Microlensing of GSC 3656-1328

ATel #943; M. Mikolajewski, T. Tomov, A. Niedzielski, A. Strobel, T. Brozek, K. Czart, C. Galan, A. Karska, E. Swiercznski, M. Wiecek, Torun Center for Astronomy, Nicolaus Copernicus University, Torun, Poland
on 14 Nov 2006; 15:09 UT
Credential Certification: Andrzej Niedzielski (aniedzi@astri.uni.torun.pl)

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In our previous report (ATEL#931) we have suggested that the bright optical transient in Cassiopeia, identified as GSC 3656-1328, can be understood as a gravitational microlensing event of a nearby star (see also ATEL#942). All available photometric data support the finding and a symmetric microlens fit best reproduces the light curve. We continue our spectroscopic and photometric monitoring of GSC 3656-1328. New spectra obtained in the 3700-7300AA region at a resolution of 4A with the 60/90cm Schmidt-Cassegrain Telescope (TSC) at Piwnice equipped with the Copernicus Cassegrain Spectrograph (CCS) between 2006 November 4 and November 10 show the same A0V-A1V spectral type as observed a few days earlier (ATEL#931). Additionally we have searched our database of prismatic spectra collected with the TSC and identified the object on a Kodak IIa-O plate obtained with the F2 objective prism (dispersion about 260A/mm at H-gamma) on 1966 November 11.917 UT. Although the plate is of moderate quality GSC 3656-1328 is easy to identify as a hot continuum object with strong Balmer absorptions similar to other nearby A0 stars. Our new photometric UBVRI observations obtained with the 60cm Cassegrain telescope and SBIG STL-1001 camera show that changes of U-B, B-V, V-R and R-I between 2006 November 02 and November 10 are below 0.07, 0.03, 0.03 and 0.01 mag, respectively i.e. within the observation uncertainties. The V magnitude is 10.48 on 2006 November 03.755 UT, 10.55 on November 03.979 UT and 11.22 on November 09.9 UT. The mean color index B-V=0.19 +/- 0.01 and remains constant. Six hours monitoring of the object on 2006 November 09 in BV filters did not show any remarkable changes.