TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  242
SUBJECT: GRB990123 Optical Observations
DATE:    99/02/01 02:19:38 GMT
FROM:    Jules Halpern at Columbia U.  <jules@astro.columbia.edu>

 I. A. Yadigaroglu, J. P. Halpern, R. Uglesich, & J. Kemp (Columbia U.)
 report on behalf of the MDM Observatory GRB follow-up team:

"We imaged the field of GRB 990123 in the R band on Jan. 30.52 using the
 MDM Observatory 2.4m telescope.  A total of 40 minutes exposure was
 obtained in seeing of 0.9-1.0 arcsec.  The optical transient is clearly
 detected at magnitude R = 23.01 +/- 0.24 (referenced to the comparison 
 star of GCN #207, assuming r - R = 0.4).  An independent calibration
 using a Landolt standard also gives a consistent magnitude.  The OT 
 position, measured with respect to the USNO-A2.0 reference system,
 is (J2000) RA 15:25:30.34, Dec +44:45:59.2 with an uncertainty of
 0.3 arcseconds in radius.  This position is consistent with that of the
 original detection of the OT (GCN #206).  The temporal power-law decay 
 slope connecting our observation to the first Palomar detection is 
 alpha_r = -1.15 +/- 0.07, consistent with all previous observations,
 which gave alpha_r = -1.13 +/- 0.03 (GCN #240).

 Thus, the OT appears to be unresolved, at a fixed position, and following
 a power-law decay in time.

 However, we see no object corresponding to a suggested intervening
 galaxy that was estimated to lie 1.8 arcsec north of the OT (GCN #206).
 In fact, there is no other object on our image within 5 arcsec of the
 OT, to a limiting magnitude of approximately R = 24.  Since this galaxy
 was only reported to be seen marginally on one POSS II red plate with
 R = 21.5 +/- 0.5 (GCN #213), we conclude that it probably does not
 exist.  If true, this eliminates one of the arguments for hypothesizing
 that the burst is lensed."

 The MDM image will be posted in the near future at
 http://cba.phys.columbia.edu/grb/

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