2025ApJ...987..133X

We present the discovery of extended GeV gamma-ray emissions spatially correlated with the unidentified very-high-energy (VHE; > 100 GeV) gamma-ray source HESS J1503-582, utilizing data from Pass 8 of the Fermi Large Area Telescope. The GeV gamma-ray spectrum of HESS J1503-582 is optimally characterized by a power-law model with an index of 1.90, spanning the energy range from 1 GeV to 1 TeV. The gamma-ray emission is spatially coincident with the “Forbidden Velocity Wing” (FVW) 319.8+0.3, a characteristic 21 cm H I line structure that extends to velocities beyond those allowed by Galactic rotation. Taking into account the fact that several FVWs have been identified as old supernova remnants (SNRs), a potential SNR associated with FVW 319.8+0.3 is discussed to explain the multiwavelength data of HESS J1503-582. While both leptonic and hadronic models are capable of accounting for the multiwavelength observations of HESS J1503-582, the cutoff energies and total energy contents of particles challenge the acceleration capability of an old SNR with a slow expanding shell. While a hypothetical OB association or a pulsar halo could potentially account for the high total energy contents of particles and the low magnetic field strength, the absence of any detections of such objects challenges these proposed scenarios.