The bright new variable in Cassiopeia - a microlensing event?

ATel #931; M. Mikolajewski, T. Tomov, A. Niedzielski, K. Czart, C. Galan - Torun Center for Astronomy, Nicolaus Copernicus University, 87-100 Torun, Poland
on 3 Nov 2006; 20:56 UT
Credential Certification: Andrzej Niedzielski (aniedzi@astri.uni.torun.pl)

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We report BV photometry and optical spectroscopic observations of a bright new variable in Cassiopeia, identified as GSC 3656-1328 (CBET # 711, # 712). According to SIMBAD database GSC 3656-1328 is a 11.4 star in V with B-V of about 0.2. Our spectra in the region 3700-7300AA and at a resolution of 4A were obtained with the 60/90cm Schmidt-Cassegrain telescope of Torun Observatory between 2006 November 02.790 UT and November 03.713 UT. They show a typical A0V-A1V star without remarkable spectral variations or line shifts. The Balmer absorptions show an average radial velocity of -45 +/-15 km/s. The brightness estimate: V=10.15 and B=10.35 was obtained with the 60cm Cassagrain telescope of Torun Observatory on 2006 November 02.765 UT. The SIMBAD data show that before the event GSC 3656-1328 was a slightly reddened A0V-A1V star at a distance of about 1 kps. The only observed change was a sudden rapid increase and then a decrease of the brightness with an amplitude about 4 mag without any spectral changes. It is difficult to associate such an observed phenomenon with any type of variable stars. A possible explanation of the GSC 3656-1328 behavior could be a gravitational microlensing event. If that is the case, this would be the closest microlensing event ever observed.