Astronomers Probe Enigmatic Source of Ultra-High Energy

Astronomers from the University of Maryland and the Michigan Technological University have delved into the enigmatic ultra-high energy gamma-ray source called LHAASO J2108+5157. Their research findings, disclosed on August 31st via the pre-print server arXiv, hold the potential to unveil the true essence of this celestial phenomenon.

This source emits gamma radiation with photon energies spanning from 100 GeV to 100 TeV, categorizing it as a very-high energy (VHE) gamma-ray source. Moreover, if photon energies surpass 0.1 PeV, these sources are designated as ultra-high energy (UHE) gamma-ray sources. Despite their prominence, the precise nature of these sources remains an unsolved puzzle. Hence, astronomers persistently search for novel objects of this nature to fathom their characteristics, ultimately shedding light on their underlying properties.

Headed by University of Maryland’s Sajan Kumar, a team of astronomers decided to scrutinize one such UHE gamma-ray source, identified as LHAASO J2108+5157. This source is characterized by a point-like appearance with an angular extent of less than 0.39 degrees, and it is notably associated with the molecular cloud [MML2017]4607, located at a staggering distance of 10,700 light years from Earth.

Earlier observations of LHAASO J2108+5157 had failed to identify any X-ray counterparts. It was determined that the nearest X-ray source is the eclipsing binary RX J2107.3+5202, situated approximately 0.3 degrees away. Given the absence of robust evidence for potent pulsars or supernova remnants in the proximity of LHAASO J2108+5157, the origin of its gamma-ray emission remains ambiguous, with plausible explanations involving both hadronic and leptonic models.

To shed more light on the emission of ultra-high energy gamma-rays, Kumar’s team conducted observations of LHAASO J2108+5157 using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the High-Altitude Water Cherenkov Observatory (HAWC).

The observations yielded no substantial emissions in the immediate vicinity of LHAASO J2108+5158. Additionally, astronomers carried out spectral analysis within a circular region with a radius of 0.09 degrees centered on the position of LHAASO J2108+5157. The results established differential flux upper limits at energy levels of 1.0, 3.98, and 15.38 TeV. These findings remained consistent with prior investigations.

The obtained upper limits dismiss the hadronic model and point toward a leptonic origin for the emission, spanning from a few TeV to several hundred TeV in energy. Nonetheless, the researchers noted a recent discovery of a new molecular cloud in the vicinity of LHAASO J2108+5157, providing additional insight into the origin of the observed gamma-ray emission.

The astronomers concluded that the morphology of this newly identified cloud closely aligns with the gamma-ray emission from LHAASO J2108+5157, observed up to 2 GeV by Fermi-LAT and also detected by LHAASO. This suggests a higher likelihood that the gamma rays are generated through the hadronic channel, with the molecular cloud serving as the primary target for cosmic ray particles accelerated by unidentified PeVatrons.

They further emphasized the necessity for future observations by the Cherenkov Telescope Array (CTA) and X-ray band analyses to comprehensively decipher the nature of LHAASO J2108+5157.

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