Spectral evolution of the Atoll source 4U 1728-34 with RXTE and INTEGRAL: evidence for hard X-ray tail

We report the temporal and spectral results on the INTEGRAL and RXTE 2006–2007 observation campaign of the Atoll source 4U 1728–34 (GX 354‐0). The source shows, more than once, spectral evolution as revealed by the hardness intensity diagram. The soft state is well described by a Comptonization with an electron temperature of 3 keV and a high optical depth of 6. In the hard spectral state the emission extends to above 100 keV and it can be described by Comptoniziation (with higher temperature of 10 keV) plus a power law component with Γ of 1.8, which is evidence for non‐thermal emission from the source.