A team of international astronomers reported the discovery of a new ultra-high energy (UHE) gamma-ray source in the Galactic plane named LHAASO J0341+5258. It is an extended source with a width of 0.29° and emission from it reaches up to 200 TeV. Their study recently appeared in Arxiv.
Cosmic rays are charged particles, mainly protons and helium nuclei, that arrive isotropically (i.e. exhibiting the same behaviour in all directions) from space and continuously bombard Earth’s atmosphere. They were discovered by Victor Hess in 1912, when he measured an increasing radiation level in the atmosphere with altitude, using his balloon to reach a height of 5.3 km. He rightly postulated the extraterrestrial origin of cosmic rays and was awarded the Nobel Prize in Physics in 1936 for his discovery.
Decades of measurements helped to construct the energy spectrum of cosmic rays observed from Earth. It is one of the most famous plots of modern physics, exhibiting a remarkable power law in energy over several orders of magnitude (see inset in Figure 1). This power law has a break in energy at a few Peta-electronvolts (PeV, 1015 eV), which is referred to as the knee. Below the knee, cosmic rays are believed to be of Galactic origin, but the sources where they are produced are still unknown. Sources capable of accelerating particles up to at least PeV energies are called PeVatrons, and astronomers are actively on the hunt for these extreme accelerators within our Galaxy in order to shed more light on their properties.
Now, by using China’s Large High Altitude Air Shower Observatory (LHAASO) a team of international astronomers found a new source of this type called LHAASO J0341+5258. It 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° 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,” they said.
It was found from observations that LHAASO J0341+5258 is an extended source with angular size of approximately 0.29°. The gamma-ray emission from this source reaches values of nearly 200 TeV (0.2 PeV).
Their results showed that the integrated energy flux of gamma-ray emissions for LHAASO J0341+5258 above 25 TeV is 1.44 × 10¯14 (cm¯2s¯1), which accounts for about 20 percent of the flux from the Crab Nebula. In addition, the energy spectrum of this source can be described by a power-law, even though there is a hint of curvature at around 50 TeV.
Moreover, they 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, they 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,” they explained.
Finally, they added that the 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.
Featured image: The significance map of LHAASO J0341+5258 above 25 TeV. The green circle marks the position of 4FGL J0340.4+5302, and the blue cross marks the position of the pulsar PSR J0343+5312. Credit: Cao et al.
Reference: LHAASO Collaboration, “Discovery of a new γ-ray source LHAASO J0341+5258 with emission up to 200TeV”, arXiv:2107.02020 [astro-ph.HE] arxiv.org/abs/2107.02020
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