Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection

We report results of a one-year monitoring of the low-mass X-ray binary (LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time intervals were covered by INTEGRAL, during which the source showed strong spectral evolution. We studied the broad-band X-ray spectra in detail by fitting several models in the different sections of the hardness-intensity diagram. The soft states are characterized by prominent blackbody emission plus a contribution from a Comptonized emission. The hard states are characterized by the presence of an excess flux with respect to the Comptonization model above 50 keV, while the soft component is fainter. To obtain an acceptable fit to the data, this excess is modelled either with a power law with photon index Gamma similar to 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the few LMXBs of atoll type showing non-thermal emission at high energy. From our analysis, it is also apparent that the presence of the hard tail is more prominent as the overall spectrum becomes harder. We also discuss alternative models which can describe these hard states.