What is the estimated temperature for a newly formed neutron star core immediately following collapse?

Up to $100$ billion Kelvin ($10^{11} ext{ K}$)

When a massive star undergoes a supernova explosion, the resulting core collapse forms an extremely dense remnant, such as a neutron star. Immediately after this catastrophic event, the core is subjected to unimaginably high temperatures, estimated to reach up to $100$ billion Kelvin, or $10^{11} ext{ K}$. This extreme heat is not generated by steady-state nuclear fusion, like in a living star, but is instead a direct consequence of the massive, violent conversion of gravitational potential energy into thermal energy during the instantaneous collapse phase. This temperature is transient, as the remnant begins to cool almost immediately.