The Andromeda galaxy is our nearest neighboring large galaxy. It has a mysterious dominant source of high-energy X-ray emission called Swift J0042.6+4112. Recent observations by the NuSTAR (Nuclear Spectroscopic Telescope Array) mission have pinpointed a pulsar that is for this high-energy radiation. A pulsar is the dense remnant of a dead star that is highly magnetized and spinning. The strong pulsar in Andromeda is likely in a binary system in which material from a stellar companion gets pulled onto the pulsar. X-rays are radiated by the material as it heats up.
The highest energy x-rays are color code blue in the NuSTAR image above, and the pulsar is shown as a blue dot. It appears brighter in high-energy X-rays than anything else in the galaxy.
Image Credit: NASA