EMU Discovery of Thunder: A Bow-shock PWN Powered by PSR J1631–4722 Escaping the Nimbus SNR (G336.7+0.5)

We report the discovery of a bow-shock pulsar wind nebula (PWN), dubbed Thunder, powered by the radio pulsar PSR J1631−4722 and projected within the Galactic supernova remnant (SNR) G336.7+0.5 (Nimbus). The system was first identified in observations from the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe survey and further characterized using MeerKAT Galactic Plane Survey data, together with follow-up observations at 5.5 and 9 GHz obtained with the Australia Telescope Compact Array. Assuming a distance of 7 kpc, the radio images resolve an elongated ∼80″ (2.7 pc) cometary nebula, indicative of a high-velocity pulsar. An X-ray counterpart extending ∼50″ (1.7 pc) is detected in archival XMM-Newton data. The flat radio spectrum ( α = −0.27 ± 0.05) and hard X-ray photon index (Γ = 1.6 ± 0.4) indicate synchrotron emission from relativistic particles injected in the pulsar wind. Polarization analysis reveals a highly ordered magnetic field aligned with the nebular flow, with fractional polarization reaching up to 30% in the tail. An equipartition estimate gives a PWN magnetic field strength of B eq ≈ 54–140 μ G. Pulsar timing over a ∼2.2 yr baseline reveals strong timing noise and a small spin glitch with amplitude Δ ν / ν = 1.10 × 10 −8 . The SNR shows no clear diffuse X-ray counterpart. The morphology and multiwavelength properties of the Nimbus–Thunder system, together with evolutionary models, constrain the system age to ∼30–45 kyr, placing the remnant in the late Sedov phase, approaching the transition to the radiative stage.