Significant Cocoon Emission and Photosphere Duration Stretching in GRB 211211A: A Burst from a Neutron Star-Black Hole Merger

The radiation mechanism (thermal photosphere or magnetic synchrotron) and the progenitor of gamma-ray bursts (GRBs) are under hot debate. Recently discovered, the prompt long-duration (similar to 10 s, normally from the collapse of massive stars) property of GRB 211211A strongly conflicts with its association with a kilonova (normally from the merger of two compact objects, NS-NS, NS-BH, or NS-WD, duration less than or similar to 2 s). In this paper, we find that the probability photosphere model with a structured jet can satisfactorily explain this peculiar long duration, through the duration stretching effect (similar to 3 times) on the intrinsic longer (similar to 3 s) duration of an NS-BH merger, the observed empirical 2SBPL spectrum (with soft low-energy index alpha of similar to-1), and its evolution. In addition, much evidence of the NS-BH merger origin is found, especially the good fit of the afterglow-subtracted optical-near-IR light curves by the significant thermal cocoon emission and the sole thermal "red" kilonova component. Finally, a convincing new explanation for the X-ray afterglow plateau is revealed.