Magnetic field at a jet base: extreme Faraday rotation in 3C 273 revealed by ALMA
Magnetic field at a jet base: extreme Faraday rotation in 3C 273 revealed by ALMA
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DOI：
10.1051/0004-6361/201832587
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发表时间：
2018-03
影响因子：
6.5
通讯作者：
T. Hovatta;S. O’Sullivan;I. Martí-Vidal;T. Savolainen;A. Tchekhovskoy;A. Tchekhovskoy
中科院分区：
文献类型：
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作者：
T. Hovatta;S. O’Sullivan;I. Martí-Vidal;T. Savolainen;A. Tchekhovskoy;A. Tchekhovskoy
目标。我们在阿尔马上研究了类星体3C 273在1 mm波段的偏振行为，总带宽为7.5 GHz，223-243 GHz，分辨率为0.8“"，相当于3C 273距离处的2.1 kpc。通过这些观测，我们能够探测靠近喷流底部的光学薄偏振发射，并限制磁场结构。方法.我们利用简单的线性拟合和法拉第旋转测度合成计算了法拉第旋转测度。此外，我们模拟了分数阶斯托克斯Q和U参数的宽带行为（qu-fitting）。通过蒙特卡罗模拟对阿尔马偏振观测的系统不确定性进行了评估。结果我们发现3C 273的未分辨核是1.8%的线偏振。当我们假设一个单一的偏振组件和一个外部RM屏幕时，我们在1 mm波段上检测到一个非常高的旋转测量（RM），为（5.0 ± 0.3）× 105 rad m−2。这导致>40°的固有电矢量位置角的旋转，这显著高于通常针对毫米波长所假设的旋转。偏振分数作为波长的函数而增加，根据我们的qu-fitting，这可能是由于我们的光束内不同法拉第深度的多个偏振分量或内部法拉第旋转。由于我们有限的波长覆盖范围，我们无法区分这些情况，需要额外的多频和高角分辨率观测来确定法拉第活动区磁场的位置和结构。将我们的RM估计值与在较低频率下获得的值进行比较，RM作为观测频率的函数而增加，遵循指数为2.0 ± 0.2的幂律，与围绕锥形扩张射流的鞘层一致。我们还检测到约0.2%的圆偏振，但需要进一步的观察来证实这一结果。
Aims. We studied the polarization behavior of the quasar 3C 273 over the 1 mm wavelength band at ALMA with a total bandwidth of 7.5 GHz across 223–243 GHz at 0.8′′ resolution, corresponding to 2.1 kpc at the distance of 3C 273. With these observations we were able to probe the optically thin polarized emission close to the jet base, and constrain the magnetic field structure. Methods. We computed the Faraday rotation measure using simple linear fitting and Faraday rotation measure synthesis. In addition, we modeled the broadband behavior of the fractional Stokes Q and U parameters (qu-fitting). The systematic uncertainties in the polarization observations at ALMA were assessed through Monte Carlo simulations. Results. We find the unresolved core of 3C 273 to be 1.8% linearly polarized. We detect a very high rotation measure (RM) of (5.0 ± 0.3) × 105 rad m−2 over the 1 mm band when assuming a single polarized component and an external RM screen. This results in a rotation of >40° of the intrinsic electric vector position angle, which is significantly higher than typically assumed for millimeter wavelengths. The polarization fraction increases as a function of wavelength, which according to our qu-fitting could be due to multiple polarized components of different Faraday depth within our beam or to internal Faraday rotation. With our limited wavelength coverage we cannot distinguish between the cases, and additional multifrequency and high angular resolution observations are needed to determine the location and structure of the magnetic field of the Faraday active region. Comparing our RM estimate with values obtained at lower frequencies, the RM increases as a function of observing frequency, following a power law with an index of 2.0 ± 0.2, consistent with a sheath surrounding a conically expanding jet. We also detect ~0.2% circular polarization, although further observations are needed to confirm this result.