If a pulsar is orbiting a black hole, its radio signal will look normal whenever the pulsar passes in front of the black hole. But when the black hole eclipses the pulsar, the radio beam will reach us via a region of space-time that is steeply curved by the immense gravity¹. Black holes and neutron stars (including pulsars) can have hot matter around them, especially when one of these objects is in a binary system with another star². Gravitational waves can be used to detect merging black holes, and pulsars can be used to observe the mergers of supermassive black holes^{3 4}.