THE CIRCULAR POLARIZATION OF SAGITTARIUS A* AT SUBMILLIMETER WAVELENGTHS
THE CIRCULAR POLARIZATION OF SAGITTARIUS A* AT SUBMILLIMETER WAVELENGTHS
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DOI：
10.1088/0004-637x/745/2/115
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发表时间：
2011-05
期刊：
影响因子：
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通讯作者：
D. Muñoz;D. Marrone;J. Moran;R. Rao
中科院分区：
文献类型：
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作者：
D. Muñoz;D. Marrone;J. Moran;R. Rao
我们报告了在1.3 mm波长（230 GHz）和860 μm（345 GHz）的1.2% ± 0.3%水平上首次探测到来自致密射电源和超大质量黑洞候选者Sgr A* 的亚毫米波长圆偏振发射，具有相同的旋向性，左旋圆偏振（LCP），如在所有较低频率（1.4-15 GHz）下观察到的。使用亚毫米阵列在多个时期进行的观测也显示出在两个波长处约6%的同时线性偏振（LP）。这些特性与波长大于1 cm（频率低于30 GHz）的特性明显不同，其中弱圆偏振（CP）（≤ 0.5%）在LP上占主导地位，而LP在类似的分数限制下未检测到。我们描述了一组广泛的测试，以确保我们的测量的准确性。我们在任何其他源中都没有发现CP，包括明亮的类星体1924-292，它在天空中的轨迹与Sgr A* 相同，因此应该受到来自仪器框架的相同系统误差的影响。由于相对论性同步加速器等离子体预计将产生很少的CP，所观察到的CP可能产生接近事件视界的法拉第转换过程。我们用一个简单的近似来证明与法拉第转换相关的相移几乎与频率无关，这是使CP的旋向性与频率无关的充分条件。由于τ = 1表面的大小在1.4和345 GHz之间变化超过一个数量级，因此磁场必须在这样的尺度上一致地产生LCP。为了提高我们对SgrA* 环境的理解，未来的关键测量包括确定法拉第旋转是否偏离波长的λ2依赖性，以及通量密度的圆形和线性分量是否相关。
We report the first detections of circularly polarized emission at submillimeter wavelengths from the compact radio source and supermassive black hole candidate Sgr A* at a level of 1.2% ± 0.3% at 1.3 mm wavelength (230 GHz) and 1.6% ± 0.3% at 860 μm (345 GHz) with the same handedness, left circular polarization (LCP), as observed at all lower frequencies (1.4–15 GHz). The observations, taken with the Submillimeter Array in multiple epochs, also show simultaneous linear polarization (LP) at both wavelengths of about 6%. These properties differ sharply from those at wavelengths longer than 1 cm (frequencies below 30 GHz), where weak circular polarization (CP) (∼0.5%) dominates over LP, which is not detected at similar fractional limits. We describe an extensive set of tests to ensure the accuracy of our measurements. We find no CP in any other source, including the bright quasar 1924-292, which traces the same path on the sky as Sgr A* and therefore should be subject to identical systematic errors originating in the instrument frame. Since a relativistic synchrotron plasma is expected to produce little CP, the observed CP is probably generated close to the event horizon by the Faraday conversion process. We use a simple approximation to show that the phase shift associated with Faraday conversion can be nearly independent of frequency, a sufficient condition to make the handedness of CP independent of frequency. Because the size of the τ = 1 surface changes by more than an order of magnitude between 1.4 and 345 GHz, the magnetic field must be coherent over such scales to consistently produce LCP. To improve our understanding of the environment of SgrA* critical future measurements includes determining whether the Faraday rotation deviates from a λ2 dependence in wavelength and whether the circular and linear components of the flux density are correlated.