WASHINGTON — An international team of astronomers found new evidences that a few ultraluminous X-ray (ULXs) sources, glowing with X-ray light equal to the total output at all wavelengths of millions of suns, are relatively small neutron stars, and one of the neutron star consumes more matter than expected.
The study, published in the latest released journal Nature Astronomy, showed that a newly characterized ULX in M51 galaxy, based on images taken by NASA’s Chandra X-ray Observatory and the Hubble Space Telescope, has provided new clues about how these objects can shine so brightly.
Previously, three ULXs have been identified as neutron stars because of their regular “pulsations” in the X-ray emission, a behavior exhibited by neutron stars but not black holes.
At first, researchers thought many of ULXs were black holes with masses between about a hundred and a hundred thousand times that of the sun, while the neutron stars are only 1.5 times the mass of the sun.
Neutron stars are extremely dense objects (a teaspoon would weigh more than a billion tons,) and tend to pull surrounding materials away from companion stars.
As this material falls toward the neutron star, it heats up and glows with X-rays. It won’t stop pulling until the resulting X-ray light becomes so intense that pushes the matter away, a point called the Eddington limit.
But this newly identified neutron star in M51 is surpassing the Eddington limit for a common neutron star, pulling more matter than it should do, according to astronomers.
Then, they discovered an unusual dip in this ULX’s X-ray spectrum that is likely caused by a process when charged particles circle around in a magnetic field, resulting in a “cyclotron line.”
According to the astronomers, if the charged particles are positively charged protons, the magnetic fields around the neutron star are extremely strong, much stronger than the strongest magnetic fields produced by neutron stars.
They explained that this may help break the Eddington limit, reducing the pressure from a ULX’s X-rays that pushes away matter, and allowing the neutron star to consume more matter than expected.
But if the charged particles are negatively charged electrons, the magnetic field strength around the neutron star would be about 10,000 times less strong, and thus not powerful enough for the flow onto this neutron star to break the Eddington limit.
To further address this mystery, the researchers are planning to acquire more X-ray data on the ULX in M51 and in other ULXs to determine the cyclotron line’s origin.
Scientists from American space agency NASA, European Space Agency, and those from Britain, France and Italy have participated in the research.