Gamma rays with energies above one hundred trillion electronvolts are like stones thrown with incredible force. For a long time, it was believed that only protons, heavy particles accelerated in cosmic accelerators, could launch such 'stones.' But new work shows that sometimes light electrons are enough.

The authors examined three objects: the microquasar SS 433, the center of our Galaxy, and TeV halos — clouds around pulsars. In all three cases, the observations are explained by simple models where electrons, not protons, produce the gamma rays. The model matched especially well with the size of SS 433: the calculated width of the source coincided with the measured one.

To pinpoint a proton accelerator, additional clues are needed. For example, a burst of neutrinos — particles that are born only together with protons. Or a coincidence of gamma-ray emission with gas clouds: protons colliding with gas produce a characteristic signal.

Ultra-high-energy gamma rays are like the signature of an invisible artist. Previously, it was thought that only protons could leave this signature. Now it's clear: electrons can sign their names too.