*2.2. Post-Merger Models*

Within the standard fireball model for GRBs, a photospheric blackbody precursor can be produced when the fireball changes from optically thick regime to optically thin (see Section 7.3.3 of Reference [29], and references therein). The luminosity is determined by the transition radius. However, it has been found that, to fit the observation, the fireball Lorentz factor should be ∼30, much smaller than the expected value (>100) [40,69]. Therefore, this is unlikely to be responsible for the precursors, and we mainly focus on the SBO model for post-merger models.
