Islands for black holes in a hybrid quantum state

Abstract

Following our previous work on hybrid quantum states in the Russo-Susskind-Thorlacius model, we study its most interesting solution representing a completely regular spacetime with the structure of causal diamond, containing an apparent horizon and radiation at infinity. Adapting recent computations of radiation entropy in terms of the entropy of entanglement, we find that this entropy follows a Page curve. This confirms our previous result [], which was obtained by directly calculating the thermodynamic entropy of radiation at infinity. We also investigate the presence of a possible island in these systems and find that it does not seem to play a role in contributing to the generalized black hole entropy. Published by the American Physical Society 2025