Speaker
Description
It is difficult to identify hadronic PeVatrons (the PeV particle accelerator) from the ultra-high-energy (UHE, $E$ > 100 TeV) gamma-ray sources, which is however crucial in revealing the origin of cosmic rays. As an endeavor in this regard, we focus in this work on the UHE gamma-ray source 1LHAASO J1857+0203u, which may be associated with supernova remnant (SNR) G35.6−0.4 and H II region G35.6−0.5. We analyze the LHAASO WCDA and KM2A data, and report the point-like nature with a significance of 10.1$\sigma$ above 100 TeV. While in the energy range of 1−30 TeV, it shows an extension of $r_{39}=0.18^\circ$. The spectra measured by WCDA and KM2A can be smoothly connected, with power-law spectral indexes of $\sim$2.5 and $\sim$3.2, respectively. Given the low mass of the environmental molecular clouds, it is unlikely that the TeV gamma-ray emissions arise from the clouds illuminated by the protons escaped from SNR G35.6−0.4. In the scenario that HII region G35.6−0.5 can accelerate particles to the UHE band, the observed GeV-TeV gamma-ray emission could be well explained by a hadronic model with a power-law spectral index of $\sim$2.0 and a cutoff energy of $\sim$260 TeV. However, a pulsar-wind-nebula origin cannot be ruled out.
