A long-duration gamma-ray burst with a peculiar origin

It is generally believed that long-duration gamma-ray bursts (GRBs) are associated with massive star core collapse¹, whereas short-duration GRBs are associated with mergers of compact star binaries². However, growing observations^3-6 have suggested that oddball GRBs do exist, and several criteria (prompt emission properties, supernova/kilonova associations and host galaxy properties) rather than burst duration only are needed to classify GRBs physically⁷. A previously reported long-duration burst, GRB 060614 (ref. ³), could be viewed as a short GRB with extended emission if it were observed at a larger distance⁸ and was associated with a kilonova-like feature⁹. As a result, it belongs to the type I (compact star merger) GRB category and is probably of binary neutron star (NS) merger origin. Here we report a peculiar long-duration burst, GRB 211211A, whose prompt emission properties in many aspects differ from all known type I GRBs, yet its multiband observations suggest a non-massive-star origin. In particular, substantial excess emission in both optical and near-infrared wavelengths has been discovered (see also ref. ¹⁰), which resembles kilonova emission, as observed in some type I GRBs. These observations point towards a new progenitor type of GRBs. A scenario invoking a white dwarf (WD)-NS merger with a post-merger magnetar engine provides a self-consistent interpretation for all the observations, including prompt gamma rays, early X-ray afterglow, as well as the engine-fed^11,12 kilonova emission.