Spectrum and Morphology of the Ultra-High-Energy Source LHAASO~J2018+3651

The LHAASO~J2018+3651 region is one of the brightest sources in the sky at TeV energies. Photons with energies up to ∼0.27 PeV from this region have been detected with the Large High Altitude Air Shower Observatory (LHAASO) and here we present a detailed study of this region using more data from LHAASO. This analysis resolves the region into six sources: LHAASO~J2018+3641, LHAASO~J2019+3649, LHAASO~J2021+3657, LHAASO~J2016+3712, LHAASO~J2013+3610 and LHAASO~J2028+3655. An investigation of the morphology and spectrum for LHAASO~J2018+3641, LHAASO~J2019+3649 and LHAASO~J2021+3657 are the focus and we find that the spectrum above TeV deviates significantly from a single power-law, and is best described by a power-law with exponential cutoff. The three sources may be associated with PSR~J2021+3651 and its X-ray pulsar wind nebula. Additionally, LHAASO~J2016+3712 is tentatively associated with the evolved supernova remnant CTB 87.