TITLE: GCN GRB OBSERVATION REPORT NUMBER: 666 SUBJECT: Possible GRB-related phenomenon DATE: 00/05/09 23:38:25 GMT FROM: Fredrick J. Vrba at USNO [WARNING: This is not a typical GCN reporting observations associated with any GRB detection. The uniqueness of this transient object (see below) and its remote chance of being GRB-related along with its rapid fading prompt this GCN.] R. Hudec, V. Simon (Astronomical Institute Ondrejov), M. Tichy and J. Ticha (Klet Observatory), W. Li, A. Filippenko and M. Papenkova (UCB), F. Vrba and B. Canzian (USNO Flagstaff) report: We have obtained seven CCD observations with the 0.8 automatic KAIT, 0.6 m Klet, and 1.0 m Flagstaff telescopes covering the position of the optical transient detected in the field of NGC 3432 (IAUC 7415: R.A. 10h 52m41.40s, Decl. +36o40'08.5", equinox 2000.0). The object is about 123" east and 180" north of the diffuse nucleus and superimposed on an HII region (or the spiral arm) of this interacting dwarf galaxy at D = 7.8 Mpc. The object was discovered on May 3.228 UT at nearly mag 17.4 (unfiltered image) and since then (our last measurement is from May 9.181 UT) has faded by nearly 3.5 magnitudes, similar to rapid GRB optical afterglow declines. Analysis of the data taken at our observatories is in progress to produce a final light curve. Preliminary analysis indicates that a power law fits the data well with a slope of -3.3 +/- 0.1, if T_0 is assumed to be May 0.0 UT for instance, (based on differential photometry with respect only to a nearby star so far), although without a known GRB time the choice of this T_0 is totally arbitrary. A spectrum of the object (IAUC 7417) shows its distance to be consistent with that of NGC 3432 and thus not a foreground or background object. While this spectrum is consistent with that of a classical nova, the the brightness at this distance would make the nova extraordinarily bright. The object might yet be a (probably absorbed) optical afterglow of a not-observed GRB (due to different beaming opening angles in gamma rays and in the optical). Even if this not a GRB optical afterglow (an extraordinary bright nova, for instance ), the object is still worthy of further study, but is rapidly fading into the background of NGC 3432. Tonight might be the last opportunity to observe it. The discovery FITS frame is available by anonymous ftp at flipper.berkeley.edu, in the directory /pub/bait/ngc3432. Finding charts from UT May 07 and 09 showing the object fading are avaiable in postscript format by anonymous ftp at ftp.nofs.navy.mil in the directory /pub/outgoing/fjv/ngc3432.dir.