There’s something ‘up there’ that doesn’t quite add up. Something that systematically breaks the laws of physics and that, for all we know, shouldn’t even exist. Astronomers call them ‘ultraluminous X-ray sources’ (ULX). And it is not for less, because with their excessive brightness they emit about 10 million times more energy than the Sun. Several of these strange objects are known, but it was thought that their impossible brightness was a kind of mirage. Detailed analysis of one of them, however, has shown the opposite: ULXs are very real, and they really break the limits set by Physics.
The extreme brightness of these objects, in effect, breaks into pieces a law, known as ‘the Eddington limit‘ which precisely regulates how bright an object can get in relation to its size. According to scientists, if something were to breach this limit, the energy released would blow it apart. Which, of course, is not the case with ULXs, which according to NASA “regularly exceed the Eddington limit between 100 and 500 times, leaving scientists baffled.”
The latest observations with X-ray telescopes carried out by the North American agency and recently published in ‘The Astrophysical Journal’ confirm, indeed, that the extraordinary brightness of a particular ULX, called M82 X-2, is absolutely real, and not some kind of optical illusion as some previous theories suggested. And they also confirm, of course, that it greatly exceeds the Eddington limit. One hypothesis suggests that this ‘impossible’ brightness is due to the ULX’s strong magnetic fields. But scientists can only prove this idea 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.
This is how the Eddington limit works
When emitted by an object, particles of light, called photons, exert a small push ‘outwards’. So if any cosmic object (for example, an ULX) emits a sufficient amount of light per square meter, the outward push of photons can overcome the inward pressure of gravity trying to compress it. That is precisely the Eddington limit. And when it is reached, the light from the object acquires enough force to push any gas or material that tries to fall towards it.
This change is very significant, because the material that falls on an ULX is, at the same time, the source of its brightness. It’s the same thing that happens with black holes: when their strong gravity pulls in surrounding gas and dust, those materials accelerate, heat up, and begin to radiate light.
they are not black holes
That’s why scientists initially thought ULXs were black holes surrounded by glowing rings of gas. But in 2014, data from the NuSTAR Telescope Network revealed that M82 X-2 is not a black hole, but a neutron star. Like black holes, neutron stars form when a star dies and collapses, squeezing one or more solar masses into an area not much larger than an average-sized city.
This incredible density also creates an exceptional gravitational pull on the surface of the neutron star, about 100 trillion times stronger than that of Earth. Thus, the gas and other material dragged by that enormous gravity are accelerated at millions of kilometers per hour, releasing tremendous energy when they strike the surface of the star. (A simple candy falling on the surface of a neutron star would hit it with the energy of a thousand hydrogen bombs.) And that produces the high-energy X-ray light detected by NASA with NuSTAR.
Now, the researchers have returned to study M82 X-2, and apart from confirming that its brightness is not an illusion, they have discovered that this neutron star is ‘parasitizing’ a nearby star, from which it steals about 9 billion trillions of tons of material, equal to one and a half times the mass of the Earth, every year.
Knowing how much material hits the neutron star’s surface, the scientists were able to estimate how bright the ULX should be, and their calculations matched independent measurements of its brightness. The work therefore confirmed that the M82 X-2 exceeds the Eddington limit.
But how is this possible? Something, then, must be happening to the M82 X-2 so that it can happily ‘skip’ the laws of physics. In an attempt to find an explanation, scientists propose that the strong magnetic field of the neutron star could be changing the shape of the atoms, allowing the star to stay in one piece even beyond the Eddington limit.
“These observations – says Matteo Bachetti, an astrophysicist at the Italian National Institute for Astrophysics and lead author of the study – allow us to see the effects of these incredibly strong magnetic fields that we could never reproduce on Earth with current technology. 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 decide to show us its secrets.