Cao Z.Aharonian F.AxikeguBai Y.X.Bao Y.W.Bastieri D.Bi X.J.Bi Y.J.Bian W.Bukevich A.V.Cao Q.Cao W.Y.Cao Z.Chang J.Chang J.F.Chen A.M.Chen B.Q.Chen E.S.Chen H.X.Chen L.Chen L.Chen L.Chen M.J.Chen M.L.Chen Q.H.Chen S.Chen S.H.Chen S.Z.Chen T.L.Chen Y.Cheng N.Cheng Y.D.Chu M.C.Cui M.Y.Cui S.W.Cui X.H.Cui Y.D.Dai B.Z.Dai H.L.Dai Z.G.DanzengluobuDong X.Q.Duan K.K.Fan J.H.Fan Y.Z.Fang J.Fang J.H.Fang K.Feng C.F.Feng H.Feng L.Feng S.H.Feng X.T.Feng Y.Feng Y.L.Gabici S.Gao B.Gao C.D.Gao Q.Gao W.Gao W.K.Ge M.M.Ge T.T.Geng L.S.Giacinti G.Gong G.H.Gou Q.B.Gu M.H.Guo F.L.Guo J.Guo X.L.Guo Y.Q.Guo Y.Y.Han Y.A.Hannuksela O.A.Hasan M.He H.H.He H.N.He J.Y.He Y.Hor Y.K.Hou B.W.Hou C.Hou X.Hu H.B.Hu Q.Hu S.C.Huang C.Huang D.H.Huang T.Q.Huang W.J.Huang X.T.Huang X.Y.Huang Y.Huang Y.Y.Ji X.L.Jia H.Y.Jia K.Jiang H.B.Jiang K.Mahidol University2025-02-012025-02-012025-01-20Astrophysical Journal Vol.979 No.1 (2025)0004637Xhttps://repository.li.mahidol.ac.th/handle/20.500.14594/103149Identifying Galactic PeVatrons (PeV particle accelerators) from ultrahigh-energy (UHE, >100 TeV) γ-ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.6−0.4 or H ii region G35.6−0.5. We perform detailed analysis on the very-high-energy and UHE γ-ray emissions toward this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6σ above 100 TeV, indicating the presence of a PeVatron. It has an extent of ∼0 . ° 18 with a power-law (PL) spectral index of ∼2.5 at 1-25 TeV and pointlike emission with a PL spectral index of ∼3.2 above 25 TeV. Using archival CO and H i data, we identify some molecular and atomic clouds that may be associated with the TeV γ-ray emissions. Our modeling indicates that the TeV γ-ray emissions are unlikely to arise from clouds illuminated by the protons that escaped from SNR G35.6−0.4. In the scenario in which H ii region G35.6−0.5 could accelerate particles to the UHE band, the observed GeV-TeV γ-ray emission could be well explained by a hadronic model with a PL spectral index of ∼2.0 and cutoff energy of ∼450 TeV. However, an origin in an evolved pulsar wind nebula cannot be ruled out.Earth and Planetary SciencesPhysics and AstronomyArticleSCOPUS10.3847/1538-4357/ad991d2-s2.0-8521612517515384357