# The LHAASO PeVatron Bright Sky: What We Learned
## 1 Introduction
### Status of the Field before the LHAASO Results  
One of the main channels for investigating the nature of galactic CRs and their features is the nonthermal HE (\(E>100\) MeV) \(\gamma\)-ray emissions, produced either by electrons, mainly through Bremsstrahlung and inverse Compton (IC) processes, and by protons via pion decay from p-p and p-\(\gamma\) interactions. The difference between the two main types of processes, leptonic or hadronic, forms the base of CR acceleration phenomena. The \(\gamma\)-raydetection of the so-called \"pion bump\" at about 100 MeV (due to \(\pi^{0}\) rest mass) is one of the direct proofs that confirm the presence of energized CRs in a source, and until recently, a detection of ultra-high energy (UHE, \(E>100\) TeV, this energy threshold for the UHE definition is valid in the \(\gamma\)-ray context. If we consider the particle context, the UHE range begins above E \(>10^{17-18}\) eV. \(\gamma\)-ray photons was considered unquestionable evidence of freshly accelerated CRs. Indeed, at these energies, electron IC should be limited by the Klein-Nishina regime (In the relativistic regime, \(hv_{i}>m_{e}c^{2}\), the Compton scattering is inelastic and characterized by \(\sigma_{KN}\), derived by the quantum electrodynamics, and not by \(\sigma_{T}\)) and UHE \(\gamma\)-ray should only be explained through the decay of neutral pions produced by collisions of PeV protons (CRs) with target protons (for recent reviews see [10; 11]). According to this view, the term \"PeVatrons\" has been coined to indicate galactic CR accelerators (sources emitting \(\gamma\)-ray at E \(>100\) TeV). In the standard paradigm, supernova remnants (SNRs) are the main contributors to galactic CRs [8]. We know that they energize CRs at the low \(\gamma\)-ray energies (MeV-GeV) thanks to their detection around the \"pion-bump\" energies by AGILE and Fermi-LAT [12; 13], likely even through re-acceleration [14], but in no SNRs were detected \(\gamma\)-ray photons with E \(>100\) TeV. According to theoretical models, the reason for this is that SNRs can accelerate CRs at these energies only in the first 100 years of their life, and all the known SNRs are older [15; 16]. The only way we can detect their UHE emission is in the case CRs are trapped in near giant molecular clouds (GMCs), emitting UHE photons for \(t>100\) yrs [11; 17; 18]. The lack of SNRs at UHE supports the possibility that other sources can accelerate CRs, and this possibility has been widely investigated. In 2011, the Fermi-LAT satellite observed extended \(\gamma\)-ray emissions associated with the superbubble surrounding the Cygnus OB2 region, then called \"Cygnus Cocoon\", for the first time confirming the hypothesis proposed by [19], that star forming regions are able to accelerate particles. Deeper analysis of this region, and further very-high energy (VHE, \(100\) GeV \(<E<100\) TeV) detections from other massive star clusters (MSCs), supported this possibility (see Sections 2.2 and 3.10 for more details). Later, the detection of several pulsar wind nebulae (PWNe) at \(E>56\) TeV with different instruments [20; 21; 22; 23], and in particular of the Crab Nebula at \(E>150\) TeV [20; 23], opened the possibility that the PWNe, leptonic accelerators \"by definition\", could also accelerate hadrons and, consequently, CRs. Moreover, UHE \(\gamma\)-ray detection from the Crab Nebula made it evident that leptons are able to emit beyond the Klein-Nishina limit. Several works have shown that leptonic emission through IC can predominate at  
Figure 1: Left: energy spectrum of HE CRs obtained from air-shower measurements (Figure from [6]). Right: proton spectrum measured by CALET, where the not “pure” PL behavior of the CR spectrum before the knee is evident (Figure from [9]). these energies in radiation-dominated environments [24] or in the surroundings of high spin-down powers pulsars [25; 26]. In this new context, another fundamental channel comes into play: neutrino detection.