A CHANDRA X-RAY STUDY OF CYGNUS A. II. THE NUCLEUS

ANDREW J. YOUNG; ANDREW S. WILSON; YUICHI TERASHIMA; KEITH A. ARNAUD; DAVID A. SMITH

摘要

We report Chandra Advanced CCD Imaging Spectrometer and quasi-simultaneous Rossi X-Ray Timing Explorer (RXTE) observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (>a few keV) X-ray emission is spatially unresolved with a size approx<1″ (1.5 kpc, H_0 = 50 km s~(-1) Mpc~(-1)) and coincides with the radio and near-infrared nuclei. In contrast, the soft (<2 keV) emission exhibits a bipolar nebulosity that aligns with the optical bipolar continuum and emission-line structures and approximately with the radio jet. In particular, the soft X-ray emission corresponds very well with the [O Ⅲ] λ5007 and Hα + [N Ⅱ] λλ6548, 6583 nebulosity imaged with Hubble Space Telescope. At the location of the nucleus, there is only weak soft X-ray emission, an effect that may be intrinsic or result from a dust lane that crosses the nucleus perpendicular to the source axis. The spectra of the various X-ray components have been obtained by simultaneous fits to the six detectors. The compact nucleus is detected to 100 keV and is well described by a heavily absorbed power-law spectrum with Γ_h = 1.52_(-0.12)~(+0.12) (similar to other narrow-line radio galaxies) and equivalent hydrogen column N_H(nuc) = 2.0_(-0.2)~(+0.1) 10~(23) cm~(-2). This column is compatible with the dust obscuration to the near-infrared source for a normal gas-to-dust ratio. The soft (<2 keV) emission from the nucleus may be described by a power-law spectrum with the same index (i.e., Γ_l = Γ_h), although direct fits suggest a slightly larger value for Γ_l. Narrow emission lines from highly ionized neon and silicon, as well as a "neutral" Fe Kα line, are detected in the nucleus and its vicinity (r approx< 2 kpc). The equivalent width (EW) of the Fe Kα line (182_(-54)~(+40) eV) is in good agreement with theoretical predictions for the EW versus N_H(nuc) relationship in various geometries. An Fe K edge is also seen. The RXTE observations indicate a temperature of kT = 6.9_(1.0)~(+0.3) keV for the cluster gas (discussed in Paper III of this series) and cluster emission lines of Fe Kα and Fe Kβ and/or Ni Kα. We consider the possibility that the extended soft X-ray emission is electron-scattered nuclear radiation. Given that 1% of the unabsorbed 2-10 keV nuclear radiation would have to be scattered, the necessary gas column [N_H(scattering) approx= 3.5 x 10~(22) cm~(-2)] would absorb the X-rays rather than scatter them if the gas is cold. Thus, the scattering plasma must be highly ionized. If this ionization is achieved through photoionization by the nucleus, the ionization parameter ξ > 1 ergs cm s~(-1) and the electron density n_e approx= 6 cm~(-3) given the observed distance of the soft X-ray emission from the nucleus. The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light, strongly suggesting that the polarized optical light is scattered by dust. The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 ergs cm s~(-1) approx< ξ approx< 300 ergs cm s~(-1); these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas. A collisionally ionized thermal model of the extended soft X-rays cannot be ruled out but is unattractive in view of the low metal abundance required (Z = 0.03 Z_☉). The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies. By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object, we estimate M_B = ―22.4 for Cygnus A, near the borderline between Seyfert galaxies and QSOs.

机译：我们报告了Chandra高级CCD成像光谱仪和附近同时强大的射电星系天鹅座A的准同时Rossi X射线定时资源管理器（RXTE）观测结果，本文重点关注了活动核的性质。在钱德拉（Chandra）观测中，硬的（>几个keV）X射线发射在空间上未解析，大小约为<1英寸（1.5 kpc，H_0 = 50 km s〜（-1）Mpc〜（-1））并且重合带有无线电和近红外核。相比之下，软发射（<2 keV）表现出双极性雾度，该双极性雾度与光学双极性连续体和发射线结构以及无线电射流大致对齐。特别是，软X射线的发射与哈勃太空望远镜拍摄的[OⅢ]λ5007和Hα+ [NⅡ] λλ6548、6583的雾度非常吻合。在原子核的位置，只有微弱的软X射线发射，该效应可能是固有的，也可能是由与垂直于源轴的原子核交叉的尘埃带引起的。通过同时拟合六个检测器，获得了各种X射线分量的光谱。致密核被检测到100 keV，并由Γ_h= 1.52 _（-0.12）〜（+0.12）（类似于其他窄线射电星系）和等效氢柱N_H（ nuc）= 2.0 _（-0.2）〜（+0.1）10〜（23）cm〜（-2）。对于正常的气尘比，该色谱柱与粉尘对近红外光源的相容性兼容。来自核的软发射（<2 keV）可以用具有相同指数（即Γ_1=Γ_h）的幂律谱描述，尽管直接拟合表明Γ_1的值略大。在原子核及其附近（r约<2 kpc）中检测到了来自高度离子化的氖和硅的窄发射线以及“中性” FeKα线。 FeKα线的等效宽度（EW）（182 _（-54）〜（+40）eV）与各种几何结构中EW与N_H（nuc）关系的理论预测都非常吻合。还可以看到Fe K边缘。 RXTE观测表明，团簇气体（在本系列论文的第三部分中讨论）和团簇发射线FeKα和FeKβ和/或NiKα的温度为kT = 6.9_（1.0）〜（+0.3）keV。我们认为扩展的软X射线发射是电子散射核辐射的可能性。考虑到必须吸收未吸收的2-10 keV核辐射的1％，必要的气柱[N_H（散射）大约= 3.5 x 10〜（22）cm〜（-2）]会吸收X射线。而不是在气体冷的情况下将它们撒散。因此，散射等离子体必须高度电离。如果通过原子核的电离实现该电离，则在观察到的软X射线发射距离的情况下，电离参数ξ> 1 ergs cm s〜（-1），电子密度n_e大约= 6 cm〜（-3）。从细胞核。从X射线观察推断出的电子柱密度太低，不足以说明扩展的光散射光，这强烈表明偏振光被灰尘散射。在软X射线光谱中存在高度电离的Ne线需要20 ergs cm s〜（-1）约<ξ约<300 ergs cm s〜（-1）;这些线可能比扩展的软连续体更靠近原子核，或者起源于较低密度的气体。不能排除扩展的软X射线的碰撞电离热模型，但鉴于所需的低金属丰度（Z = 0.03Z_☉），因此没有吸引力。天鹅座A核的硬X射线与远红外比率类似于在塞弗特1和未遮盖的射电星系中看到的比率。通过这些类物体的硬X射线光度与核光学绝对量之间的相关性，我们估算出天鹅绒A的M_B = ―22.4，接近塞弗特星系和QSOs的边界。

A CHANDRA X-RAY STUDY OF CYGNUS A. II. THE NUCLEUS

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