Gamma-ray bursts are thought to be the most powerful explosions in the Universe, yet the cause of these high-energy flashes remains a mystery. Blindingly bright for space-based gamma-ray detectors the burst sources are so faint at visible wavelengths that large telescopes and sensitive cameras are required to search for them.

Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31 ,left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy.