Speaker
Description
The origin of Galactic sources capable of accelerating cosmic rays up to PeV energies (PeVatrons) remains a central question in high-energy astrophysics. Recently, LHAASO has detected ultra-high-energy ($E > 100$ TeV) gamma rays from approximately 50 sources, pinpointing locations of PeVatron candidates; however, about half of them remain unidentified due to the lack of observations at other wavelengths. In this study, we focus on 1LHAASO J0007+5659u, one of the dark gamma-ray sources detected only by LHAASO/KM2A. Using the Nobeyama 45-m radio telescope, we conducted $^{12}$CO, $^{13}$CO, and C$^{18}$O ($J=1\text{–}0$) observations over an area of $0.5^\circ \times 0.5^\circ$ centered on the source. We identified three molecular cloud. One of which is located near the center of the gamma-ray emission. These clouds are detected at $-38$ to $-35$ km s$^{-1}$ and $-4.2$ to $-2.3$ km s$^{-1}$. Their physical properties are estimated as follows: distances of $\sim0.4$ or $\sim3$ kpc, sizes of $\sim0.2$ or $\sim2$ pc, and number densities of $10^{2}$–$10^{3}$ cm$^{-3}$. These molecular clouds can serve as targets for proton–proton interactions, producing hadronic gamma rays via neutral pion decay. The total energy of cosmic-ray protons required to explain the observed gamma-ray flux is estimated to be $10^{43}$–$10^{45}$ erg.
We also performed similar observations toward 1LHAASO J1740+0948u, 1LHAASO J1959+1129u, and 1LHAASO J0216+4237u, but found no significant CO emission. In this contribution, we present these results and discuss the nature of the gamma-ray emission.
