{
  "bibcode": "1999GCN...248....1H",
  "body": "I. A. Yadigaroglu & J. P. Halpern (Columbia U.)\nreport on behalf of the MDM Observatory GRB follow-up team:\n\n\"We imaged the field of GRB 990123 in the R band on Feb. 3.54 UT using\nthe MDM Observatory 2.4m telescope.  A total of 60 minutes exposure was\nobtained in seeing of 1.2 arcsec.  An object is detected at magnitude \nR = 23.9 +/- 0.25 (referenced to the comparison star of GCN #207).\nHowever, its position is approximately 0.6 arcsec north of the previous \nposition.  The new position is (J2000) RA 15:25:30.343, Dec +44:45:59.86, \nwhereas the position on Jan. 30.52 was (J2000) RA 15:25:30.330,\nDec +44:45:59.27.  These positions are measured with respect to the \nsame set of comparison stars, and they each have a statistical uncertainty\nof 0.3 arcseconds in radius, but negligible systematic difference.\nA possible extension in a Jan. 27 K-band image 0.5 arcsec to the north \nof the OT was described by Djorgovski et al. (GCN #243).  Our new R-band\nimage is consistent with that report.\n\nThe optical transient has faded at an accelerated rate since our last\nreported observation on Jan. 30.52 (GCN #242).  A continuation of the \nalpha_r = -1.13 decay (Bloom et al. GCN #240) would have predicted an\nR magnitude of 23.47 on Feb. 3.54.  Since the position of the optical\ncentroid has shifted, we conclude that our measured R = 23.9 represents\nan upper limit to the magnitude of both the OT and any coincident galaxy,\nand that we are beginning to detect either an intervening galaxy or\nthe host galaxy of the burst.  We note that the accelerated decay of\nthe OT could be an indication that the cooling frequency has passed below\nthe optical band, or that a jet which initially was highly collimated \ntoward us has begun to spread.  An alternative interpretation in which\nthe initial OT has disappeared, and at the same time been replaced by\na fainter lensed component, seems less likely.\n\nThe new MDM image will be posted in the near future at\nhttp://cba.phys.columbia.edu/grb/\n\nThis message may be cited.",
  "circularId": 248,
  "createdOn": 918072158000,
  "email": "jules@astro.columbia.edu",
  "subject": "GRB990123, Optical observation",
  "submitter": "Jules Halpern at Columbia U.  <jules@astro.columbia.edu>",
  "eventId": "GRB 990123"
}