A Neutron Capture Explanation for the 10 MeV Emission Line Seen in GRB 221009A
Abstract
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 structure jet. Disk winds could be another possible site.
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