GCN Circular 206

GRB 990123 Optical Follow-Up

J. S. Bloom, R. R. Gal, L. L. Lubin, J. Mulchaey, S. C. Odewahn (CIT), S.
R. Kulkarni
report on behalf of the Caltech-NRAO-CARA GRB Collaboration:

"We obtained a single B-band image (300-s) at the Palomar 200-inch of the
field of GRB 990123 localized by BeppoSAX (GCN #200).  Using images
obtained of the Landolt standard field 98, we derived a photometric
zeropoint for the GRB data.  The transient first reported in Odewahn,
Bloom, Kulkarni (GCN #202) is well-detected at B = 18.93 +/- 0.03 mag (Jan
23.578 UT).  The uncertainty includes the systematic and statistical
uncertainties though no color term was applied.  For reference, we find
the following B = 19.59 +/- 0.04 for the object at position ra:
15:25:32.7, dec: +44:44:29.7 (J2000).

Absolute astrometry was obtained by comparison of 34 objects near the
optical transient with the USNO-A2.0 Catalogue.  The r.m.s. uncertainties
of the astrometry are 0.28 arcsec (ra) and 0.26 arcsec (dec).  We find the
position of the optical transient to be, ra: 15:25:30.34, dec: +44:45:59.1
(J2000).  Based on the POSS-II F-Plate of this field, the purported host
galaxy (see GCN #202) appears offset from the OT by 1.8 arcsec (+/- 0.4
arcsec) to the South.  Further analysis is underway.

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