M87的边缘增亮喷流（7 Schwarzschild半径尺度）
The limb-brightened jet of M87 down to 7 Schwarzschild radii scale
摘要：
M87是距离我们最近的射电星系之一，其喷流的尺度从次pc到kpc。由于它的接近和中心黑洞的大质量，它是研究喷流形成的最佳射电源之一。我们的目标是研究在$\sim7-100$史瓦西半径（$R_{\rm sch}$）的BH投影分离的喷流底部附近的物理条件。全球毫米VLBI阵列（GMVA）在86 GHz（$λ=3.5\,$毫米）的观测提供了一个$\sim50μ$角分辨率为，这对应于一个空间分辨率只有$7~R_{\rm sch}$和达到小空间尺度。利用2004- 2015年获得的5个M87的GMVA数据集，给出了86 GHz高角分辨率VLBI图。特别是，我们着重分析的亮温，射流脊线，和喷流对喷流比。图像显示了一个抛物线扩展的边缘增亮射流，它来自一个$\sim(8-13) R_{\rm sch}$大小的VLBI核心。观测到的任何时期的核心亮度温度是$\sim(1-3)\times10^{10}\,$ K，这是低于均分亮度温度，并建议在喷流基地的磁能优势。我们估计的直径的射流在其基础上是$\sim5 R_{\rm sch}$假设一个自相似的射流结构。这表明喷流鞘可能锚定在吸积盘的最内部。图像叠加揭示了次pc尺度上边缘增亮喷流中心的微弱发射。我们讨论其物理意义的范围内的脊椎鞘结构的射流。
英文摘要：
M87 is one of the nearest radio galaxies with a prominent jet extending from sub-pc to kpc-scales. Because of its proximity and large mass of the central black hole, it is one of the best radio sources to study jet formation. We aim at studying the physical conditions near the jet base at projected separations from the BH of $\sim7-100$ Schwarzschild radii ($R_{\rm sch}$). Global mm-VLBI Array (GMVA) observations at 86 GHz ($λ=3.5\,$mm) provide an angular resolution of $\sim50μ$as, which corresponds to a spatial resolution of only $7~R_{\rm sch}$ and reach the small spatial scale. We use five GMVA data sets of M87 obtained during 2004--2015 and present new high angular resolution VLBI maps at 86GHz. In particular, we focus on the analysis of the brightness temperature, the jet ridge lines, and the jet to counter-jet ratio. The imaging reveals a parabolically expanding limb-brightened jet which emanates from a resolved VLBI core of $\sim(8-13) R_{\rm sch}$ size. The observed brightness temperature of the core at any epoch is $\sim(1-3)\times10^{10}\,$K, which is below the equipartition brightness temperature and suggests magnetic energy dominance at the jet base. We estimate the diameter of the jet at its base to be $\sim5 R_{\rm sch}$ assuming a self-similar jet structure. This suggests that the sheath of the jet may be anchored in the very inner portion of the accretion disk. The image stacking reveals faint emission at the center of the edge-brightened jet on sub-pc scales. We discuss its physical implication within the context of the spine-sheath structure of the jet.
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