2018A&A...615A.152S

Context. In the hunt for rare time-domain events, it is important to consider confusing exotic extragalactic phenomena with more common Galactic foreground events.
Aims. We show how observations from multiple wavebands, in this case optical and X-ray observations, can be used to facilitate the distinction between the two.
Methods. We discovered an extremely bright and rapid transient event during optical observations of the M 31 galaxy taken by the intermediate Palomar Transient Factory (iPTF). The persistent optical counterpart of this transient was previously thought to be a variable star in M 31 without any dramatic flux excursions. The iPTF event initially appeared to be an extraordinarily rapid and energetic extragalactic transient, which had a ~=3mag positive flux excursion in less than a kilosecond; one of the exciting possibilities was this event could be a very fast nova in M 31. The nature of the source was resolved with the help of Chandra archival data, where we found an X-ray counterpart and obtained its X-ray spectrum.
Results. We find the X-ray spectrum of the quiescent emission can be described by 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-ray luminosity, which is calculated assuming the source is located in M 31 (∼3x10³⁶erg/s), and the color temperature exclude any type of known accreting compact object or active star in M 31. We argue instead that the optical transient source is an M-type main-sequence, active star located in the disk of the Milky Way at a distance of ∼0.5-1kpc. Its persistent X-ray luminosity is in the ~=1.3-5x10³⁰erg/s range and it has the absolute optical magnitude of 9.5-11.0mag in the R band. The observed optical flare has the equivalent duration of ~=95 min and total energy of ~=(0.3-1)x10³⁵erg in the R band, which places it among the brightest flares ever observed from an M-type star. This case can serve as an example for the classification of Galactic and extragalactic events in upcoming high-cadence time-domain projects, such as the Zwicky Transient Facility and the Large Synoptic Survey Telescope.