Redefining the High-Energy gamma-ray morphology of the W51 complex with Fermi: detection of two extended components associated with W51B and W51C

The W51 giant cloud is one of the largest star-forming regions (SFR) in the Galaxy. Several experiments have detected gamma-ray emissions from the W51 complex: Fermi (Jogler and Funk 2016), MAGIC (Aleksic et al. 2012), HESS (H.E.S.S. Collaboration et al. 2018), HAWC (Albert et al. 2020), and more recently, LHAASO (Cao et al. 2024). This complex contains two star-forming regions, known as W51A and W51B, as well as the middle-aged supernova remnant (SNR) W51C. This last has been detected in the High-Energy (HE) gamma-ray band and is associated with the latest $\textit{Fermi}$ catalogue source 4FGL J1923.2+1408e. The spectral cutoff detected by LHAASO at $\sim$400 TeV might provide the first evidence of SNRs accelerating Cosmic Rays up to PeV energies. However, LHAASO's limited angular resolution precludes a precise morphological analysis, and the two young stellar clusters contained within W51B (G48.9-0.3 and G49.2-0.3) are also valid PeVatron candidates accountable for the Ultra-High-Energy gamma-ray (UHE) emission. We analyzed 16 years of $\textit{Fermi}$-LAT data on the W51 region and found an $8\sigma$ excess, improperly fitted by the current model adopted for 4FGL J1923.2+1408e in the 4FGL catalogue. We performed a binned Likelihood analysis to identify the best-fitting model for the emitting region. We identified two overlapping extended elliptic sources, spatially coincident with the radio emission regions of W51B and W51C. Eventually, we disentangled the HE gamma-ray emission from the W51 complex in two distinct spectral components between 1 GeV and 1 TeV: a softer LogParabola (LP) component ($\Gamma$=1.8) associated with W51C and a harder Power Law (PL) spectrum ($\Gamma$=2.4) associated with the W51B star-forming region. We performed a multi-wavelength modelling of the two sources, including the Very-high-Energy and Ultra-High-Energy data from MAGIC and LHAASO. We confirmed the hadronic nature of both components. We studied the combination of two acceleration sites: the W51C old SNR shock, accelerating protons up to the TeV range, and the Super Bubbles (SB) originating from the stellar clusters contained in W51B, likely responsible for the PeV emission detected by LHAASO.