# The LHAASO PeVatron Bright Sky: What We Learned
## 2 PeVatron Candidate Source Typologies
### The Standard Candidates: Supernova Remnants  
Since the beginning of CR study, SNRs have been considered \"the source\" of galactic CRs, because they respect most of the theoretical and observational requirements of hadronic PeVatrons (see [8], for a recent review). First of all, their power is able to sustain the CR flux measured in our galaxy: \(\dot{E}_{SN}\approx 10^{42}\) erg s\({}^{-1}\), consequently, \(E_{CR}\approx 10\%\dot{E}_{SN}\)[79]. Thus, they are ideal laboratories for particle acceleration using the DSA mechanism [80; 81; 82]; the first-order Fermi acceleration mechanism works very well in their environment, producing the required pure power-law spectrum. Various experimental results have supported this hypothesis with indirect proofs. First of all, there have been several detections of their extended emission at HE and VHE \(\gamma\)-ray, as well as their very frequent association with high-density MCs. Then, the presence of non-thermal and fast variable X-ray emissions coincident with TeV \(\gamma\)-rays indicates the presence of magnetic field amplification, a fundamental condition of reaching VHE energies [83; 84; 85; 86]. The modified Balmer line detection is evidence of the CR back-reaction to the DSA [87], and finally their spatial distribution is compatible with the CR distribution and enough SNRs exist to explain the anisotropy [11]. Furthermore, a first real breakthrough came in 2011, when the AGILE satellite showed, for the first time, direct proof of the presence of CRs in a shock of a SNR, the SNR W44 [12; 88], which was then confirmed using Fermi-LAT in the same source W44 and in other similar middle-aged SNRs, such as IC443, W51c, and W49b [13; 89; 90]. All these sources were detected at E \(<\) 100 MeV with a spectrum clearly correlated to the presence of a \"pion bump\".  
However, these detections confirmed that SNRs can accelerate CRs with low energies (\(E<\) 100 GeV) but a \(\gamma\)-ray clear detection at UHE is still missing. Theoretical works have shown that this could be due to the intrinsic properties of SNRs, which accelerate PeV particles only in the first 100 years of their evolution [15; 16; 91] and only very rarely (1 every 10\({}^{4}\) yrs) can high-power (\(E_{SN}\sim\) 5-10 \(\times\) 10\({}^{51}\) erg) explosions accelerate CRs to PeV energies at the beginning of their Sedov phase [92]. The only way to detect UHE \(\gamma\)-rays from SNRs is if the accelerated CRs, after their escape from the sources, are trapped in near GMCs thanks to suppression of the diffusion [11; 17; 18; 93]. The LHAASO results show that about 6 of the its 12 PeVatrons could be associated with (middle-aged) SNRs. Consequently, if a deeper study of these UHE emission regions could confirm their SNR nature, we would know if the emission is actually from near GMCs or, in case the UHE emission comes from the SNR itself, if we need to improve our knowledge of SNR microphysics [5; 92].  
Anyway, there are also other difficulties related to the link between SNRs and CR acceleration. The predicted spectrum does not match the observed ones, requiring modifications to the original DSA theory (see [8; 10], for a recent review). Moreover, if SNRs are the only CR sources, there are several composition issues that are not easy to explain: the most popular example is the \({}^{22}Ne/^{20}Ne\) abundance, which in Galactic CRs is about five-times larger than the one in Solar CRs and which cannot be explained in a SNR context. In the face of these many challenges, the search for other galactic CR contributors is well motivated.