Detection of HC3N maser emission in NGC 253

We report the detection of maser emission from the J = 4-3 transition of HC₃N at 36.4 GHz toward the nearby starburst galaxy NGC 253. This is the first detection of maser emission from this transition in either a Galactic or extragalactic source. The HC₃N maser emission has a brightness temperature in excess of 2500 K and is offset from the center of the galaxy by approximately 18'' (300 pc), but close to a previously reported class I methanol maser. Both the HC₃N and methanol masers appear to arise near the interface between the galactic bar and the central molecular zone, where it is thought that molecular gas is being transported inwards, producing a region of extensive low-velocity shocks.