A heatwave of accretion energy traced by masers in the G358-MM1 high-mass protostar

A ring of maser emission seemingly expanding at 0.05 c is actually tracing the propagation of heat through the circumstellar medium around a high-mass protostar rather than subluminal motion. The heatwave is a manifestation of an accretion burst. High-mass stars are thought to accumulate much of their mass via short, infrequent bursts of disk-aided accretion(1,2). Such accretion events are rare and difficult to observe directly but are known to drive enhanced maser emission(3-6). In this Letter we report high-resolution, multi-epoch methanol maser observations toward G358.93-0.03, which reveal an interesting phenomenon: the subluminal propagation of a thermal radiation 'heatwave' emanating from an accreting high-mass protostar. The extreme transformation of the maser emission implies a sudden intensification of thermal infrared radiation from within the inner (40-mas, 270-au) region. Subsequently, methanol masers trace the radial passage of thermal radiation through the environment at >= 4% of the speed of light. Such a high translocation rate contrasts with the <= 10 km s(-1) physical gas motions of methanol masers typically observed using very-long-baseline interferometry (VLBI). The observed scenario can readily be attributed to an accretion event in the high-mass protostar G358.93-0.03-MM1. While being the third case in its class, G358.93-0.03-MM1 exhibits unique attributes hinting at a possible 'zoo' of accretion burst types. These results promote the advantages of maser observations in understanding high-mass-star formation, both through single-dish maser monitoring campaigns and via their international cooperation as VLBI arrays.