GCN Circular 6947

J. Gorosabel (IAA-CSIC Granada), A. de Ugarte Postigo (ESO Santiago), M.
Jel�nek (IAA-CSIC), M. Vuckovic (Katholieke Universiteit Leuven), E.
Rubio-Herrera, P. van Ham, F. van Weerdenburg, P. van Oers (Univ. of
Amsterdam) and A. J. Castro-Tirado (IAA-CSIC) report:

"Following the detection of GRB 071018 by SWIFT (Krim et al. GCNC 6932)
we have obtained R & I-band observations with the 1.2m Mercator
telescope at Observatorio del Roque de los Muchachos in La Palma. The
frames were obtained starting on 19.2916 Oct (i.e. 22.4 hr after the BAT
trigger). The result of the Mercator Telescope follow-up observation,
referred to the two optical sources reported by Lin et al. (GCNC 6936)
and the two XRT sources reported by Beardmore et al. (GCNC 6943) is the
following one:

The optical source # 1 at RA(J2000) = 10:58:11.3 Dec(J2000)= +53:50:56 is
well detected in our R & I-band frames with R = 20.6 +/- 0.4 based on
the photometry given in the USNO-B1.0 catalogue. This is consistent with
the value reported by SARA (Updike et al. GCNC 6944) about 3 hours
later. We note that there is no "quiescent" counterpart to this object
in the SDSS but the lack of an X-ray counterpart argues against this
variable object being the optical afterglow.

The location of the optical source # 2 at RA(J2000)= 10:58:21.459
Dec(J2000)= +53:51:09.85 is covered in our images and we do not see any
object at this position with a limiting magnitude of R ~ 22.0 (besides
the one present in the SDSS).

The location of the XRT source # 1 at RA(J2000) = 10 58 02.99 Dec(J2000)
= +53 49 21.9 is covered in our images and we do not see any object at
this position with a limiting magnitude of R ~ 22.0.

The XRT source # 2, outside the refined BAT error box (Sato et al. GCNC
6933), is also outside the field of view of the Mercator frames.

Further observations are encouraged."