We report the first detection of a substantial brightening event in an isotopologue of a key molecular ion, HCO, within a protoplanetary disk of a T Tauri star. The H13CO J=3-2 rotational transition was observed three times toward IM Lup between 2014 July and 2015 May with the Atacama Large Millimeter/submillimeter Array. The first two observations show similar spectrally integrated line and continuum fluxes, while the third observation shows a doubling in the disk-integrated J=3-2 line flux compared to the continuum, which does not change between the three epochs. We explore models of an X-ray active star irradiating the disk via stellar flares, and find that the optically thin H13CO emission variation can potentially be explained via X-ray-driven chemistry temporarily enhancing the HCO abundance in the upper layers of the disk atmosphere during large or prolonged flaring events. If the HCO enhancement is indeed caused by an X-ray flare, future observations should be able to spatially resolve these events and potentially enable us to watch the chemical aftermath of the high-energy stellar radiation propagating across the face of protoplanetary disks, providing a new pathway to explore ionization physics and chemistry, including electron density, in disks.
