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GCN Circular 6769

Orphan GRB candidate in IC 4779 at TAROT La Silla observatory
2007-08-31T21:03:38Z (17 years ago)
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:

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

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

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