Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157
Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157
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银河系 PeVatron 候选者 LHAASO J2108 5157 的多波长研究
DOI：
10.1051/0004-6361/202245086
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发表时间：
2023
期刊：
影响因子：
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通讯作者：
et al.
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作者：
Project CTA-LST;Jurysek J.;Balbo M.;Eckert D.;Tramacere A.;et al.
最近，大型高空空气簇射天文台（LHAASO）合作发现了几个新的超高能（UHE）γ射线源。这代表着在寻找所谓的银河系PeVatrons方面向前迈出了一步，PeV能量的银河系宇宙射线的神秘来源。然而，多TeV的γ射线发射并不一定证明源中存在强子加速器;事实上，这种发射也可以解释为辐射主导环境中电子的逆康普顿散射。只有考虑到多波长数据和源的详细形态，才有可能明确区分两种主要发射机制。它是已知的少数几个没有甚高能量（VHE）对应物的UHE源之一。牛顿在2021年总共进行了3.8小时，并使用大型望远镜原型（LST-1）以TeV的能量进行了49小时的高质量数据。此外，我们分析了12年的费米LAT数据，以更好地限制其高能（HE）对应物4FGL J2108.0+5155的发射。我们使用naima和jetset软件包检查轻子和强子的情况下的多波长emission的source.ResultsWe发现一个过剩（3.7σ）在LST-1的数据在energiesE> 3 TeV。 进一步分析整个LST-1能量范围，假设一个点状源，得到一个暗示（2.2σ）的硬发射，它可以用一个单一的幂律来描述，光子指数为Γ = 1.6 ± 0.2，范围为0.3 - 100 TeV。 在XMM-牛顿数据中，我们没有发现任何可能与超新星遗迹（SNR）或脉冲星风星云（PWN）有关的显着扩展发射，这对相对论电子可能的同步辐射产生了很大的限制。我们在Fermi-LAT数据中发现了一个新的潜在硬源，其显著性为4σ，光子指数为Γ = 1.9 ± 0.2，它与LHAASO J2108+5157在空间上不相关，但将其包含在源模型中，我们能够改进HE对应物4FGL J2108.0+ 5155的光谱表示。结论LST-1和LHAASO观测可以解释为相对论电子的反康普顿主导轻子发射，截止能量为TeV。 同步辐射的X射线上限所施加的源中的低磁场与PWN或TeV晕的假设是兼容的。此外，HE对应物的光谱特性与Geminga类脉冲星一致，这将能够为VHE-UHE发射提供动力。然而，在UHE源附近缺乏脉冲星是对PWN/TeV晕场景的挑战。UHEγ射线也可以解释为π 0衰变主导的强子发射，由于相对论质子与源方向上两个已知分子云之一的相互作用。事实上，LST-1波段的硬谱与在中年SNR附近逃离冲击的质子是相容的，因为它们具有高的低能截止，但HEγ射线发射的起源仍然是一个悬而未决的问题。
ContextSeveral new ultrahigh-energy (UHE)γ-ray sources have recently been discovered by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These represent a step forward in the search for the so-called Galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeVγ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse Compton scattering from electrons in a radiation-dominated environment. A clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source.AimsWe aim to understand the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no very high-energy (VHE) counterpart.MethodsWe observed LHAASO J2108+5157 in the X-ray band withXMM-Newtonin 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good-quality data. In addition, we analyzed 12 years ofFermi-LAT data, to better constrain emission of its high-energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source.ResultsWe found an excess (3.7σ) in the LST-1 data at energiesE> 3 TeV. Further analysis of the whole LST-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of Γ = 1.6 ± 0.2 the range of 0.3 − 100 TeV. We did not find any significant extended emission that could be related to a supernova remnant (SNR) or pulsar wind nebula (PWN) in theXMM-Newtondata, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source inFermi-LAT data with a significance of 4σand a photon index of Γ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155.ConclusionsThe LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which would be able to power the VHE-UHE emission. Nevertheless, the lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHEγrays can also be explained asπ0decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. Indeed, the hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off, but the origin of the HEγ-ray emission remains an open question.