Disk-jet coupling in the low-mass X-ray binary 4U 1636-53 from INTEGRAL observations

We report on the spectral analysis of the neutron star (atoll-type) low-mass X-ray binary 4U 1636-53 as observed by the INTEGRAL and BeppoSAX satellites. The spectral behavior in three different epochs, corresponding to three different spectral states, has been investigated in depth. Spectra from two data sets show a continuum that is well described by one or two soft blackbodies plus a Comptonized component, with changes in the Comptonizing electrons and blackbody temperature and accretion rate that are typical of the spectral transition from high to low state. On one occasion the INTEGRAL spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. The total spectrum is fitted as the sum of a Comptonized component similar to the soft state and a power-law component (Gamma = 2.76), indicating the presence of a nonthermal distribution of electron velocities. In this case, a comparison with the hard tails detected in soft states from neutron star systems and some black hole binaries suggests that a similar mechanism could give rise to these components in both cases.