where ULX differs from other objects, and that's probably the root cause of the massive amount of X-rays we see from them. ”
SS 433 "steals" more substance than it can consume. Some excess material is blown away from the disk and forms two hemispheres on either side of the disk. Within each hemisphere there is a cone-like void that opens up to space. These cones aggregate high-energy X-rays into a single beam. Anyone looking directly at one of the cones will see a distinct ultra-low energy X-ray. Although only made up of gas, these cones are so thick and so large that they act like lead plates in an X-ray examination chamber, blocking X-rays from passing through them to the outside.
Scientists suspect that some ULXs may be hidden for this reason. SS 433 offers a unique opportunity to test this idea because it acts like a gyroscope, wobbling on its axis — a process that astronomers call "precession."
Most of the time, both cones of SS 433 point far from Earth. But because of the way SS 433 moves forward, one of the cones regularly tilts slightly toward Earth, so scientists can see a little bit of X-ray emanating from the top of the cone. In the new study, the scientists looked at how the X-rays seen by NuSTAR changed as SS 433 moved. They showed that if the cone continued to tilt toward Earth so that scientists could look down directly at it, they would see enough X-rays to officially refer to SS 433 as ULX.
Black holes that "eat" at extreme speeds have shaped the history of our universe. Supermassive black holes have millions to billions of times the mass of the Sun, and when they "eat, they can profoundly affect their host galaxies. Early in the history of the universe, some of these massive black holes may have "eaten" at the rate of SS 433, releasing large amounts of radiation that reshaped their surroundings. Outflows, such as cones in SS 433, redistribute the material, which could eventually form stars and other celestial bodies.
But because these fast-draining "behemoths" inhabit incredibly distant galaxies, they remain difficult to study. With SS 433, scientists have discovered a miniature example of this process, and NuSTAR has provided new insights into the activities taking place there.
"When we launched NuSTAR, I don't think anyone thought ULX would be such a rich field of research for us," said Fiona Harrison, a professor of physics at Caltech, nustar's lead researcher. "But what makes NuSTAR unique is that it can see almost all of the X-ray wavelengths emitted by these celestial bodies, which gives us insight into the extreme processes that drive them."