Neutron stars may not be as weighty as their more massive black hole cousins, but they can be just as mighty when it comes to generating awesome X-ray fireworks.
Since the 1980s, astronomers have studied sources of intense X-rays erupting from the outer regions of other galaxies. They're called ultraluminous X-ray sources, or ULXs, and they output more energy than a million suns. Usually, astronomers would observe such powerful emissions in the cores of active galaxies, where feeding supermassive black holes lurk, but ULXs are far from these behemoths. The idea was that they were being generated by smaller stellar-mass black holes — of a few tens of solar masses — feeding on the gases of unfortunate stars.
But a baffling pattern started to emerge in 2014 when NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission and other space telescopes started to study these enigmas. It turns out that ULXs may not be powered by black holes at all; rather, neutron stars appear to be the culprit.
"It was a huge surprise," says Fiona Harrison, principal investigator of the NuSTAR mission and professor of physics at Caltech, in Pasadena, California. "At first people thought there was something wrong with the observation."
Far from being wrong, in a new study co-authored by Harrison and published in the journal Nature Astronomy, a neutron star has been confirmed to be the engine behind a ULX in the famous Whirlpool galaxy, also known as M51. The galaxy is located 28 million light-years from Earth. It's the fourth time that astronomers have identified a ULX powered by a neutron star.
While studying archival data from NASA's Chandra X-ray Observatory, the researchers noticed a mysterious dip in the ULX light spectrum. When they investigated, they deduced that it must have been caused by cyclotron resonance scattering, a phenomenon that occurs in highly magnetic environments and is caused by charged particles, such as electrons and protons, spiraling around the magnetic field.
Here's the kicker: Black holes don't have magnetic fields, whereas neutron stars are famous for being magnetic powerhouses, so the fact that the spectrum of this ULX has the fingerprint of cyclotron resonance scattering is a big clue that a black hole isn't powering it, but a neutron star is.