Speaker
Description
The origin of hard X-ray emission in black hole X-ray binaries during the hard spectral state is commonly attributed to thermal Comptonization in a hot inner flow, yet the possible contribution of jet synchrotron emission remains uncertain. We present a multi-wavelength study of the 2021 outburst of GX 339-4, focusing on five hard-state observations covering luminosities of ~1-10% of the Eddington limit. Our dataset combines NICER and NuSTAR X-ray observations with quasi-simultaneous optical and ultraviolet coverage from Swift/UVOT and the Las Cumbres Observatory, enabling broadband spectral modeling from the optical to hard X-ray band. We find that standard reflection-based models do not provide a fully self-consistent description of the spectra, motivating the exploration of alternative emission components. Using a phenomenological model that incorporates an optically thin synchrotron component extending into the X-ray band, we obtain significantly improved fits and physically plausible evolution of the spectral parameters. Applied to the full broadband dataset, the model successfully reproduces the spectra at all epochs and reveals the presence of a synchrotron cooling break between the optical and X-ray bands. Our results highlight the potential role of jet emission in shaping hard X-ray spectra and provide new constraints on the interplay between the jet, accretion flow, and reflection in the hard state.