The Puzzling Ultra-Luminous X-ray Sources Breaking the Eddington Limit

Category Astronomy

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Ultra-luminous X-ray sources (ULXs) produce extreme amounts of energy and exceed the Eddington limit by 100 to 500 times, causing scientists to be puzzled. A recent study using NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) confirmed that ULXs definitely break the Eddington limit. This could be due to the ULXs' high magnetic fields, which distort the atoms and reduce the photons' ability to push away, thus allowing for such bright cosmic objects.

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Exotic cosmic objects known as ultra-luminous X-ray sources (ULXs) produce about 10 million times more energy than the Sun. They’re so radiant, in fact, that they appear to surpass a physical boundary called the Eddington limit, which puts a cap on how bright an object can be based on its mass. Ultra-luminous X-ray sources (ULXs, for short) regularly exceed this limit by 100 to 500 times, leaving scientists puzzled.

ULXs are particularly common in galaxies that are undergoing star formation.

In a recent study published in The Astrophysical Journal, researchers report a first-of-its-kind measurement of a ULX taken with NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). The finding confirms that these light emitters are indeed as bright as they seem and that they break the Eddington limit. A hypothesis suggests this limit-breaking brightness is due to the ULX’s strong magnetic fields. But scientists can test this idea only through observations: Up to billions of times more powerful than the strongest magnets ever made on Earth, ULX magnetic fields can’t be reproduced in a lab.

The first ULX was discovered in 1979 by the Einstein Observatory.

Particles of light, called photons, exert a small push on objects they encounter. If a cosmic object like a ULX emits enough light per square foot, the outward push of photons can overwhelm the inward pull of the object’s gravity. When this happens, an object has reached the Eddington limit, and the light from the object will theoretically push away any gas or other material falling toward it.

That switch – when light overwhelms gravity – is significant, because material falling onto a ULX is the source of its brightness. This is something scientists frequently observe in black holes: When their strong gravity pulls in stray gas and dust, those materials can heat up and radiate light. Scientists used to think ULXs must be black holes surrounded by bright coffers of gas. But in 2014, NuSTAR data revealed that a 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 bigger than a mid-size city.

Neutron stars can spin hundreds of times a second.

This incredible density also creates a gravitational pull at the neutron star’s surface about 100 trillion times stronger than the gravitational pull on Earth’s surface. Gas and other material dragged in by that gravity accelerate to millions of miles per hour, releasing tremendous energy when they hit the neutron star’s surface. (A marshmallow dropper in comparison has a puny force about a million billion trillion times weaker than a neutron star’s gravity.) .

Neutron stars can have magnetic fields up to a hundred billion times stronger than that of the Earth.

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