BeppoSAX view of the NS-LMXB GS 1826–238

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica sez. di Roma, via Fosso del Cavaliere, 100, 00133 Roma, Italy
e-mail: Massimo.Cocchi@iasf-roma.inaf.it
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica sez. di Palermo, Italy
e-mail: farinelli@ifc.inaf.it
³ Dipartimento di Fisica, Universitá di Ferrara, Italy
⁴ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica sez. di Milano, Italy
e-mail: ada@iasf-mi.inaf.it

Received: 1 December 2010
Accepted: 8 March 2011

Abstract

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 (kT_e ~ 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 We analysed a total of six data sets, collected from 1997 to 2000; data analysis of two of them was still unpublished.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.