利用空间VLBI揭示OJ 287在1.7 GHz处的弯曲喷流结构与偏振
Unveiling the Bent Jet Structure and Polarization of OJ 287 at 1.7 GHz with Space VLBI
AI总结：
利用RadioAstron空间VLBI在1.68GHz对OJ287进行高分辨率观测，解析了其弯曲喷流结构，结合多频数据获得了核心子组分的磁场强度，并揭示了核心区域环形磁场与下游极向磁场主导的分布特征。
摘要：
我们展示了2016年4月16日通过RadioAstron空间VLBI观测获得的OJ287在1.68GHz的总强度和线偏振图像。观测使用由VLBA和EVN组成的地面阵列进行。地面-空间干涉条纹的最大投影基线长度达到5.6个地球直径，实现了530微角秒的角分辨率。凭借这一在低频下前所未有的分辨率，OJ287逐渐弯曲的喷流结构在距射电核心10秒差距的投影距离内被成功解析。通过与MOJAVE和VLBA-BU-BLAZAR监测项目中时间相近的15、43、86GHz VLBI观测进行比较，我们获得了显示不透明核心和光学薄喷流组分的谱指数图。光学厚核心的亮温度为10^13 K，在更高频率下进一步被解析为C1和C2两个子组分。这些子组分表现出从光学厚到光学薄的转变，SSA转折频率估计分别为33和11.5GHz，转折通量密度分别为4和0.7Jy。假设多普勒增亮因子为10，SSA值提供了基于SSA的磁场强度估计：C1为3.4G，C2为1.0G。基于能量均分假设的磁场强度估计分别为2.6G和1.6G。积分线偏振度约为2.5%，电矢量位置角与核心区域的局部喷流方向高度对齐。这种对齐表明存在主导的环形磁场，这与需要螺旋磁场锚定于黑洞能层或吸积盘的喷流形成模型一致。在更下游，喷流似乎主要由极向磁场贯穿。
英文摘要：
We present total intensity and linear polarization images of OJ287 at 1.68GHz, obtained through space-based VLBI observations with RadioAstron on April 16, 2016. The observations were conducted using a ground array consisting of the VLBA and the EVN. Ground-space fringes were detected with a maximum projected baseline length of 5.6 Earth's diameter, resulting in an angular resolution of 530 uas. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ287 has been resolved up to 10 pc of the projected distance from the radio core. In comparison with close-in-time VLBI observations at 15, 43, 86 GHz from MOJAVE and VLBA-BU-BLAZAR monitoring projects, we obtain the spectral index map showing the opaque core and optically thin jet components. The optically thick core has a brightness temperature of 10$^{13}$ K, and is further resolved into two sub-components at higher frequencies labeled C1 and C2. These sub-components exhibit a transition from optically thick to thin, with a SSA turnover frequency estimated to be 33 and 11.5 GHz, and a turnover flux density 4 and 0.7 Jy, respectively. Assuming a Doppler boosting factor of 10, the SSA values provide the estimate of the magnetic field strengths from SSA of 3.4 G for C1 and 1.0 G for C2. The magnetic field strengths assuming equipartition arguments are also estimated as 2.6 G and 1.6 G, respectively. The integrated degree of linear polarization is found to be approximately 2.5 %, with the electric vector position angle being well aligned with the local jet direction at the core region. This alignment suggests a predominant toroidal magnetic field, which is in agreement with the jet formation model that requires a helical magnetic field anchored to either the black hole ergosphere or the accretion disk. Further downstream, the jet seems to be predominantly threaded by a poloidal magnetic field.
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