Gamma-ray emission from particle illumination and shock-cloud interaction in the W51 complex
- 1Istituto di Astrofisica e Planetologia Spaziali – INAF
Abstract
In the current era of very high-energy (VHE) and ultrahigh-energy (UHE) γ-ray astronomy, understanding Galactic PeVatrons and their acceleration mechanisms remains a primary objective. Recent LHAASO observations of the W51 complex make it an ideal laboratory for investigating the origin of UHE emission, particularly due to the presence of a massive and dense molecular environment surrounding multiple potential particle accelerators. In this work, we study two hadronic scenarios for the W51 complex. First, we model the direct interaction between the supernova remnant (SNR) W51C and the nearby clouds in W51B, incorporating fresh particle acceleration, shock-driven adiabatic compression, and reacceleration of permeating Galactic cosmic rays. Second, we explore an accelerator-independent illumination scenario in which the W51B cloud acts as a long-term confinement region for high-energy particles injected during an earlier epoch. We find that the direct shock-cloud interaction scenario successfully reproduces the giga-electronvolt emission observed by Fermi-LAT, but fails to account for the UHE emission detected by LHAASO. In contrast, the illumination scenario naturally explains the UHE spectrum, indicating that dense molecular clouds can efficiently confine and sustain energetic hadronic populations over long timescales. Although the inferred injection history is compatible with a young SNR origin, the source-independent nature of the illumination framework also permits other accelerators within the W51 complex. Our results therefore identify dense molecular environments as the key structures sustaining historical PeVatron activity and shaping the observed UHE γ-ray emission.
Versions
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BibTeX
@article{alan2026gammaray,
title = {{Gamma-ray emission from particle illumination and shock-cloud interaction in the W51 complex}},
author = {Sunny Alan and Cardillo Martina},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202661081},
url = {https://doi.org/10.1051/0004-6361/202661081},
note = {CC BY},
}
RIS
TY - JOUR
TI - Gamma-ray emission from particle illumination and shock-cloud interaction in the W51 complex
AU - Sunny Alan
AU - Cardillo Martina
PY - 2026
DA - 2026/10/23
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202661081
UR - https://doi.org/10.1051/0004-6361/202661081
AB - In the current era of very high-energy (VHE) and ultrahigh-energy (UHE) γ-ray astronomy, understanding Galactic PeVatrons and their acceleration mechanisms remains a primary objective. Recent LHAASO observations of the W51 complex make it an ideal laboratory for investigating the origin of UHE emission, particularly due to the presence of a massive and dense molecular environment surrounding multiple potential particle accelerators. In this work, we study two hadronic scenarios for the W51 complex. First, we model the direct interaction between the supernova remnant (SNR) W51C and the nearby clouds in W51B, incorporating fresh particle acceleration, shock-driven adiabatic compression, and reacceleration of permeating Galactic cosmic rays. Second, we explore an accelerator-independent illumination scenario in which the W51B cloud acts as a long-term confinement region for high-energy particles injected during an earlier epoch. We find that the direct shock-cloud interaction scenario successfully reproduces the giga-electronvolt emission observed by Fermi-LAT, but fails to account for the UHE emission detected by LHAASO. In contrast, the illumination scenario naturally explains the UHE spectrum, indicating that dense molecular clouds can efficiently confine and sustain energetic hadronic populations over long timescales. Although the inferred injection history is compatible with a young SNR origin, the source-independent nature of the illumination framework also permits other accelerators within the W51 complex. Our results therefore identify dense molecular environments as the key structures sustaining historical PeVatron activity and shaping the observed UHE γ-ray emission.
SN - 0004-6361
SN - 1432-0746
ER -