Black holes are difficult to study because their gravity is so powerful that even light cannot escape it. But the gas and dust attracted to a black hole swirl around it in a flat ‘accretion disk’ before they are sucked in – and the particles in this disk glow because they are heated by friction. This radiation allows us to study the structure and formation of the disk, and can reveal clues about the black hole itself.

Spruit and colleagues observed X-rays and visible light from the disk around the black hole XTE J118+480. They found that the intensity of both signals varied periodically, but every jump in the optical output lagged behind the corresponding increase in the X-ray output by half a second.

Spruit and colleagues initially thought that the well-known ‘X-ray reprocessing’ effect could explain their observations. In this process, X-rays from the inner portion of the accretion disk heat up gas and dust at the outer edge of the disk, making it emit visible light. But the team ruled this out when they found that the optical signal changed in intensity every few tens of milliseconds, whereas the X-ray intensity changed only every few seconds.

The researchers studied the fluctuations in the intensity of the visible light to determine the size of the region that emitted it. Since the intensity changed significantly on timescales of around hundred milliseconds, the region that produced the light could not be larger than the distance light can travel in that time – that is, about 30 000 kilometres.

To propose an alternative to X-ray reprocessing, Spruit and colleagues combined the estimate of the size of the emitting region with luminosity measurements of the visible light. Their calculations led them to suggest that the light is cyclosynchrotron radiation – radiation generated by charged particles accelerated in a circular path by a strong magnetic field. According to the team, the strength of the magnetic field at around 20 000 kilometres from the black hole would generate light at the luminosity that they observed.