A Galactic Plane Monitor at 200 MHz

Abstract We present the Galactic Plane Monitor (GPM), a dedicated low-frequency radio transient survey conducted with the Murchison Widefield Array at 200 MHz. The GPM targets an underexplored region of transient parameter space spanning timescales from seconds to weeks, where recent discoveries of unusual Galactic radio sources—including long-period transients and other coherent emitters—suggest a rich and largely uncharted population. Leveraging the MWA’s wide field of view, we carried out 72 observing epochs between June 2022 and March 2025, searching ∼ 5,000 sq. deg. of the Southern Galactic plane. We developed a multi-stage processing pipeline to search for variability across a broad range of timescales, from intra-snapshot (4 s to minutes), to inter-snapshot (minutes to hours), and inter-epoch (days to years). This approach combines image differencing, statistical variability metrics, and visual inspection to robustly identify transient candidates while mitigating the effects of ionospheric variability and imaging artefacts. In parallel, we produced deep mosaics using a “lucky imaging” strategy to maximise sensitivity to faint persistent sources. We present the detections of five long-period radio transients, seven eclipsing binary millisecond pulsars, seven intrinsically variable pulsars, and the detection of refractive interstellar scintillation and night-time interplanetary scintillation. The GPM represents a key step toward fully mapping the dynamic low-frequency radio sky and informs the design of next-generation transient surveys with the SKA Observatory.