Mrk 501在X射线与γ射线历史低活动期间的多信使表征
Multi-messenger characterization of Mrk 501 during historically low X-ray and $γ$-ray activity
AI总结：
本研究利用多波段观测数据，分析了耀变体Mrk 501在历史低活动期的辐射特征，验证了轻子模型的适用性，明确了γ射线区位置，并提出基线与可变辐射分别源于定常激波和行进/膨胀激波的解释。
摘要：
我们利用2017年至2020年的多波段观测数据研究了Mrk 501的宽波段辐射，观测动用了大量仪器，其中包括MAGIC、Fermi-LAT、NuSTAR、Swift、GASP-WEBT和OVRO。Mrk 501呈现出极低的宽波段活动水平，这或许有助于揭示其基线辐射特征。尽管如此，我们在所有波段都探测到了显著的流量变化，其中变化幅度最大的出现在X射线和甚高能（VHE）γ射线波段。我们测得X射线与甚高能γ射线之间存在显著相关性（>3σ），这支持了轻子模型可用于解释辐射的可变部分——即便在低活动期也是如此。当我们将数据范围从2008年拓展至2020年时，这一结论得到了进一步支持，并且我们首次识别出Swift-XRT与Fermi-LAT数据之间的显著相关性。此外我们还发现高能γ射线与射电辐射之间存在相关性，其中射电辐射滞后超过100天，这表明γ射线辐射区位于喷流中射电亮区的上游。另外，2017年中至2019年中，Mrk 501的X射线和甚高能γ射线活动处于历史低位，其能量>0.2 TeV的甚高能流量稳定在蟹状星云辐射水平的5%。这段持续2年的低态（可能是Mrk 501的基线辐射）的宽波段谱能量分布（SED），既可以用单区轻子模型描述，也可以用满足IceCube中微子流量约束的（轻）强子模型描述。我们探究了SED向低态演化的时间过程，发现稳定的基线辐射可能归因于定常激波，而可变辐射则归因于额外的膨胀激波或行进激波。
英文摘要：
We study the broadband emission of Mrk 501 using multi-wavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi-LAT, NuSTAR, Swift, GASP-WEBT, and OVRO. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wavebands, with the highest occurring at X-rays and very-high-energy (VHE) $γ$-rays. A significant correlation ($>$3$σ$) between X-rays and VHE $γ$-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between Swift-XRT and Fermi-LAT. We additionally find correlations between high-energy $γ$-rays and radio, with the radio lagging by more than 100 days, placing the $γ$-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE $γ$-rays from mid-2017 to mid-2019 with a stable VHE flux ($>$0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2-year-long low-state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED towards the low-state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock.
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