  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 H$^{13}$CO$^+$ $J=3-2$ rotational transition was observed
three times toward IM Lup between July 2014 and May 2015 with the Atacama Large
Millimeter 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
H$^{13}$CO$^+$ 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 a 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.
