If you’re looking for some of the brightest objects in the cosmos, squint no further than ultra-bright X-ray sources, or ULXs.
These places of overwhelming luminosity have long intrigued astronomers because they appear exceed what is known as the Eddington limit, which limits the luminosity of an object by up to 500 times depending on its mass.
Many scientists, however, have simply attributed this to an optical illusion. But now, in a first-of-its-kind study published in The Astrophysical Journalastronomers used NASA’s Nuclear Spectroscopic Telephone Array (NuSTAR) to observe a ULX-designated M82 X-2 that is ten millions times brighter than the Sun, and confirmed that it does indeed exceed Eddington’s limit – no trick of the light required.
Better still, they believe they have discovered the mechanism behind their staggering luminosity: magnetic fields so ridiculously strong that they are impossible to imitate in a laboratory.
“These observations allow us to see the effects of these incredibly strong magnetic fields that we could never replicate on Earth with current technology,” Matteo Bachetti, an astrophysicist at the Cagliari Astronomical Observatory in Italy, said in a statement from the agency. Nasa.
But first, let’s unpack the Eddington limit. In a nutshell, it describes a delicate balance between the outward thrust of an object’s light-producing radiation and the inward pull of its gravity, as in a star. If bright enough, the outgoing photons of light can actually overwhelm the object’s gravity, preventing wayward matter from being pulled into its orbit and suspending them in equilibrium.
As such, astronomers thought ULXs were black holes that surrounded themselves with enough gas and dust that gradually warmed up over time, eventually emitting light. This explanation would prevent Eddington’s limit from being challenged.
But in 2014, it was discovered that M82 X-2 was actually a neutron star, the incredibly dense core of a once massive star that collapsed in on itself without forming a black hole. As some of the densest objects in the universe, neutron stars exert a gravitational pull about 100 trillion times stronger than Earth’s.
So instead, the light from the ULX can be produced by gas and dust blasted onto the neutron star’s surface at millions of miles per hour.
It’s a ton of light, to put it mildly. According to NASA, a marshmallow-sized object hitting such a star would release the energy of a thousand hydrogen bombs.
Proving that this would be enough to produce enough light to make up an ULX, however, is a whole other Pandora’s box.
For their study, the astronomers determined that M82 X-2 was siphoning about 1.5 Earth masses of material per year from a nearby star — a lot, in other words. Through clever calculations, they estimated that all that mass bombarding the star’s surface with neutron would be bright enough to match the luminosity of actual ULX observations, proving that it does indeed exceed Eddington’s limit.
Previous hypotheses suggested that ULXs only appeared exceptionally bright due to the buildup of gas and dust that formed cones that amplified their underlying light source, especially if pointed toward Earth.
Now astronomers are more confident in a recent hypothesis suggesting that, at least with this ULX, the ridiculously strong magnetic field of its neutron star could distort the shape of nearby atoms, allowing them to pass through the otherwise crushing thrust of the ‘star. emitting photons and come crashing down on its surface.
Whatever the cause, astronomers are at least armed with sufficient evidence that Eddington’s limit has been exceeded – but only further observations will be able to substantiate their findings for ULXs in general.
“That’s the beauty of astronomy,” Bachetti said. “We can’t really set up experiments to get quick answers. We have to wait for the universe to reveal its secrets to us.”
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