Known as ultraluminous X-ray sources, or ULXs, these objects were long considered to be black holes. Recent research has identified three of them as extremely dense neutron stars, and now this new finding brings the total of known ULXs to four. The discovery also provides clues about how these objects can shine so brightly.
In the 1980s, astronomers found extremely bright X-ray sources in the outer regions of galaxies, far from the supermassive black holes in their hearts. It wasn't until 2014that observations from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and other space telescopes revealed that some of these objects are actually neutron stars, the dense cores left behind after stars explode in fiery supernovas. [The Strangest Things in Space … Ever]
Using NASA's Chandra X-ray Observatory, astronomers targeted a ULX in the Whirlpool Galaxy, a galaxy with a pronounced spiral structure that lies approximately 28 million light-years away. They found an unusual dip in the light spectrum streaming from the object, which they identified as coming from charged particles circling a magnetic field. Because black holes don't have magnetic fields, the dip signaled that the ULX was instead a neutron star, the scientists said.
Neutron stars are extremely dense, city-size objects with masses about 1.5 times that of the sun. A teaspoon of material from a neutron star can weigh up to a billion tons, scientists said in a statement. Their extreme gravity can strip material away from a companion star.
That material heats up and emits X-rays as it is pulled into the neutron star, and eventually, that X-rays light overpowers the star's gravity and pushes material away — a point known as the Eddington limit. For the ULX neutron stars, these X-rays are far stronger than their cousins, and scientists aren't sure why. [Related: How Big Are Neutron Stars, Really?]
"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," Murray Brightman, lead author of the new study and a researcher at the California Institute of Technology, said in the same statement.
"But ULXs are somehow breaking this limit to give off such incredibly bright X-rays, and we don't know why," Brightman said.
The charged particles circling the magnetic field reveal signatures in a star's spectrum of light known as cyclotron lines, which can provide information about the strength of the stellar magnetic field. But there's a catch. Researchers must know whether the lines are caused by positively charged protons or negatively charged electrons. Currently, they don't have enough information to determine which set of particles are involved.
"If the cyclotron line is from protons, then we would know that these magnetic fields around the neutron star are extremely strong and may in fact be helping to break the Eddington limit," Brightman said. A strong magnetic field could help reduce the pressure from the X-rays that push away the ULX's matter, allowing the star to gobble down more material than a typical neutron star and making it shine so brightly.
If the cyclotron limit is from circling electrons, however, the magnetic-field strength of the ULX would not be especially strong, and the field would play no role in the extreme light pouring from the star.
The researchers plan to acquire more X-ray data on the ULX in the Whirlpool Galaxy and hunt down more cyclotron lines in other ULXs, in hopes of figuring out how neutron stars are overcoming these limits to burn so brilliantly.
"The discovery that these very bright objects, long thought to be black holes with masses up to 1,000 times that of the sun, are powered by much less massive neutron stars, was a huge scientific surprise," said Fiona Harrison, the principal investigator for the NuSTAR mission and a researcher at Caltech. "Now we might actually be getting firm physical clues as to how these small objects can be so mighty."