Evidence for a gamma-ray molecular target in the enigmatic PeVatron candidate LHAASO J2108+5157
Journal article, 2023

Context.
Peta-eV (PeV) astronomy emerged in 2021 with the discovery of ultra-high-energy gamma-ray sources associated with powerful natural particle accelerators known as PeVatrons. In order to determine the nature of their emission, namely whether it has a hadronic or leptonic origin, it is essential to characterise the physical parameters of the environment where it originates.
Aims.
We unambiguously confirm the association of molecular gas with the PeVatron candidate LHAASO J2108+5157 using unprecedented high angular-resolution (17′) 12,13CO(J = 1 → 0) observations carried out with the Nobeyama 45m radio telescope.
Methods.
We characterised a molecular cloud in the vicinity of the PeVatron candidate LHAASO J2108+5157 by determining its physical parameters from our 12,13CO(J = 1 → 0) line observations. We used an updated estimation of the distance to the cloud, which provided a more reliable result. The molecular emission was compared with excess gamma-ray images obtained with Fermi-LAT at energies above 2 GeV to search for spatial correlations and test a possible hadronic (π0 decay) origin for the gamma-ray emission.
Results.
We find that the morphology of the spatial distribution of the CO emission is strikingly similar to that of the Fermi-LAT excess gamma ray. By combining our observations with archival 21 cm HI line data, the nucleons (HI + H2) number density of the target molecular cloud is found to be 133.0 ± 45.0 cm-3, for the measured angular size of 0.55 ± 0.02 at a distance of 1.6 ± 0.1 kpc. The resulting total mass of the cloud is M(HI + H2) = 7.5±2.9×103M⊙. Under a hadronic scenario, we obtain a total energy of protons of Wp = 4.3 ± 1.5 × 1046 erg with a cutoff of 700±300 TeV, which reproduces the sub-PeV gamma-ray emission.
Conclusions.
We identified a molecular cloud in the vicinity of LHAASO J2107+5157 as the main target where cosmic rays from an unknown PeVatron produce the observed gamma-ray emission via π0 decay.

ISM: molecules

Gamma rays: ISM

Radio lines: ISM

ISM: clouds

ISM: individual objects: LHAASO J2108+5157

Methods: data analysis

Author

Eduardo De La Fuente

University of Tokyo

University of Guadalajara

Iván Toledano-Juárez

University of Guadalajara

K. Kawata

University of Tokyo

Miguel A. Trinidad

Universidad de Guanajuato

Mitsuyoshi Yamagishi

University of Tokyo

S. Takekawa

Kanagawa University

Daniel Tafoya

Chalmers, Space, Earth and Environment, Onsala Space Observatory

M. Ohnishi

University of Tokyo

A. Nishimura

National Astronomical Observatory of Japan

S. Kato

University of Tokyo

T. Sako

University of Tokyo

M. Takita

University of Tokyo

H. Sano

Gifu University

R. K. Yadav

National Astronomical Research Institute of Thailand

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 675 L5

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/202346681

More information

Latest update

8/4/2023 7