It turns out that neutron stars — not massive black holes — power at least some ultraluminous X-ray sources.

When astronomers first discovered so-called ultraluminous X-ray (ULX) sources in the 1980s — sources millions of times brighter than the Sun — they didn't know what these objects were. A stellar-mass black hole couldn't feed enough to produce so many X-rays without blowing its food source apart. Intermediate-mass black holes with the mass of hundreds or thousands of Suns could explain the radiation, but their existence was (and is) still unproven.

But a new explanation is making headway. Pulsations observed by the NuSTAR, Chandra, and XMM-Newton X-ray observatories have provided evidence that at least some ULXs are powered not by black holes at all but by neutron stars. Most recently, a new kind of measurement has added a fourth object to the list of neutron star ULXs.

Murray Brightman (Caltech) and colleagues measured the spectrum for a ULX in the Whirlpool Galaxy. They found a dip in brightness at 4,500 electron volts (0.3 nanometer), which corresponds to a phenomenon called cyclotron resonance scattering, when charged particles racing in circles within a magnetic field deflect radiation. This feature has only ever been seen in neutron stars, never in black holes.

But the source of the neutron star's power is still a puzzle. If the charged particles in question are heavier protons, then the field could be 100 trillion Gauss — a trillion times stronger than your typical fridge magnet. The magnetic field could be strong enough to drive a beam of particles and radiation, producing the kind of power that we see in X-ray observations. But if lighter electrons are instead involved, the field would be 1,000 times weaker, and scientists would have to go back to the drawing board.

The researchers are planning additional observations of the ULX in M51, as well as of other ULXs, to resolve the question of the magnetic field and the source of ULX power.

"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," says coauthor Fiona Harrison (Caltech). "Now we might actually be getting firm physical clues as to how these small objects can be so mighty."