Astronomers detect ultra-high energy gamma-ray source

Astronomers from the Chinese Academy of Sciences (CAS) and elsewhere report the detection of a new ultra-high energy (UHE) gamma-ray source in the Galactic plane. The newly identified source, designated LHAASO J0341+5258, is extended and emission from it reaches up to 200 TeV. The finding is detailed in a paper published July 5 on arXiv.org.

Sources emitting gamma radiation with photon energies between 100 GeV and 100 TeV are called very-high energy (VHE) gamma-ray sources, while those with photon energies above 0.1 PeV are known as ultra-high energy (UHE) gamma-ray sources. The nature of these sources is still not well understood; therefore, astronomers are constantly searching for new objects of this type to characterize them, which could shed more light on their properties in general.

Now, a team of astronomers led by Zhen Cao of CAS has found a new source of this type using China's Large High Altitude Air Shower Observatory (LHAASO). LHAASO J0341+5258 was detected during an obervational campaign between December 2019 and November 2020.

"Since December 2019, the half of LHAASO-KM2A experiment has monitored the sky in the declination band from -15 degrees to 75 degrees above tens of TeV with high duty cycle. An excess with a pre-trial significance of 8.2σ was detected from the direction of LHAASO J0341+5258 using events with energy above 25 TeV," the researchers wrote in the paper.

The observations found that LHAASO J0341+5258 is an extended source with angular size of approximately 0.29 degrees. The gamma-ray emission from this source reaches values of nearly 200 TeV (0.2 PeV).

The results show that the integrated energy flux of gamma-ray emissions for LHAASO J0341+5258 above 25 TeV accounts for about 20 percent of the flux from the Crab Nebula. The astronomers added that energy spectrum of this source can be described by a power-law, even though there is a hint of curvature at around 50 TeV.

Furthermore, it was found that LHAASO J0341+5258 is positionally coincident with a known GeV gamma-ray source 4FGL J0340.4+5302. Therefore, the researchers assume that both sources may have a unified origin.

Summing up the results, the authors of the paper noted that LHAASO J0341+5258 could be an extended emission of a pulsar wind nebula (PWN) and/or a pulsar halo. However, the challenge of this scenario is the lack of a reported powerful pulsar.

"Interestingly, such a pulsar could be the gamma-ray source 4FGL J0340.4+5302 with a characteristic spectrum below 1 GeV. The detection of pulsed radio emission from this source would support the IC [inverse Compton] origin of the UHE gamma-ray emission," the scientists explained.

They added that hadronic origin of the UHE emission from LHAASO J0341+5258 can be interpreted as an 'echo' from molecular clouds. This is the result of interactions of protons with dense gas regions in the proximity of an old supernova remnant.