# The LHAASO PeVatron Bright Sky: What We Learned
## 2 PeVatron Candidate Source Typologies
### TeV Halos  
The discovery of TeV halos surrounding pulsars can be attributed to the extended TeV emission detected around the Geminga and Monogem (B0654+14) pulsars by MILAGRO [116] and HAWC [117]. The extension of these \(\gamma\)-ray sources is about 2\({}^{\circ}\), indicating that CR propagation near these pulsars is more constrained with respect to that in the ISM [118]. TeV halos can represent a local environment for studying CR propagation with small-scale diffusion models [119]. Their emission is due to a slow diffusion of the electrons in the ISM around the pulsars (several hundred times smaller than the galactic diffusion coefficient [45]). Although we do not know the nature of the environment that implies a slowing down of the diffusion, we know that the \(\gamma\)-ray emission is due to the IC scattering of electrons accelerated by the pulsar in the galactic and cosmological radiation fields [119]. The discovery of this new class of objects is also important because they could allow the discovery of a population of \"invisible\" pulsars that have no beamed radio or HE beamed emissions. Moreover, the spectral study of TeV halos is an alternative method for constraining the injection of PWNe electrons because their spectrum only depends on the electron spectral cutoff.  
The large extension (\(\sim\)1 degree) of many of the LHAASO sources and their correlations with pulsars increased the interest around this new class of sources (see Figure 19). The only firm TeV halo detected by LHAASO is LHAASO J0621+3755 [45], which is not treated in this review because of its low UHE \(\gamma\)-ray significance (\(<\)5 \(\sigma\) at 100 TeV). It is one of the currently known TeV halos, together with the Geminga halo, Monogem halo, and HESS J1831-098 [119]. The main problem related to TeV halo detection is the possibility of confusing them with extended SNRs [118]. In order to verify the TeV halo origin of a source, beyond the co-location of the bulk of its emission with a pulsar, this pulsar needs to have a spin-down luminosity sufficient to generate a halo and needs to be enough old to allow electrons to diffuse in the ISM (bow-shock nebula [70]). Furthermore, the extension of the halo has to be larger than the possible PWN X-ray emission, otherwise the emission  
Figure 5: The LHAASO candidate PeVatron with a PWN as a potential TeV candidate in the PWN parameter space, according to the model in [26]. All the sources could be PWN, without violating PWN physics, apart from LHAASO J2032+4127, which is likely associated with the Cygnus Cocoon [Figure from [26]].  
could be a \(\gamma\)-ray PWN, and its morphology must be explained by slow-diffusion, otherwise this could indicate a hadronic accelerator [119].  
The low angular resolution of the EAS experiments does not allow resolving the TeV halo morphology and, unfortunately, the size of these objects is challenging for current IACTs because of their small field of view. Indeed, Geminga and Monogem are not detected by these instruments, despite their high TeV flux. The future ASTRI Mini-Array, with a FoV of \(10^{\circ}\) at E \(>10\) TeV, and CTA SSTs in the southern hemisphere, with a FoV \(>8^{\circ}\), could finally resolve the morphology of some of these objects and detect a high-resolution spectrum. In the ASTRI Mini-Array core science paper [62], a first simulation of the Geminga halo was developed, building a radial profile and showing a high-significance spectrum up to 50 TeV with a single observation. Multiple observations will allow the ASTRI Mini-Array to better track the source morphology.