Wide band observations of the X-ray burster GS 1826-238

M. Cocchi¹, R. Farinelli², A. Paizis³ and L. Titarchuk^2,4,5,6

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 Dipartimento di Fisica, Universitá di Ferrara, Italy
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, sez. di Milano, Italy
⁴ Center for Earth Observing and Space Research, George Mason University, Fairfax, VA, USA
⁵ High Energy Space Environment Branch, US Naval Reserch Laboratory, Washington, DC, USA
⁶ Gravitational Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Received: 30 June 2009
Accepted: 19 November 2009

Abstract

Context. GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass binary system. Thermal Comptonisation by a hot electron cloud ( keV) is a widely accepted mechanism accounting for its high energy emission, while the nature of most of its soft X-ray output is not completely understood. A further low energy component is typically needed to model the observed spectra: pure blackbody and Comptonisation-modified blackbody radiation by a lower temperature (a few keV) electron plasma were suggested to explain the low energy data.

Aims. In order to better characterise the nature of the low energy emission and the bolometric output of the source, the steady emission of GS 1826-238 is studied by means of sensitive, broad band (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory.

Methods. In this data analysis, the newly developed, up-to-date Comptonisation model comptb is applied for the first time to study effectively the low-hard state variability of a low-luminosity neutron star in a low-mass X-ray binary system. Three joint ISGRI/JEM-X data sets (two from observations performed in 2003 and one from 2006) were analysed.

Results. We confirm that the 3-200 keV emission of GS 1826-238 is characterised by Comptonisation of soft seed photons by a hot electron plasma. A single spectral component is sufficient to model the observed spectra. At lower energies, no direct blackbody emission is observed and there is no need to postulate a low temperature Compton region. Compared to the 2003 measurements, the plasma temperature decreased from ~20 to ~14 keV in 2006, together with the seed photons temperature. The source intensity was also found to be ~ lower in 2006, whilst the average recurrence frequency of the X-ray bursts significantly increased. Possible explanations for this apparent deviation from the typical limit-cycle behaviour of this burster are discussed.