It is difficult to distinguish the hadronic process from the leptonic one in γ-ray observation, which is however crucial in revealing the origin of cosmic rays. As an endeavor in this regard, we focus in this work on the complex γ-ray emitting region, which partially overlaps with the unidentified TeV source HESS J1858+020 and includes supernova remnant (SNR) G35.6−0.4 and H ii region G35.6−0.5. We reanalyze CO line, H i, and Fermi-LAT GeV γ-ray emission data of this region. The analysis of the molecular and H i data suggests that SNR G35.6−0.4 and H ii region G35.6−0.5 are located at different distances. The analysis of the GeV γ-rays shows that GeV emission arises from two point sources: one (SrcA) coincident with the SNR, and the other (SrcB) coincident with both HESS J1858+020 and H ii region G35.6−0.5. The GeV emission of SrcA can be explained by the hadronic process in the SNR–molecular cloud association scenario. The GeV-band spectrum of SrcB and the TeV-band spectrum of HESS J1858+020 can be smoothly connected by a power-law function, with an index of ∼2.2. The connected spectrum is well explained with a hadronic emission, with the cutoff energy of protons above 1 PeV. It thus indicates that there is a potential PeVatron in the H ii region and should be further verified with ultrahigh-energy observations with, e.g., LHAASO.

The supernova remnant (SNR) G106.3+2.7 was recently found to be one of the few potential Galactic hadronic PeVatrons. Aiming to test the solidity of the SNR’s association with the molecular clouds (MCs) that are thought to be responsible for hadronic interaction, we performed a new CO observation with the IRAM 30 m telescope toward its “belly” region, which is coincident with the centroid of the γ-ray emission. There is a filament structure in the local standard of rest velocity interval −8 to −5 km s−1 that nicely follows the northern radio boundary of the SNR. We have seen asymmetric broad profiles of 12CO lines, with widths of a few km s−1, along the northern boundary and in the “belly” region of G106.3+2.7, but similar 12CO-line profiles are also found outside the SNR boundary. Further, the low 12CO J = 2–1/J = 1–0 line ratios suggest the MCs are cool. Therefore, it is still uncertain whether the MCs are directly disturbed by the SNR shocks, but we do find some clues that the MCs are nearby and thus can still be illuminated by the protons that escaped from the SNR. Notably, we find an expanding molecular structure with a velocity of ∼3.5 km s−1 and a velocity gradient of the MCs across the SNR from ∼−3 to −7 km s−1, which could be explained as the effect of the wind blown by the SNR’s progenitor star.

Very-high-energy (VHE) observations have revealed approximately 100 TeV sources in our Galaxy, and a significant fraction of them are under investigation to understand their origin. We report our study of one of them, HESS J1844−030. It is found to be possibly associated with the supernova remnant (SNR) candidate G29.37 + 0.1, and detailed studies of the source region at radio and X-ray frequencies have suggested that this SNR is a composite one, containing a pulsar wind nebula (PWN) powered by a candidate young pulsar. As the GeV source 4FGL J1844.4−0306 is also located in the region with high positional coincidence, we analyze its γ-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. We determine the GeV γ-ray emission is extended, described with a log-parabola function. The obtained spectrum can be connected to that of the VHE source HESS J1844−030. Given these properties and those from multifrequency studies, we discuss the origin of the γ-ray emission by considering that the two γ-ray sources are associated. Our modeling indicates that while the TeV part would have either a hadronic (from the SNR) or a leptonic origin (from the putative PWN), the GeV part would arise from a hadronic process. Thus we conclude that 4FGL J1844.4−0306 is likely the GeV counterpart to G29.37 + 0.1.

Galactic supernova remnants (SNRs) play an important role in our understanding of supernovae and their feedback on the interstellar environment. SNR G352.7-0.1 is special for its thermal composite morphology and double-ring structure. We have performed spectroscopic mapping of the 12CO and 13CO J = 2–1 lines toward G352.7-0.1 with the Atacama Pathfinder Experiment telescope. Broad 12CO lines are found in the northeastern ring at a local-standard-of-rest velocity range of ∼−50 to −30 km s−1, suggesting that the remnant is interacting with molecular clouds at ∼−51 km s−1. Thus, we adopt a distance of ∼10.5 kpc for this SNR. The momentum and kinetic energy of the shocked gas along the line of sight are estimated to be ∼102 M ☉ km s−1 and ∼1046 erg, respectively. We also find an expanding structure around the remnant, which is possibly related to the wind-blown bubble of the progenitor star. From Fermi-LAT data in the energy range 0.1–500 GeV, we find no gamma-ray counterparts to G352.7-0.1.

We report a possible γ-ray enhancement event detected from Tycho’s supernova remnant (SNR), the outcome of a type Ia supernova explosion that occurred in the year 1572. The event lasted for 1.5 yr and showed a factor of 3.6 flux increase mainly in the energy range of 4–100 GeV, while notably accompanied with two 478 GeV photons. Several young SNRs (including Tycho’s SNR) were previously found to show peculiar X-ray structures with flux variations in one- or several-year timescales, such an event at γ-ray energies is for the first time seen. The year-long timescale of the event suggests a synchrotron radiation process, but the hard γ-ray emission requires extreme conditions of either ultrahigh energies for the electrons up to ∼10 PeV (well above the cosmic-ray knee energy) or high inhomogeneity of the magnetic field in the SNR. This event in Tycho’s SNR is likely analogous to the γ-ray flares observed in the Crab Nebula, the comparably short timescales of them both requiring a synchrotron process, and similar magnetohydrodynamic processes such as magnetic reconnection would be at work as well in the SNR to accelerate particles to ultrarelativistic energies. The event, if confirmed, helps reveal the more complicated side of the physical processes that can occur in young SNRs.