These objects are more than 100 times brighter than they could be. Observations from the agency’s NuSTAR X-ray telescope support a possible solution to this puzzle.
Exotic cosmic objects known as ultraluminous X-ray sources produce about 10 million times more energy than the Sun. They are so radiant, in fact, that they appear to exceed a physical limit called the Eddington limit, which limits the brightness of an object in function of its mass. Ultraluminous X-ray sources (ULX for short) regularly exceed this limit by 100 to 500 times, leaving scientists baffled.
In a recent study published in The Astrophysical Journal, researchers report the first-of-its-kind measurement of an ULX taken with NASA’s Nuclear Spectroscopic Telescope Array ( NuSTAR ). The finding confirms that these light emitters are as bright as they appear and that they exceed the Eddington limit. One hypothesis suggests that this boundary-breaking brightness is due to the ULX’s strong magnetic fields. But scientists can prove this idea only through observations: Up to billions of times more powerful than the strongest magnets ever made on Earth, ULX magnetic fields cannot be reproduced in a laboratory.
Particles of light, called photons, exert a small push on the objects they encounter. If a cosmic object like an ULX emits enough light per square foot, the outward pull of photons can overcome the inward pull of the object’s gravity. When this happens, an object has reached the Eddington limit and, in theory, light from the object will push any gas or other material that falls towards it. That change, when light overcomes gravity, is significant, because the material falling on an ULX is the source of its glow. This is something scientists often see in black holes: When stray gas and dust are pulled in by their strong gravity, those materials can heat up and radiate light. Scientists used to think that ULXs must be black holes surrounded by glowing chests of gas. But in 2014, NuSTAR data revealed that an ULX by the name of M82 X-2 is actually a less massive object called a neutron star. Like black holes, neutron stars form when a star dies and collapses, packing more than the mass of our Sun into an area not much larger than an average-sized city.
This incredible density also creates a gravitational pull on the neutron star’s surface about 100 trillion times stronger than the gravitational pull on Earth’s surface. The gas and other material pulled by that gravity is accelerated at millions of miles per hour, releasing tremendous energy when they hit the surface of the neutron star. (A marshmallow falling on the surface of a neutron star would hit it with the energy of a thousand hydrogen bombs.) This produces the high-energy X-ray light that NuSTAR detects. The recent study pointed to the same ULX at the heart of the 2014 discovery and found that, like a cosmic parasite, M82 X-2 is stealing about 9 trillion trillion tons of material per year from a neighboring star, or about 1 1/2 times the mass of the Earth. Knowing how much material hits the neutron star’s surface, scientists can estimate how bright the ULX should be, and their calculations match independent measurements of its brightness. The work confirmed that M82 X-2 exceeds the Eddington limit.
If scientists can confirm the brightness of more ULXs, they may confirm a persistent hypothesis that would explain the apparent brightness of these objects without the ULXs having to exceed the Eddington limit. That hypothesis, based on observations of other cosmic objects, postulates that strong winds form a hollow cone around the light source, concentrating most of the emission in one direction. If pointed directly at Earth, the cone could create a kind of optical illusion, falsely making it appear as if the ULX is exceeding the brightness limit. Even if that’s the case for some ULXs, an alternate hypothesis supported by the new study suggests that strong magnetic fields distort roughly spherical atoms into elongated, stringy shapes. This would reduce the ability of photons to push atoms away, and ultimately increase the maximum possible brightness of an object.
“These observations allow us to see the effects of these incredibly strong magnetic fields that we could never reproduce on Earth with current technology,” said Matteo Bachetti, an astrophysicist at the Cagliari Observatory of the National Institute for Astrophysics in Italy and lead author of the recent study. . “This is the beauty of astronomy. By observing the sky, we expand our ability to investigate how the universe works. On the other hand, we can’t really set up experiments to get quick answers; we have to wait for the universe to show us its secrets.” More about the mission A Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory in Southern California for the agency’s Science Mission Directorate in Washington, NuSTAR was developed in partnership with the Danish Technical University and the Space Agency. Italian (ASI). The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. NuSTAR’s 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 at the Space Flight Center. Goddard of NASA. ASI provides the mission ground station and a mirror data file.