A Shell Collision Scenario for EP251222b/GRB 251222A: Connecting Prompt Emission and Double-Peaked Afterglow

Abstract: GRB 251222A is a long-duration gamma-ray burst at a spectroscopic redshift of $z=3.171$. It was detected by the EP as the soft X-ray transient EP251222b before and during the gamma-ray emission. We performed a multi-wavelength (EP/WXT, fermi/GBM, swift/XRT and optical/NIR) investigation of this event. Its prompt soft X-ray and gamma-ray features, together with two afterglow rebrightenings, allow us to examine a possible connection between the prompt ejecta and the afterglow. We consider a multiple-shell scenario in which a faster, narrower shell catches up with a slower, wider shell. We assume that the selected pulse groups trace different ejecta and that the late X-ray flare marks their collision. Under these assumptions, we represent the post-collision outflow by an effective merged core and an outer annular component. A joint fit to the multi-band afterglow gives a Lorentz factor of $Γ<em>c\approx293$ for the merged core and $Γ</em>{1,w}\approx228$ for the outer component. The sample-wise collision mapping gives an energy release of $$E_{\rm diss}=(1.6<sup>{+2.1}_{-0.9})\times10<sup>{52}$$ erg and a dynamical dissipation efficiency of $0.37<sup>{+0.07}_{-0.14}\%$. EP251222b can therefore be used to test a prompt--afterglow connection through shell collisions, although it does not uniquely establish this interpretation.