Detection of a Rare Supersoft Outburst Event during a Suzaku Observation of 1E0102.2-7219

We report on the detection of a transient X-ray source toward the Small Magellanic Cloud (SMC) using the X-ray Imaging Spectrometer (XIS) on board the Suzaku telescope. The source was detected at the edge of the XIS image during a routine observation of the calibration source 1E0102.2-7219, a supernova remnant in the SMC. We constrained the source position using ray-tracing simulations. No such transient source was found at the position in the other Suzaku observations, nor in all of the available archived images of other X-ray missions for the last ∼28 years. The XIS spectrum can be explained by a single blackbody with a temperature of ∼72 eV, and an interstellar extinction of ∼4.9×10²⁰ H atoms cm^-2, consistent with the value to the SMC. An additional absorption edge at ∼0.74 keV was also confirmed, which is presumably due to absorption by helium-like oxygen ions. Assuming that the source is at the distance of the SMC, the X-ray luminosity in the 0.2-2.0 keV band is ∼10³⁷ erg s^-1 and the radius of the source is ∼10⁸cm. The XIS light curve shows about a two-fold decline in X-ray flux during the 24 ks observation. Together with the archived data, the X-ray flux in the burst is at least three-orders of magnitude brighter than the undetected quiescent level. All of these properties are often seen among supersoft sources (SSSs). We conclude that the transient source is another example of SSS in the SMC.