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| In this illustration of an ultra-luminous X-ray source, two rivers of hot gas are pulled onto the surface of a neutron star. Strong magnetic fields, shown in green, may change the interaction of matter and light near neutron stars’ surface, increasing how bright they can become. Credit: NASA/JPL-Caltech |
A bizarre 'ultraluminous X-ray source' shines millions of times brighter than
the sun, breaking a physical law called the Eddington limit, a new study
finds.
Exotic cosmic objects known as ultra-luminous X-ray sources 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.
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.
Breaking the Limit
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.
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 dropped 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 NuSTAR detects.
The recent study targeted the same ULX at the heart of the 2014 discovery
and found that, like a cosmic parasite, M82 X-2 is stealing about 9 billion
trillion tons of material per year from a neighboring star, or about 1 1/2
times the mass of Earth. Knowing the amount of material hitting 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 M82 X-2 exceeds the Eddington limit.
No Illusions
If scientists can confirm of the brightness of more ULXs, they may put to
bed a lingering hypothesis that would explain the apparent brightness of
these objects without ULXs having to exceed the Eddington limit. That
hypothesis, based on observations of other cosmic objects, posits 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 sort of optical illusion, making it falsely appear as though
the ULX were exceeding the brightness limit.
Even if that’s the case for some ULXs, an alternative hypothesis supported
by the new study suggests that strong magnetic fields distort the roughly
spherical atoms into elongated, stringy shapes. This would reduce the
photons’ ability to push atoms away, ultimately increasing an object’s
maximum possible brightness.
“These observations let us see the effects of these incredibly strong
magnetic fields that we could never reproduce on Earth with current
technology,” said Matteo Bachetti, an astrophysicist with the National
Institute of Astrophysics’ Cagliari Observatory in Italy and lead author on
the recent study. “This is the beauty of astronomy. Observing the sky, we
expand our ability to investigate how the universe works. On the other hand,
we cannot really set up experiments to get quick answers; we have to wait
for the universe to show us its secrets.”
Reference:
Matteo Bachetti et al, Orbital Decay in M82 X-2, The Astrophysical Journal
(2022).
DOI: 10.3847/1538-4357/ac8d67
Tags:
Space & Astrophysics