TITLE: GCN CIRCULAR NUMBER: 6769 SUBJECT: Orphan GRB candidate in IC 4779 at TAROT La Silla observatory DATE: 07/08/31 21:03:38 GMT FROM: Alain Klotz at CESR-CNRS Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report: During the night 30-31 August 2007, we imaged six times a 2x2 degrees field centered on RA=18h47.3min DEC=-63d20m (J2000,0) to search supernovae. Dates of beginning of exposures are following: #1 2007-08-31T00:20:38.69 exposure=120s, filter=R #2 2007-08-31T00:22:44.56 exposure=120s, filter=R #3 2007-08-31T01:16:47.89 exposure=120s, filter=R #4 2007-08-31T01:18:53.76 exposure=120s, filter=R #5 2007-08-31T02:22:15.39 exposure=120s, filter=R #6 2007-08-31T02:24:21.26 exposure=120s, filter=R We detect a bright like-star object R=11.7 only in the #5 image, at coordinates: RA=18h50m32s0 DEC=-63d01m00s3 (J2000,0) This object is at 18 arcsec S-E of the IC 4779 nucleus (redshift z=0.016752 from NED). None is seen at this place in other images, implying R>18.5. Screen copies of images are available at: http://www.cesr.fr/~klotz/ic4779/ All the evidence available at that time is in favor of a real astronomical optical transient (hereafter OT). The 2D profile of the OT is very close to that of the PSF derived from nearby stars of same magnitude (OT spreads on about 25 pixels). An artificial satellite or a plane flashlight are rejected because none is seen in the #6 image. We suggest that if this OT is an astronomical transient, one hypothesis is an optical flash occurring when a short burst (SHGRB) is active in gamma wavelengths. This idea is supported by the fact the #6 image taken just after the #5 one does not show the OT. Moreover, the low distance of IC 4779 is compatible with a host galaxy of SHGRB. We remind that, due to their very short durations (<0.2s) no optical telescope had ever observed a SHGRB during its gamma emission phase. We are currently regarding other possibilities than a GRB (a cosmic trace is almost excluded at the current step of analysis). Complementary observations are strongly encouraged. Some suggestions to do to confirm a SHGRB are: - Searching in large field surveys archives at the #5 image date. Following table indicates the expected magnitudes reached for various duration of the optical transient during the exposure: R=11.7 if duration=120 sec. R=9.0 if duration=10 sec. R=6.5 if duration=1 sec. - Deep image at OT position (mag lim > 23) as soon as possible with a very large telescope. - Searching in the high-energy satellites archive to find a SHGRB signal at the #5 image date. FITS files are available asking to alain.klotz@free.fr This message may be cited.