用LOFAR在144 MHz下表征BL Lac的扩展形态
Characterising the extended morphologies of BL Lacs at 144 MHz with LOFAR
摘要：
我们提出了一个形态和光谱研究的样本99 BL湖使用LOFAR两米巡天第二次数据发布（LDR 2）。在BL Lac周围的千兆赫频率下已经确定了扩展发射，但是使用LDR 2，现在可以在144 MHz下系统地研究它们的形态，预计会有更多的扩散发射。LDR 2揭示了存在的扩展无线电结构周围66/99的BL Lac核，与角度范围高达115角秒，对应的空间范围为410千秒差距。扩展发射很可能是与喷流中相对论体运动有关的非对称扩散发射和束发射。扩展发射的空间范围和亮度与AGN统一方案一致，其中BL Lac对应于低激发射电星系，其喷流轴沿视线对齐。虽然在大部分BL Lac周围检测到扩展发射，但144-1400 MHz频谱指数的中位数和144 MHz处的核心优势表明，核心分量平均占总低频通量密度的42%。与144 MHz扩展通量密度和伽马射线光子通量（r = 0.42）相比，144 MHz核心通量密度和伽马射线光子通量（r = 0.69）之间存在更强的相关性。这表明，无线电到伽马射线的联系在低射频下减弱，因为产生伽马射线通量的粒子群与产生与空间扩展特征相关的扩散同步辐射的电子不同。
英文摘要：
We present a morphological and spectral study of a sample of 99 BL Lacs using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lacs, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115 arcseconds, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulk motion in jets. The spatial extents and luminosities of the extended emission are consistent with the AGN unification scheme where BL Lacs correspond to low-excitation radio galaxies with the jet axis aligned along the line-of-sight. While extended emission is detected around the majority of BL Lacs, the median 144-1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ~42% on average to the total low-frequency flux density. A stronger correlation was found between the 144 MHz core flux density and the gamma-ray photon flux (r = 0.69) compared to the 144 MHz extended flux density and the gamma-ray photon flux (r = 0.42). This suggests that the radio-to-gamma-ray connection weakens at low radio frequencies because the population of particles that give rise to the gamma-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially-extended features.
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