RONCHINI, SAMUELESAMUELERONCHINISTRATTA, Maria GiulianaMaria GiulianaSTRATTAROSSI, AndreaAndreaROSSIKann, David AlexanderDavid AlexanderKannOGANESYAN, GORGOROGANESYANDALL'OSSO, SimoneSimoneDALL'OSSOBranchesi, MaricaMaricaBranchesiDE CESARE, GIOVANNIGIOVANNIDE CESARE2024-12-232024-12-2320230004-6361http://hdl.handle.net/20.500.12386/35552A large fraction of gamma-ray bursts (GRBs) shows a plateau phase during the X-ray afterglow emission, whose physical origin is still debated. In this work we define a sample of 30 GRBs with simultaneous X-ray and optical data during and after the plateau phase. Through a time-resolved spectral analysis of the X-ray plateaus, we test the consistency of the unabsorbed optical fluxes with those obtained via X-ray-to-optical spectral extrapolation by assuming a synchrotron spectrum. Combining X-ray with optical data, we find that 63% (19/30) GRBs are compatible with a single synchrotron spectrum thus suggesting that both the optical and X-ray radiations are produced from a single emitting region. For these GRBs we derive the temporal evolution of the break frequency and we compare it with the expectations predicted by several models. For 11/30 GRBs the optical emission is above the predicted range of values extrapolated from the X-rays in at least one temporal bin of the light curve. These GRBs may not be explained with a single zone emission, indicating the necessity of invoking two cooperating processes in order to explain the broad band spectral behaviour during X-ray plateaus. We discuss our findings in the framework of different scenarios invoked to explain the plateau feature, including the energy injection from a spinning-down magnetar and the high latitude emission from a structured jet.STAMPAenArticle10.1051/0004-6361/2022453482-s2.0-85165542696PPRN:22543335https://www.aanda.org/articles/aa/full_html/2023/07/aa45348-22/aa45348-22.htmlhttps://publons.com/wos-op/publon/59771727/FIS/05 - ASTRONOMIA E ASTROFISICA