Shared cloud interactions unveil a candidate binary-system supernova pair with no known analogue
- Miltiadis Michailidis
- Marianne Lemoine-Goumard
- Reinhold Willcox
- Stefano Gabici
- Niccolò Di Lalla
- Nicola Omodei
2026-07-21
IC 443 is one of the most extensively studied supernova remnants in the Galaxy, yet the surrounding region remains shrouded in mystery. Here we show that 16 years of Fermi -LAT observations uncover extended gigaelectronvolt gamma-ray emission from G189.6+3.3, a source long hidden in the shadow of the much brighter IC 443. The gamma-ray morphology is consistent with the X-ray shell detected by eROSITA, indicating an origin of the emission in the remnant. Spectral and spatial analyses identify distinct hadronic and leptonic gamma-ray components associated with regions containing and lacking molecular gas, respectively, revealing a clear spatial segregation of these emission mechanisms. In the northern shell boundary, hadronic gamma rays coincide with an H α filament linked to the S249 ionized hydrogen cloud. Morphological studies, crushed-cloud modelling and ultraviolet observations of this region support gamma-ray production through proton re-acceleration in compressed post-shock gas. The shared-cloud interaction positions G189.6+3.3 at the same distance as IC 443, with a linear separation between the two remnants and a time delay in their explosion events, bolstering the hypothesis that these two remnants are part of a binary system, both resulting from separate supernova events.