中子星X射线双星EXO 0748-676附近电离等离子体的性质
Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748-676
摘要：
我们提出了一个大的XMM牛顿观测EXO 0748-676，一个明亮的倾斜LMXB的光谱分析。特别是，我们专注于浸渍现象的结果，在靠近源的电离气体的属性的变化。利用RGS收集的高分辨率光谱，我们探索了两个简单的几何场景，我们推导出吸收材料的物理量，如密度，大小和质量。我们发现，连续吸收的中性气体，并通过碰撞（温度T~70 eV）和光电离（电离参数log ~2.5）吸收剂。还检测到来自OVII和OVIII的发射线。这是第一次在低质量X射线双星中发现碰撞电离吸收体的证据。碰撞电离的吸收体可能是密集的（n>10^14 cm^-3）细丝，位于距离r>10^11 cm处。在浸渍期间，光电离吸收剂显著增加其柱密度（因子2- 4），同时变得较少电离。这加强了这样一种观点，即碰撞圆盘的吸积流中较冷的物质在下降时经过我们的视线。我们发现，从中子星到撞击区域的距离（~ 5 × 10^10 cm）与中子星的洛希瓣的大小相似。在持续状态期间观察到的气体可以具有扁平的几何形状。最后，我们探讨的可能性存在的材料形成一个初始的，较热的一部分的环双星盘。
英文摘要：
We present the spectral analysis of a large set of XMM-Newton observations of EXO 0748-676, a bright dipping LMXB. In particular, we focus on the dipping phenomenon as a result of changes in the properties of the ionised gas close to the source. Using the high-resolution spectra collected with the RGS, we explored two simple geometrical scenarios for which we derived physical quantities of the absorbing material like the density, size, and mass. We find that the continuum is absorbed by a neutral gas, and by both a collisionally (temperature T~70 eV) and photoionised (ionisation parameter logξ~2.5) absorbers. Emission lines from OVII and OVIII are also detected. This is the first time that evidence of a collisionally ionised absorber has been found in a low-mass X-ray binary. The collisionally ionised absorber may be in the form of dense (n>10^14 cm^-3) filaments, located at a distance r>10^11 cm. During dips, the photoionised absorber significantly increases its column density (factor 2--4) while becoming less ionised. This strengthens the idea that the colder material of the accretion stream impinging the disc is passing on our line of sight during dips. We find that the distance from the neutron star to the impact region (~ 5x10^10 cm) is similar to the size of the neutron star's Roche lobe. The gas observed during the persistent state may have a flattened geometry. Finally, we explore the possibility of the existence of material forming an initial, hotter portion of a circumbinary disc.
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