The Hard X-Ray Tails in Neutron Star Low-Mass X-Ray Binaries: BeppoSAX Observations and Possible Theoretical Explanation of the Case of GX 17+2

We report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the observations, the source exhibited a power-law-like hard (>30 keV) X-ray tail, which was described in a previous work with a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of low-mass X-ray binaries including both Z and atoll classes have shown that dynamical (bulk) Comptonization of soft photons is a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and exploit the broadband capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure that originates at the surface of the neutron star (NS). As a result, the high radiation pressure stops the bulk inflow, and consequently, this radiation feedback from the NS surface leads to quenching of the dynamical (bulk) Comptonization.