Determine whether the prompt emission requires multiple ejecta shells

Determine whether the multiple prompt pulses of EP251222b/GRB 251222A require two physically distinct ejecta shells rather than internal velocity structure within a single shell.

Background

The shell-collision interpretation assumes that the earlier and later prompt pulse groups originate from physically distinct ejecta shells. However, multiple pulses can also arise from internal velocity structure within a single shell, so the pulse-to-shell identification is not uniquely supported by the available observations. Resolving this issue is essential for testing whether the late soft X-ray flare and the double-peaked afterglow are consequences of a shell collision.

The paper suggests that improved pulse sampling, spectral-hardness resets, and lag distributions could help discriminate between multiple-shell and single-shell interpretations.