Enlarge / M82, the site of what's likely to be a giant flare from a magnetar. (credit: NASA, ESA and the Hubble Heritage Team) Gamma rays are a broad category of high-energy photons, including everything with more energy than an X-ray. While they are often created by processes like radioactive decay, few astronomical events produce them in sufficient quantities that they can be detected when the radiation originates in another galaxy. That said, the list is larger than one, which means detecting gamma rays doesn't mean we know what event produced them.