摘要
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 M31 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 M31 without any dramatic flux excursions. The iPTF event initially appeared to be an extraordinarily rapid and energetic extragalactic transient, which had a approximate to 3 mag positive flux excursion in less than a kilosecond; one of the exciting possibilities was this event could be a very fast nova in M31. 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 approximate to 7 MK, typical for coronal emission from an active star. The combination of the X-ray luminosity, which is calculated assuming the source is located in M31 (similar to 3 x 10(36) erg s (1)), and the color temperature exclude any type of known accreting compact object or active star in M31. 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 similar to 0 : 5-1 kpc. Its persistent X-ray luminosity is in the approximate to 1 : 3-5 x 10(30) erg s (1) range and it has the absolute optical magnitude of 9.5-11.0 mag in the R band. The observed optical flare has the equivalent duration of approximate to 95 min and total energy of approximate to(0 : 3-1) x 10(35) erg 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.