TITLE:   GCN CIRCULAR
NUMBER:  918
SUBJECT: Optical Observations of GRB 001018 at Keck
DATE:    01/01/21 23:33:36 GMT
FROM:    Josh Bloom at CIT  <jsb@astro.caltech.edu>

Optical Observations of GRB 001018 at Keck

J. S. Bloom, S. G. Djorgovski, F. A. Harrison, P. Mao (Caltech), and
D. Stern (JPL) report on behalf of the larger Caltech-NRAO-CARA GRB
Collaboration:

"We observed the field of GRB 001018 (Hurley et al., GCN #853) with the
Keck Telescopes on the nights of 29 December 2000 UT and 1 January 2001 UT
with LRIS (Oke et al. 1995) and ESI (Sheinis et al. 2000), respectively.  
About 1 arcsec from the position of the radio transient reported by Taylor
et al. (GCN #857), we find a faint compact source, likely an unresolved
galaxy.

An astrometric plate solution was obtained using 16 USNO A2.0 stars in
common with a 20s R-band exposure with LRIS. The total r.m.s errors on the
resulting tie to the ICRS were 0.31", 0.27" (ra,dec), including the
systematic uncertainties in the USNO A2.0 catalogue described by Deutsch
1998.  Including the uncertainties in the radio position and the nearby
galaxy position, the centroid of the galaxy is 0.46" +/- 0.32" E, 0.89"
+/- 0.27" N of Taylor's radio transient.  Given the proximity to the radio
transient, this galaxy may be related to the radio transient.
 
We obtained photometric zeropoints using the Landolt standard star fields
of PG1047+003 (R-band) and SA 104 (I-band).  For the galaxy, we find that
I=24.03 +/- 0.10 mag (1 Jan 01 UT) and R=24.60 +/- 0.09 mag. This
detection is consistent with the upper limit placed by Bloom et al. (GCN
#915). For comparison, we find for stars A (ra=13:14:09.9, dec=+11:48:56,
J2000) and B (ra=13:14:09.4, dec=+11:48:23, J2000) that I(A) = 18.42 +/-
0.03 mag, R(A) = 18.54 +/- 0.03 mag, I(B) = 18.82 +/- 0.03 mag, and R(B) =
19.91 +/- 0.03 mag."

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