Ponente
Descripción
W51 is one of the most massive and active star-forming complexes in the Milky Way. Multiple experiments, including Fermi-LAT, MAGIC, HESS, HAWC, and most recently LHAASO, have detected gamma-ray emission associated with this region. The complex hosts two major star-forming regions, W51A and W51B, and the middle-aged supernova remnant (SNR) W51C. The latter has been associated with the high-energy (HE) gamma-ray source 4FGL J1923.2+1408e. The characteristic pion-decay spectral feature shown by Fermi-LAT data suggests that protons accelerated by the SNR shock are interacting with dense molecular material. The detection by LHAASO of a spectral cutoff near 400 TeV indicates the presence of cosmic-ray accelerations reaching PeV energies. However, due to its limited angular resolution, LHAASO did not manage to unambiguously resolve the emission’s morphology, leaving open the possibility that young stellar clusters (YSC) within W51B (notably G48.9-0.3 and G49.2-0.3) might also contribute to the ultra-high-energy (UHE) gamma-ray signal.
In this work, we present an analysis of 16 years of Fermi-LAT data targeting the W51 region. Through a binned likelihood analysis, we refined the spatial and spectral modelling of the gamma-ray emission, using asymmetric spatial distributions between 10 GeV and 1 TeV. We tested multiple spatial configurations, including templates based on radio observations, to explore correlations between the gamma-ray emission and molecular cloud distributions near W51C and W51B. Our results reveal two extended, elliptical sources aligned with the respective radio structures. We managed to disentangle the high-energy gamma-ray emission between 1 GeV and 1 TeV into two components: a softer LogParabola spectrum associated with W51C and a harder Power Law component associated with W51B.
Our multi-wavelength modelling confirms the hadronic origin of the emission. The W51C component revealed the presence of hadronic acceleration characterised by a few per cent cosmic-ray (CR) acceleration efficiency and an energy cutoff at the TeV level. However, since the estimated shock size is smaller than the identified gamma-ray region, we modelled the HE emission also assuming a CR escape scenario. Regarding the gamma-ray emission associated with W51B, we identified 3 spatially coincident stellar clusters with the necessary properties required to explain the UHE emission detected by LHAASO: G48.9-0.3, G49.2-0.3 and G49.0-0.3. We jointly fitted the Fermi-LAT and LHAASO data and identified the acceleration and diffusion properties of the three clusters.
