约束$z$的无线电特性=6.44 QSO VIK J2318 $-$ 3113
Constraining the Radio Properties of the $z$=6.44 QSO VIK J2318$-$3113
摘要：
最近在快速ASKAP连续巡天（RACS）中发现了红移为$z=6.44$的类恒星天体（QSO）VIKING J231818.3 $-$ 311346（以下简称VIK J2318 $-$ 3113），发现了它的无线电噪声性质，使其成为迄今为止已知的最遥远的此类天体之一。通过使用来自银河系和质量大会23 $^\mathrm{h}$场的多次无线电调查以及专门后续行动的数据，我们能够将VIK J2318 $-$ 3113的无线电频谱限制在观测范围$\sim$ 0.1--10 GHz内。在高频（观测帧中为0.888- 5.5 GHz），类星体呈现陡峭的谱（$α_{\rm r}$ =1.24，有$S_ν\propto ν^{-α_{\rm r}}$），而在低频（观测帧中为0.4- 0.888 GHz），它几乎是平坦的。整个频谱可以用一个曲线函数来模拟，其静止帧周转率约为5 GHz，或者用一个平滑变化的双幂律来模拟，该双幂律在静止帧中断频率约为20 GHz以下是平坦的，而在静止帧中断频率以上则明显变陡。根据所采用的模型，我们估计VIK J2318 $-$ 3113的射电喷流必须有几百年的历史，在营业额的情况下，或少于几个$\times$ 10 $^4$年，在频谱中断的情况下。在两个频率（888 MHz和5.5 GHz）下进行了多次观测，我们进一步研究了先前报道的该源的无线电变化。我们发现，轻微显着的通量密度的变化是一致的折射星际闪烁的预期，即使相对论效应的源的方向可能仍然发挥不可忽视的作用。需要进一步的无线电和X射线观测来最终辨别这种变化的性质。
英文摘要：
The recent detection of the quasi-stellar object (QSO) VIKING J231818.3$-$311346 (hereafter VIK J2318$-$3113) at redshift $z=6.44$ in the Rapid ASKAP Continuum Survey (RACS) uncovered its radio-loud nature, making it one of the most distant known to date in this class. By using data from several radio surveys of the Galaxy and Mass Assembly 23$^\mathrm{h}$ field and from dedicated follow-up, we were able to constrain the radio spectrum of VIK J2318$-$3113 in the observed range $\sim$0.1--10 GHz. At high frequencies (0.888--5.5 GHz in the observed frame) the QSO presents a steep spectrum ($α_{\rm r}$=1.24, with $S_ν\propto ν^{-α_{\rm r}}$), while at lower frequencies (0.4--0.888 GHz in the observed frame) it is nearly flat. The overall spectrum can be modelled by either a curved function with a rest-frame turnover around 5 GHz, or with a smoothly varying double power law that is flat below a rest-frame break frequency of about 20 GHz and which significantly steepens above it. Based on the model adopted, we estimated that the radio jets of VIK J2318$-$3113 must be a few hundred years old, in the case of a turnover, or less than few$\times$10$^4$ years, in the case of a break in the spectrum. Having multiple observations at two frequencies (888 MHz and 5.5 GHz), we further investigated the radio variability previously reported for this source. We found that the marginally significant flux density variations are consistent with the expectations from refractive interstellar scintillation, even though relativistic effects related to the orientation of the source may still play a non-negligible role. Further radio and X-ray observations are required to conclusively discern the nature of this variation.
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