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(abridged) In the hunt for rare time-domain events, confusing exoticextragalactic phenomena with more common Galactic foreground events is animportant consideration. We show how observations from multiple wavebands, inthis case optical and X-ray observations, can be used to facilitate thedistinction between the two. We discovered an extremely bright and rapidtransient event in optical observations of M31 with the intermediate PalomarTransient Factory (iPTF). The persistent optical counterpart of this transientwas previously thought to be a variable star in M31 without any dramatic fluxexcursions. The iPTF event initially appeared to be an extraordinarily rapidand energetic extragalactic transient, a very fast nova in M31 being one of theexciting possibilities, with a ~3mag positive flux excursion in less than akilosecond. The nature of the source was resolved with the help of Chandraarchival data, where we found an X-ray counterpart and obtained its X-rayspectrum. We find the X-ray spectrum of the quiescent emission can be describedby a model of optically thin plasma emission with a temperature of ~7MK,typical for coronal emission from an active star. The combination of the X-rayluminosity, calculated assuming the source is located in M31 (~3.10^36 erg/s),and the color temperature exclude any type of known accreting compact object oractive star in M31. We argue instead that the optical transient source is an Mtype main-sequence, active star located in the disk of the Milky Way at adistance of ~0.5-1 kpc. Its persistent X-ray luminosity is in the~(1.3-5).10^30 erg/s range and it has the absolute optical magnitude of9.5-11.0 mag in R-band. The observed optical flare has the equivalent durationof ~95min and total energy of ~(0.3-1).10^35 erg in R-band, which places itamong the brightest flares ever observed from an M-type star.

A multi-wavelength approach to classifying transient events in the direction of M31