The Enigma of G35.6−0.5: Supernova, HII Region, or Hidden Pulsar?

The discovery of a new ultra-high-energy gamma-ray source, 1LHAASO J1857+0203u, by the Large High Altitude Air Shower Observatory (LHAASO) suggests the presence of a PeVatron, a cosmic accelerator capable of boosting particles to peta-electron volt energies. This source is particularly interesting because it is located in a region with complex multi-wavelength features, including the supernova remnant (SNR) G35.6−0.4 and the HII region G35.6−0.5.

The study, published in "An Enigmatic PeVatron in an Area around HII Region G35.6−0.5" by Cao et al., explores three possible origins for the observed gamma-ray emission:

• HII region scenario: The gamma rays originate from the HII region G35.6−0.5, possibly accelerated by the stellar winds of massive stars. While no such stars have been directly observed yet, their presence cannot be ruled out, especially if they are embedded within molecular clouds.
• Supernova remnant scenario: The gamma rays are produced when protons that escaped from the SNR G35.6−0.4 interact with nearby molecular clouds. However, the current analysis suggests that the mass of the observed clouds is insufficient to explain the observed gamma-ray flux.
• Pulsar wind nebula (PWN) scenario: The gamma rays are emitted by a PWN powered by an as-yet undiscovered pulsar. This scenario is plausible given the potential for mature PWNe to be bright in gamma rays while remaining undetected at other wavelengths.

Further observations across the electromagnetic spectrum are needed to definitively determine the origin of the gamma-ray emission from 1LHAASO J1857+0203u.** Future observations with instruments like the Large-Sized Telescope (LACT), ASTRI, and the Cherenkov Telescope Array (CTA) could help distinguish between these scenarios thanks to their enhanced angular resolution.

Reference:

Cao, Z., Aharonian, F., Axikegu, et al. "An Enigmatic PeVatron in an Area around HII Region G35.6−0.5". Draft version December 3, 2024. *Typeset using LATEX twocolumn style in AASTeX631.*

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: LHAASO