BeppoSAX view of the NS-LMXB GS 1826-238

Context. The spectroscopic characteristics of GS 1826-238, a neutron star in a low-mass X-ray binary system, have already been studied by sensitive, wide band X-ray telescopes (e.g. BeppoSAX, RXTE, INTEGRAL). Up to now, the source has always been observed in a low-hard spectral state, with two spectral components typically detected. The persistent high-energy (>10 keV) emission is effectively explained by thermal Comptonisation by a hot electron cloud (kTe ~ 20 keV); a further low energy component, modelled either by pure blackbody emission or by Compton-modified blackbody radiation by a few keV electron plasma, is generally needed to yield acceptable fits in the soft X-ray band. Aims: The aim of the present work is to investigate the origin and the nature of the low energy emission of GS 1826-238 further, along with its contribution to the bolometric output of the source, dominated by the high-temperature thermally Comptonised radiation. Methods: This kind of investigation needs sensitive data in the widest available energy band. Simultaneous covering of both the soft X-rays (below 1 keV) and the hard X-rays (up to hundreds of keV) is crucial for an unbiased characterisation of the two spectral components, so we searched the whole BeppoSAX-NFI archive for all the available GS 1826-238observations. We analysed a total of six data sets, collected from 1997 to 2000; data analysis of two of them was still unpublished. In this study we applied both a well-established (comptt) and a more recent, updated Comptonisation model (comptb), in order to get the widest quantitative information about the physical parameters at work. Results: Our results confirm that the 0.1-200 keV emission of GS 1826-238 needs two components to be explained. In particular, two populations of soft seed photons, with different colour temperatures, are observed. One population is Comptonised to high energies by a hot electron cloud (temperatures in the range 19-24 keV, anticorrelated with the source luminosity), while the other is directly observed and can be modelled by a pure blackbody. We also propose an alternative model in which both the seed photon populations are Compton-modified by the electron plasma. This model explains the observed emission of GS 1826-238 as accurately as the traditional one and, moreover, fits well in a wider evolutionary scenario able to describe the state transitions observed in neutron-star low-mass X-ray binaries. The use of comptb also indicates that, in the case of GS 1826-238, the seed photons populations are not distributed as a pure blackbody