W-solitons as prototypical black hole microstates

We analyze a new class of static, smooth geometries in five-dimensional supergravity, dubbed W-solitons. They carry the same mass and charges as four-dimensional Reissner-Nordstrom-like black holes but replace the horizon with a Kaluza-Klein bubble supported by electromagnetic flux. These solutions provide analytically tractable prototypes of black hole microstates in supergravity, including a new, relevant neutral configuration involving a massless axion field. Focusing on photon scattering and scalar perturbations, we compute their key observables, aiming to identify mesoscopic observables. We find that W -solitons feature a single photon sphere, qualitatively similar to that of the black hole but with quantitative differences. They have only short-lived quasinormal modes, as black holes, while long-lived echo modes seen in other ultracompact horizonless objects are absent. As a result, the ringdown closely resembles that of a black hole while still showing sizable deviations. The latter are at the 0(10%) level, compatible with the recent measurement of GW250114 and potentially falsifiable in the near future. Finally, we show that W-solitons are stable under scalar perturbations. Our results underscore the qualitative similarities between W-solitons and black holes, reinforcing their relevance as smooth black hole microstate prototypes.