MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS. XI. SPECTRAL DISTRIBUTIONS
MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS. XI. SPECTRAL DISTRIBUTIONS
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DOI：
10.1088/0004-6256/147/6/143
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发表时间：
2014-03
期刊：
影响因子：
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通讯作者：
T. Hovatta;M. Aller;H. Aller;E. Clausen-Brown;D. Homan;Y. Kovalev;M. Lister;A. Pushkarev;
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作者：
T. Hovatta;M. Aller;H. Aller;E. Clausen-Brown;D. Homan;Y. Kovalev;M. Lister;A. Pushkarev;
通过VLBA实验观测活动星系核喷流（莫哈韦）项目，获得了190个河外射电喷流的毫角秒尺度谱指数分布。2006年在8.1，8.4，12.1和15.4 GHz观测到的源，我们已经确定了8.1和15.4 GHz之间的光谱指数图，以研究单个喷流特征的四个频谱。我们已经进行了详细的模拟研究的影响，图像对齐和（u，v）平面覆盖的光谱指数图，以验证我们的结果。我们利用谱指数图研究了谱指数沿喷流沿着的演化，确定了喷流不同位置的谱分布。核心光谱指数平均平坦，样品的平均值为+0.22 ± 0.03，而喷流光谱一般陡峭，平均指数为-1.04 ± 0.03。一个简单的幂律拟合往往是不够的核心区域，如预期的，如果核心是部分自吸收。当平谱射电类星体的光谱（平均值为-1.09 ± 0.04）比BL Lac天体（平均值为-0.80 ± 0.05）陡峭得多时，整体喷流光谱会以约-0.001到-0.004/去投影秒差距的速率变陡。然而，在这两种类型的对象的频谱上的平均值为0.02，在喷流组件的位置，指示粒子加速或密度增强沿着射流。组件位置的平均光谱指数为−0.81 ± 0.02，对应于电子能量分布的幂律指数为1.26。我们发现了一个显着的趋势，射流组件与线性偏振平行的射流（磁场垂直于射流）有平坦的光谱，预期的横向冲击。与类星体相比，BL拉克具有更多的垂直磁场排列的喷流成分，这可以解释它们通常更平坦的光谱。光谱随距离的整体变陡可以用辐射损失来解释，如果喷流是准直的，或者用电子光谱中高能截止的演化来解释，如果喷流是圆锥形的。这种解释是支持一个显着的相关性与年龄的组件和光谱指数，与较老的组件具有陡峭的光谱。
We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the Monitoring of Jets in Active Galactic Nuclei with the VLBA Experiments (MOJAVE) project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we have determined spectral index maps between 8.1 and 15.4 GHz to study the four-frequency spectrum in individual jet features. We have performed detailed simulations to study the effects of image alignment and (u, v)-plane coverage on the spectral index maps to verify our results. We use the spectral index maps to study the spectral index evolution along the jet and determine the spectral distributions in different locations of the jets. The core spectral indices are on average flat with a mean value of +0.22 ± 0.03 for the sample, while the jet spectrum is in general steep with a mean index of −1.04 ± 0.03. A simple power-law fit is often inadequate for the core regions, as expected if the cores are partially self-absorbed. The overall jet spectrum steepens at a rate of about −0.001 to −0.004 per deprojected parsec when moving further out from the core with flat spectrum radio quasars having significantly steeper spectra (mean −1.09 ± 0.04) than the BL Lac objects (mean −0.80 ± 0.05). However, the spectrum in both types of objects flattens on average by ∼0.2 at the locations of the jet components indicating particle acceleration or density enhancements along the jet. The mean spectral index at the component locations of −0.81 ± 0.02 corresponds to a power-law index of ∼2.6 for the electron energy distribution. We find a significant trend that jet components with linear polarization parallel to the jet (magnetic field perpendicular to the jet) have flatter spectra, as expected for transverse shocks. Compared to quasars, BL Lacs have more jet components with perpendicular magnetic field alignment, which may explain their generally flatter spectra. The overall steepening of the spectra with distance can be explained with radiative losses if the jets are collimating or with the evolution of the high-energy cutoff in the electron spectrum if the jets are conical. This interpretation is supported by a significant correlation with the age of the component and the spectral index, with older components having steeper spectra.