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R. M. Wagner (LBT Obs), F. J. Vrba, A. A. Henden, B. Canzian, C. B. Luginbuhl (USNO-Flagstaff), A. V. Filippenko, W. Li, R. Chornock, A. L. Coil (UCB), G. D. Schmidt, P. S. Smith (Steward Obs), S. G. Starrfield (ASU), S. Klose (Tautenburg Landesternwarte), A. Castro-Tirado, C. Sanchez-Fernandez, J. Gorosabel (Laboratorio des Astrofisica Espacial y Fisica Fundamental), M. Tichy, J. Ticha (Klet Obs)
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~q20.5) 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~q19.3 to 17.4 over ~q2 days and then abruptly faded to R~q20.8 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~q19 and varied by ~q0.8 mag on a timescale of ~q5-10 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 ~q 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.