The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization
The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization
复制标题
本体康普顿化框架下天鹅座 X-2 的 X 射线光谱演化
DOI：
10.1051/0004-6361/200810422
复制
发表时间：
2009
影响因子：
6.5
通讯作者：
L. Titarchuk
中科院分区：
文献类型：
--
作者：
R. Farinelli;A. Paizis;R. Landi;L. Titarchuk
语境。强有力的理论和观测支持表明，中子星 LMXB 的光谱演化（包括瞬态硬 X 射线尾部）可以通过热运动康普顿化和体运动康普顿化之间的相互作用来解释。新的 XSPEC 康普顿化模型 Comptb（包括热康普顿化和体康普顿化）的引入为这一解释提供了额外的支持。目标。我们使用 Comptb 研究了中子星 LMXB Cyg X-2 沿 Z 轨道的光谱演化。我们选择单一来源来定量追踪模型物理参数的演变。方法。我们分析了 Cyg X-2 的档案宽带 BeppoSAX 光谱。通过使用颜色-颜色和颜色-强度图中描述的 Z 轨道中源位置的信息，新提取了五个宽带光谱。结果。我们用两个 Comptb 分量拟合光源的光谱。第一个参数的体参数 δ = 0，代表整个源宽带光谱的主要组成部分，其起源与高不透明度环境中暖电子对冷种子光子的热上散射（康普顿化）有关。我们将这些种子光子的起源归因于照亮吸积盘本身和中子星表面之间的外日冕区域（过渡层）的圆盘部分。该热成分的物理特性随着时间和推断的质量吸积率大致恒定。第二个 Comptb 模型描述了源自内部过渡层和中子星表面的较热种子光子的整体康普顿化（热加体积，δ> 0）。该分量在颜色-颜色或硬度-强度图的水平分支中更为重要，并且在观察到纯黑体光谱的法向分支中逐渐消失。结论。 Cyg X-2 的光谱演化根据系统最内部环境条件的变化进行研究和解释，从而导致可变的热体积康普顿化效率。
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.