The blazar BL Lacertae, a supermassive black hole surrounded by a bright disk and jets oriented toward Earth, provided scientists with a unique opportunity to answer a longstanding question: How are X-rays generated in extreme environments like this?

NASA’s IXPE (Imaging X-ray Polarimetry Explorer) collaborated with radio and optical telescopes to find answers. The results, available on the arXiv preprint server and set to be published in the journal Astrophysical Journal Letters, show that interactions between fast-moving electrons and particles of light, called photons, must lead to this X-ray emission.

Scientists had two competing possible explanations for the X-rays, one involving protons and one involving electrons. Each of these mechanisms would have a different signature in the polarization of X-ray light. Polarization is a property of light that describes the average direction of the electromagnetic waves that make up light.