RE J1255- DETECTION OF AN EXTREMELY BRIGHT EUV TRANSIENT(266 )

During a pointed ROSAT observation in the direction of the Coma cluste r of galaxies an exceptionally bright EUV source, RE J1255+266, was de tected serendipitously. The source is located close to the Galactic No rth pole, at b(II) similar or equal to 89 degrees. Its observed EUV fl ux (62-110 eV) at the time of the detection was of order 710(-9) ergs s(-1) cm(-2), making RE J1255+266 temporarily one of the brightest EUV sources on the sky. The EUV flare of RE J1255+266 has a light curve w ith a decay time of about 0.86 days. With respect to earlier nondetect ions, the source brightened by a factor of similar to 7000. Such a beh aviour has not been observed before. Thus, it is unclear what type of source RE J1255+266 might be. Up to now no positive identification wit h any known source could be obtained. Emission at the position of the source was previously only detected in the 1987 Green Bank radio conti nuum survey. Simultaneous observations with CGRO/BATSE resulted in non -detections of the source in the 8-50 keV energy range.