An Enigmatic PeVatron in an Area around HII Region G35.6$-$0.5

TL;DR

This paper reports the detection of a Galactic PeVatron near HII region G35.6-0.5 using LHAASO data, revealing a source capable of accelerating particles to PeV energies, with implications for cosmic ray origins.

Contribution

It provides the first detailed analysis of UHE gamma-ray emissions in this region, identifying a PeVatron candidate and exploring its possible origins with multi-wavelength data.

Findings

Detection of a PeVatron with 11.6σ significance above 100 TeV.

The gamma-ray source has an extension of ~0.18° and a spectral index of ~2.5 in 1-25 TeV.

The emission can be explained by a hadronic model associated with the HII region G35.6-0.5.

Abstract

Identifying Galactic PeVatrons (PeV particle accelerators) from the ultra-high-energy (UHE, >100 TeV) $\gamma$-ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.6$-$0.4 or HII region G35.6$-$0.5. We perform detailed analysis on the very-high-energy and UHE $\gamma$-ray emissions towards this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6$\sigma$ above 100 TeV, indicating the presence of a PeVatron. It has an extension of $\sim 0.18^\circ$ with a power-law (PL) spectral index of $\sim$2.5 in 1-25 TeV and a point-like emission with a PL spectral index of $\sim$3.2 above 25 TeV. Using the archival CO and HI data, we identify some molecular and atomic clouds that may be associated with the TeV $\gamma$-ray emissions. Our modelling indicates that the TeV $\gamma$-ray emissions are unlikely to arise from the clouds illuminated by the protons that escaped from SNR G35.6$-$0.4. In the scenario that HII region G35.6$-$0.5 could accelerate particles to the UHE band, the observed GeV-TeV $\gamma$-ray emission could be well explained by a hadronic model with a PL spectral index of $\sim$2.0 and cutoff energy of $\sim$450 TeV. However, an evolved pulsar wind nebula origin cannot be ruled out.