Polarimetric Properties of Event Horizon Telescope Targets from ALMA
Polarimetric Properties of Event Horizon Telescope Targets from ALMA
复制标题
ALMA 事件视界望远镜目标的偏振特性
DOI：
10.3847/2041-8213/abee6a
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发表时间：
2021-03-01
影响因子：
7.9
通讯作者：
Valtonen, Mauri
中科院分区：
文献类型：
--
作者：
Goddi, Ciriaco;Marti-Vidal, Ivan;Valtonen, Mauri
我们介绍了在2017年4月第一次VLBI活动期间使用ALMA进行的全面极化研究的结果，该研究分别与全球mm-VLBI阵列（GMVA）和事件视界望远镜（EHT）一起在$\lambda$ 300 mm和$\lambda$1.3mm波段进行。我们确定了所有VLBI目标的极化和法拉第性质，包括Sgr A*， M87和十几个无线电响AGN。我们检测到高线性极化分数（2-15%）和大旋转测量（RM $>10^{3.3}-10^{5.5}$ rad m$^{-2}$）。对于Sgr A*，我们报告了在1.3 mm处的平均RM为$(-4.2\pm0.3) \times10^5$ rad m$^{-2}$，这与过去十年的测量结果一致，并且，这是第一次，在3 mm处的RM为$(-2.1\pm0.1) \times10^5$ rad m$^{-2}$，这表明在1.3 mm处法拉第旋转的大约一半可能发生在3 mm光球和1.3 mm光源之间。我们还报道了第一次在毫米波长上对M87核的RM的明确测量，它在一年的时间尺度上经历了显著的星等变化和符号反转，范围从-1.2到0.3美元\倍，10^5$ rad m$^{-2}$在3毫米和-4.1到1.5美元\倍，10^5$ rad m$^{-2}$在1.3毫米。考虑到这种时间的可变性，我们认为，与Sgr A*的情况不同，M87中的RM不能提供对黑洞质量吸积速率的准确估计。我们提出了一个由可变紧凑区和静态扩展区组成的双组分模型，该模型可以同时解释EHT和ALMA观测到的偏振特性。这些测量为同时获得的VLBI数据的校准、分析和解释提供了关键的约束。
We present the results from a full polarization study carried out with ALMA during the first VLBI campaign, which was conducted in Apr 2017 in the $\lambda$3mm and $\lambda$1.3mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud AGN. We detect high linear polarization fractions (2-15%) and large rotation measures (RM $>10^{3.3}-10^{5.5}$ rad m$^{-2}$). For Sgr A* we report a mean RM of $(-4.2\pm0.3) \times10^5$ rad m$^{-2}$ at 1.3 mm, consistent with measurements over the past decade, and, for the first time, an RM of $(-2.1\pm0.1) \times10^5$ rad m$^{-2}$ at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source. We also report the first unambiguous measurement of RM toward the M87 nucleus at mm wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year time-scale, spanning the range from -1.2 to 0.3 $\times\,10^5$ rad m$^{-2}$ at 3 mm and -4.1 to 1.5 $\times\,10^5$ rad m$^{-2}$ at 1.3 mm. Given this time variability, we argue that, unlike the case of Sgr A*, the RM in M87 does not provide an accurate estimate of the mass accretion rate onto the black hole. We put forward a two-component model, comprised of a variable compact region and a static extended region, that can simultaneously explain the polarimetric properties observed by both the EHT and ALMA. These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA.