A very mysterious energy source that has been investigated by astronomers

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Importance map of the LHAASO J2108+5157 area using ∼2400 days of data captured by HAWC. Credit: Kumar et al., 2023.

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Importance map of the LHAASO J2108+5157 area using ∼2400 days of data captured by HAWC. Credit: Kumar et al., 2023.

Astronomers from the University of Maryland and Michigan Technological University examined a mysterious high-energy gamma-ray source known as LHAASO J2108+5157. The study results were published on August 31 on a preprint server arXivIt can help us reveal the true nature of this source.

Sources that emit gamma rays with photon energies between 100 GeV and 100 TeV are called very high-energy (VHE) gamma-ray sources, while sources with photon energies higher than 0.1 MeV are known as VHE sources very (UHE). The nature of these sources is still not well understood; Therefore, astronomers are constantly looking for new objects of this type to characterize, which may shed more light on their properties in general.

A team of astronomers led by Sajjan Kumar of the University of Maryland decided to take a closer look at one of the UHE gamma-ray sources called LHAASO J2108+5157. It is a point-like source with an extension of less than 0.39 degrees, and is known to be associated with a molecular cloud [MML2017]4607 – is located about 10,700 light-years away.

Previous observations of LHAASO J2108+5157 did not detect any X-ray counterparts, and the closest X-ray source turned out to be the eclipsing binary RX J2107.3+5202 with a separation of about 0.3 degrees. Since no powerful pulsars or supernova remnants have been detected so far near LHAASO J2108+5157, it is difficult to determine the origin of the gamma-ray emission, as this can be explained by either hadronic or leptonic models.

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Therefore, Kumar’s team observed LHAASO J2108+5157 using the Extremely Active Radiation Imaging Telescope Array System (VERITAS) and the High-Altitude Hydroelectric Cherenkov Observatory (HAWC) in order to shed more light on the emitted UHE gamma rays.

Observations found no significant emission near the location of LHAASO J2108+5158. The astronomers also performed a spectral analysis of the circular region with a radius of 0.09 degrees around the position of LHAASO J2108+5157, and measured the upper bounds of the differential flux at 1.0, 3.98, and 15.38 TeV energies, which is consistent with previous studies.

The obtained upper bounds exclude the hadron model and suggest a leptonian origin for the emission ranging from a few TeV to hundreds of TeV energies. However, the researchers note that a new molecular cloud has recently been identified near LHAASO J2108+5157, which sheds more light on the origin of the observed gamma-ray emission.

“The shape of this new cloud is closely related to the LHAASO J2108+5157 gamma-ray emission of up to 2 GeV from Fermi-LAT and the emission detected by LHAASO. This raises the possibility of gamma-ray production through the molecular hadron channel. The astronomers concluded, “ The cloud is a prime target for cosmic ray particles that are accelerated by unidentified PeVatrons.”

They added that future observations by the CTA and analysis in the X-ray range are required in order to fully understand the nature of LHAASO J2108+5157.

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