# The LHAASO PeVatron Bright Sky: What We Learned
## 2 PeVatron Candidate Source Typologies  
Despite some features revealed in the SNRs confirming that these sources accelerate the low energy component of galactic CRs, we do not have proof of their emission of UHE \(\gamma\)-ray. Even the youngest SNRs, with a high shock velocity, amplified magnetic field, and dense environment, show spectra with a cutoff well below \(E=100\) TeV (see Figure 4). Moreover, there are other challenges related to the role of SNRs in CR acceleration; their observed \(\gamma\)-ray spectra, which are different from the theoretical ones, and some problematic issues correlated with CR abundances (see [8], and reference therein), which motivated the search for alternative sources. Furthermore, in recent years, the assumption that SNRs may not be the only source of CRs had already been supported by several detections of VHE/UHE \(\gamma\)-ray emissions from different source types before the LHAASO results. This possibility is also supported by the higher complexity of the CR spectrum shown by CALET (see Figure 1, right) which seems to be an indication that we need several kinds of source to explain all of the spectral features of CRs. Indeed, different sources can contribute to the CR spectrum at different energies [71; 75]: in this hypothesis, for example, the CR \"knee\" could be the transition region between the contribution of standard isolated SNRs and that of the SNRs in MSCs (see Figure 3). With the large amount of experimental data collected so far and with theoretical models developed in the context of CR acceleration, we have important constraints with which a certain kind of source must comply to be considered a hadronic PeVatron. From a theoretical point of view, the hadronic PeVatron requirements can be summarized in the following way [67]:  
* Energetics--Hadronic PeVatrons must explain the CR power of the Galaxy, estimated to be about \(P_{CR}\sim\)10\({}^{40}\)-10\({}^{41}\) erg/s [11];
* Power-law injected spectrum--According to DSA, the accelerated spectrum at the source must follow a power-law that, after correction for transport effects and CR anisotropy, is \(\propto E^{-2.1\div 2.3}\)[10; 11];
* Maximum energy--The acceleration efficiency at the source must be able to explain the PeV particle energies;
* Anisotropy--The source distribution must be correlated with the CR anisotropy in the arrival direction predicted by a diffusive model for CRs [11; 76];
* Composition--Galactic CR sources must explain why their composition that is slightly different with respect to solar CRs (e.g., large overabundance of light elements, overabundance of metals with respect to H and He) [10; 11]. Moreover, the composition varies with energy (e.g., different elements have different spectral indices), and this energy dependence also needs to be reproduced. From an observational point of view, a candidate hadronic PeVatron must have certain precise features [77]:  
* Evidence of UHE \(\gamma\)-ray emission (\(E>100\) TeV)--If we have CR acceleration, we expect protons that are accelerated at least up to 1 PeV and, consequently, \(\gamma\)-ray emission well above 100 TeV without a cutoff in the spectrum;
* Spectral curvature--The parent particle power-law is modified by the existence of a maximum energy and/or by some breaks due to losses;
* Correlation with target material--The p-p interaction is most favored in the presence of a dense target in the source surroundings [76; 78]
* Extended emissions with a likely energy-dependent morphology--CRs diffuse through the media surrounding the source and diffusion (as in cooling) is energy dependent;
* Multi-wavelength counterpart--Secondary non-thermal electrons produced in the p-p interaction are emitted in radio/X-ray bands. All these observational requirements can also be found for leptonic PeVatrons. Consequently, a cross-check between experimental data and theoretical models is fundamental, in order to disentangle PeVatron and CR sources (hadronic PeVatrons).