'Ultraluminous X-Ray Source' Provides New Clues About Neutron Stars

Monday, 26 February 2018 - 7:52PM
Space
Astronomy
Monday, 26 February 2018 - 7:52PM
'Ultraluminous X-Ray Source' Provides New Clues About Neutron Stars
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X-ray: NASA/CXC/Wesleyan Univ./R.Kilgard, et al; Optical: NASA/STScI
If the phrase "ultraluminous x-ray source" sounds cool to you, rest assured the actual thing is just as fascinating. 

First discovered back in the 1980s, a ULX is an extremely bright source of x-rays found on the outskirts of some galaxies, which were initially thought to be black holes. And while some likely are, at least three have been discovered to be much less massive neutron stars, which should be impossible. But they exist, and now astronomers have just found a fourth.

And this new neutron star ULX, from the M51 "Whirlpool Galaxy" 28 million lightyears away, revealed some additional evidence regarding how neutron stars - the high-density collapsed cores of stars - can shine as brightly as they do.



A study published in Nature Astronomy by Caltech researchers suggests that a phenomenon called "cyclotron resonance scattering" could explain how a neutron star could consume enough matter to give off such powerful x-rays. Because normally, stars of a certain density can only absorb so much matter from surround debris - Murray Brightman, the study's lead author and a postdoctoral scholar at Caltech, explained this in a press release:

Opening quote
"In the same way that we can only eat so much food at a time, there are limits to how fast neutron stars can accrete matter. But ULXs are somehow breaking this limit to give off such incredibly bright X-rays, and we don't know why."
Closing quote


With cyclotron resonances, charged particles will circle around in a star's magnetic field and create telltale signs in the star's light patterns. Black holes don't have magnetic fields, but neutron stars do, and it could be these strong magnetic fields which are pulling in enough matter that neutron stars could "eat" even more material, breaking the Eddington limit at which a star shouldn't be able to draw in any more matter.



The end result is that from a distance (like, say, 28 million lightyears), it's not easy to tell a neutron star from a black hole when a ULX is discovered. It was only after NASA's Chandra X-Ray Observatory discovered a dip in the ULX's light spectrum that suggested a neutron star, likely caused by the star's magnetic field. 

So there's still a lot to be learned, but we're getting better at telling the two types of cosmic monstrosities apart.
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