A region of soft X-ray emission about 25° wide extending from Gemini into Monoceros; also known as the Gemini–Monoceros X-ray enhancement. It is thought to be a supernova remnant about 1000 l.y. away with a true diameter of about 150 l.y. Pulsar PSR B0656+14 lies near its centre and is thought to be the core of the star that exploded about 100 000 years ago.