BeppoSAX view of the NS-LMXB GS 1826–238

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 We analysed a total of six data sets, collected from 1997 to 2000; data analysis of two of them was still unpublished.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

A Suzaku observation of the low-mass X-ray binary GS 1826−238 in the hard state

“…Admittedly, the present data gave only a lower limit as R out > 50 km. This allows a case of the whole disk Comptonization (R out → ∞), thus making the present result appear closer to those employed by Thompson et al (2005) and Cocchi et al (2011). Actually, the spectrum analyzed here can be fitted similarly well, with χ 2 = 1.01(292), when replacing dkbbfth in Model 4 with nthcomp[diskbb] which implies Comptonization of the entire disk.…”

“…Utilizing dkbbfth, we constructed Model 4 = nthcomp (BB) + dkbbfth. This formalism is in between that of Sakurai et al (2012) and Sakurai et al (2014) which corresponds to R in → R out , and that of Thompson et al (2005) and Cocchi et al (2011) which is equivalent to R out → ∞. Since absorption became rather unconstrained due to strong coupling with dkbbfth, the column density was fixed to N H = 0.28 × 10 22 cm −2 , as obtained with Model 1.…”

Hard X-Ray Tail Discovered in the Clocked Burster Gs 1826–238

“…Previous X-ray observations of GS 1826 − 238 have found similar behavior, with the source consistently in a hard spectral state with flux out to 100 keV (Strickman et al 1996;in't Zand et al 1999;Barret et al 2000; Thompson et al 2005;Cocchi et al 2010Cocchi et al , 2011. The high-energy sensitivity of SPI above 100 keV and the numerous observations of the Galactic Center region by INTE-GRAL allow for a study of the high-energy spectrum of this source above 100 keV.…”

A Monte Carlo estimate for the fraction of thermal Comptonized photons that impinge back on the soft source in neutron star LMXBs

“…Typically in the Rossi X-ray Timing Experiment (RXTE), Proportional Counter Array (PCA) energy band of 3-20 keV, there are two spectral models namely the "hot" and "cold" seed photon models which are degenerate i.e. they both equally fit the data of neutron star LMXBs (Mitsuda et al 1984;White et al 1986;Barret 2001;Di Salvo & Stella 2002;Lin, Remillard & Homan 2007;Cocchi et al 2011). In, Kumar & Misra (2016, hereafter Paper I), we employed both these spectral models to infer the size of the medium and fraction of photons impinging on the soft seed source, η for different QPO frequencies of the transient source 4U 1608-52.…”