The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

Abstract

Context. Strong theoretical and observational support exists that the spectral evolution of neutron-star LMXBs, including transient hard X-ray tails, can be explained by the interplay between thermal and bulk motion Comptonization. The introduction of a new XSPEC Comptonization model, Comptb, including thermal and bulk Comptonization, has provided additional support to this interpretation. Aims. We used Comptb to investigate the spectral evolution of the neutron-star LMXB Cyg X-2 along its Z track. We selected a single source to trace in a quantitative way the evolution of the physical parameters of the model. Methods. We analyzed archival broad-band BeppoSAX spectra of Cyg X-2. Five broad-band spectra were newly extracted by using information about the source position in the Z track described in the colour-colour and colour-intensity diagrams. Results. We fitted the spectra of the source with two Comptb components. The first one, with a bulk parameter δ = 0, represents the dominant component of the overall source broad-band spectrum and its origin is related to thermal upscattering (Comptonization) of cold seed photons by warm electrons in a high opacity enviroment. We attribute the origin of these seed photons to the section of the disk that illuminates the outer coronal region (transition layer) located between the accretion disk itself and the neutron-star surface. The physical properties of this thermal component are roughly constant with both time and inferred mass accretion rate. The second Comptb model describes the overall Comptonization (thermal plus bulk, δ > 0) of hotter seed photons that originate in both the inner transition layer and at the neutron-star surface. This component is more significant in the horizontal branch of the colour-colour or hardness-intensity diagram and progressively disappears towards the normal branch, where a pure blackbody spectrum is observed. Conclusions. The spectral evolution of Cyg X-2 is studied and interpreted in terms of changes in the innermost environmental conditions of the system, leading to a variable thermal-bulk Comptonization efficiency.

STABILITY OF THE PHOTON INDICES INZ-SOURCE GX 340+0 FOR SPECTRAL STATES

“…Cowley et al (1979) reported that Cyg X-2 is observed at an inclination between 65 • -75 • . Farinelli et al (2009) analyzing BeppoSAX observations of Cyg X-2 and using a sum of two COMPTB components, successfully described the spectral behavior of the source from its HB to NB. Consequently we can suggest that modulation of the flux from the TL outer part and TL innermost part (related to N Com1 and N Com2 , respectively) is a result of geometric effects.…”

High energy power-law tail in X-ray binaries and bulk Comptonization due to an outflow from a disk

“…However, to explain the high energy power-law tail by thermal Comptonization, the required electron medium temperature is ∼100 keV and the optical depth around unity (Parker, et al, 2016, Titarchuk et al, 2014. Mainly, three models are proposed for power-law tail in context of Comptonization, i) non-thermal Comptonization (Done & Kubota, 2006, Kubota & Done, 2016 ii) hybrid Comptonization model (Coppi, 1999, Gierlinski et al, 1999 iii) bulk Comptonization model (Farinelli et al, 2009, Paizis et al, 2006, Titarchuk et al, 1997. In bulk Comptonization, the soft photons are upscattered by high energetic electrons, which have both thermal and bulk motions.…”

“…In bulk Comptonization, the soft photons are upscattered by high energetic electrons, which have both thermal and bulk motions. For bulk region, Titarchuk et al (1997) had considered a free-fall converging flow of spherically accreted plasma into BH (later, a bulk Comptonization model was also developed for NS with a similar free-fall bulk region, see, e.g., Farinelli et al, 2009). However, a spherically free-fall region into BH was first proposed by Chakrabarti & Titarch (1995) to examine the HS state in BH XRBs.…”

Hard X-ray emission of Sco X-1

“…Di Salvo et al 2006), or on bulk motion of matter near the compact object (e.g. Paizis et al 2006;Farinelli et al 2009). In order to distinguish between these models one need to obtain high quality spectral energy distribution in hard X-rays because of pos-sible high energy cutoff, predicted by some models, and to relate the behaviour of the hard X-ray tail to other components of X-ray emission of the source.…”