A mysterious flash of X-rays has been spotted by NASA’s Chandra X-ray Observatory in deep space, and astronomers are at a loss to explain it. The X-ray source, located in a region of the sky known as the Chandra Deep Field-South (CDF-S), was unremarkable before October 2014 – but then it erupted and became at least 1000 times brighter in a few hours. After about a day, the source had faded to...