Exotic cosmic objects known as ultraluminous X-ray sources produce some 10 million times more energy than the Sun. In fact, they are so bright that observations indicate that they exceed a physical limit called the Eddington limit. This limit puts a cap on how bright an object can be based on its mass. Ultraluminous X-ray sources regularly exceed this limit by 100 to 500 times, which has long perplexed scientists.
The international team led by Matteo Bachetti, from the Cagliari Astronomical Observatory in Italy, has used a measurement, the first of its kind, of one of these ultraluminous X-ray sources, made by the NuSTAR (Nuclear Spectroscopic Telescope Array) space telescope. from NASA. The measurement confirms that these light emitters are indeed as bright as they appear and that they are well in excess of the Eddington limit.
One hypothesis suggests that this borderline brightness is due to the strong magnetic fields of the ultraluminous X-ray sources. But this hypothesis can only be verified by astronomical observations. The magnetic fields from an ultraluminous X-ray source cannot be reproduced in a laboratory on Earth, as they are up to billions of times stronger than the strongest magnets made in our world.
Particles of light, photons, exert a small push on the objects they encounter. If a cosmic object such as an ultraluminous X-ray source emits enough light per square meter, 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 light from the object will theoretically push any gas or other material that falls towards it.
That change (when light overcomes gravity) is significant, because material falling on an ultraluminous X-ray source is the source of its glow. This is something that is often seen with black holes: when their strong gravity pulls in gas and dust, these materials can heat up and radiate light. Ultraluminous X-ray sources used to be thought to be black holes surrounded by glowing shells of gas. But in 2014, NuSTAR data revealed that an ultraluminous X-ray source called M82 X-2 is actually a less massive object, specifically one of the type known as a neutron star. Like black holes, neutron stars form when a star dies in a supernova and its core collapses in on itself, squeezing a mass greater than that of our Sun into a sphere with a similar diameter. to what a medium or large city measures from end to end.
This incredible density also creates a gravitational pull on the surface of the neutron star some 100 trillion times stronger than the gravitational pull on the Earth’s surface. The gas and other material dragged by that gravity are accelerated to millions of kilometers per hour, releasing tremendous energy when they collide with the surface of the neutron star. For example, an object as light as the fluffy candy known by names like cloud, gumdrop, or marshmallow, dropped on the surface of a neutron star, would hit it with the energy of a thousand hydrogen bombs. These highly magnified impacts produce the high-energy X-ray light that NuSTAR detects.
The recent study focused on the same ultraluminous X-ray source that was at the heart of the 2014 find. The authors of the new study have found that, like a cosmic parasite, M82 X-2 is stealing some 9 billion trillion tons of material at year of a neighboring star, or what is the same, one and a half times the mass of the Earth. Knowing how much material hits the neutron star’s surface, one can estimate how bright the ultraluminous X-ray source should be, and the calculations agree with independent measurements of its brightness. The work thus confirms that M82 X-2 exceeds the Eddington limit.
The hypothesis supported by the new study suggests that the enormous magnetic fields prevailing in that neutron star and in others are capable of distorting roughly spherical atoms into elongated shapes. This would reduce the ability of photons to push around atoms, ultimately increasing an object’s maximum possible brightness.
In this illustration of an ultraluminous X-ray source, two rivers of hot gas are swept toward the surface of a neutron star. Strong magnetic fields, shown in green, can disrupt the interaction between matter and light near the surface of neutron stars, increasing their brightness. (Image: NASA JPL/Caltech)
