A Neutron Capture Explanation for the 10 MeV Emission Line Seen in GRB 221009A

The brightest ever gamma-ray burst (GRB) 221009A displays a significant emission line component around 10 MeV. As the GRB central engine is neutron rich, we propose that the emission line could be originally due to the 2.223 MeV gamma rays following neutron capture with protons. The measured line profile can be adequately fitted with a neutron capture model that involves thermal broadening and a bulk Doppler shift. The spectral modeling reveals a Doppler factor varying from 5.1 to 2.1 for the neutron-rich component, along with a temperature increase from 300 keV to about 900 keV, during the time interval of 280–360 s since the trigger, with about 10−2 M⊙ deuteriums produced in the process. We argue that the neutron capture can take place in the outer shell of a structured jet, and the disk wind could be another possible site.