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 (kTe ~ 20 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 ~30% 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.
Wide band observations of the X-ray burster GS 1826-238
FARINELLI, Ruben;
2010
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 (kTe ~ 20 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 ~30% 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.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
