Wagner, R. M.; Vrba, F. J.; Henden, A. A.; Canzian, B.; Luginbuhl, C. B.;
Filippenko, A. V.; Chornock, R.; Li, W.; Coil, A. L.; Schmidt, G. D.; Smith,
P. S.; Starrfield, S.; Klose, S.; Tichá, J.; Tichư, M.;
Gorosabel, J.; Hudec, R.; Simon, V.
Abstract. Photometric and spectroscopic observations of an unusual, extremely luminous, variable star located in the galaxy NGC 3432 are presented. Its photometric behavior, spectrum, and luminosity suggest that the object is a very massive, luminous blue variable star analogous to eta-Carinae and SN 1997bs in NGC 3627 and that the variations are due to repeated mass-ejection events. The new object was discovered with the 0.8-m Katzman Automatic Imaging Telescope (KAIT) on 2000 May 3.2 at an unfiltered magnitude of about 17.4. Pre-discovery images obtained by KAIT between April 10th and 24th and during the course of the 2nd Palomar Sky Survey in 1998 May show the object with R = 19.2-19.5 mag, but it is absent on a KAIT image obtained on April 29 (>19.2 mag). Optical spectra obtained at the 2.3-m Bok telescope on May 6.2 (R=20.5) show a smooth continuum and strong Balmer emission lines at wavelengths consistent with the cataloged redshift of NGC 3432 (z = 0.002). Photometric monitoring in the R-band with the USNO 1-m telescope, KAIT, and others reveals a complex light curve in which the object brightened from R=19.3 to 17.4 over delta=2 days and then abruptly faded to R=20.8 over the following 8 days. The variable then brightened to R=18.6 within 4 days of having reached minimum brightness, after which it faded to R=19 and varied by =0.8 mag on a timescale of 5-10 days until our final measurement on July 4. Subsequent optical spectra obtained in May and June at the 2.4-m Hiltner and 3-m Shane telescopes continued to show strong Balmer emission lines with a mean FWHM 1550 km/s and a distinct red asymmetry. The maximum apparent magnitude implies an absolute magnitude of at least -12 at the distance of NGC 3432.