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GRB 220426A is a bright gamma-ray burst (GRB) dominated by the photosphericemission. We perform several tests to speculate the origin of this photosphericemission. The dimensionless entropy $\eta$ is large, which is not usual if weassume that it is a pure hot fireball launched by neutrino-antineutrinoannihilation mechanism only. Moreover, the outflow has larger $\eta$ with lowerluminosity $L$ in the first few seconds, so that the trend of time-resolved$\eta-L$ can not be described as a monotonically positive correlation between$\eta$ and $L$. A hybrid outflow with almost completely thermalized Poyntingflux could account for the quasi-thermal spectrum as well as large $\eta$. Moreimportantly, the existence of magnetic field could affect the proton densityand neutron-proton coupling effect, so that it could account for the observedtrend of time-resolved $\eta-L$. The other origins for the photosphericemission, such as non-dissipative hybrid outflow or magnetic reconnection, arenot supported because their radiation efficiencies are low, which is notconsistent with non-detection of the afterglow for GRB 220426A. Therefore, wethink the hybrid outflow may be the most likely origin.

The Origin of the Photospheric Emission of GRB 220426A