{
  "bibcode": "1999GCN...218....1B",
  "body": "GRB 990123: New Optical Observations, Decay Measure\n\nJ. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn (CIT), R.\nSagar, A. K. Pandey, Neelakshi, R. K. S. Yadav (U. P. State Observatory,\nIndia) report on behalf of the Caltech-NRAO-CARA GRB collaboration:\n\n\"On 24.0 Jan 1999 UT, we imaged the optical transient (GCN #201) of GRB\n990123 (GCN #199) with the 1.04-m UPSO telescope, India.  In one 50-minute\nB-band image the transient is well-detected at B = 20.16 +/- 0.15 (Jan\n23.958 UT).  The transient is also well-detected in a 60-minute stacked\nR-band exposure at gunn-r = 20.02 +/- 0.11.  The B-band image was\ncalibrated to the zeropoint of GCN #206 with a set of secondary stars in\nthe OT field.  The R-band photometry was tied the gunn-r system using the\nPalomar 60-inch photometry (#207). The error associated with the\ncalibration between the two somewhat different bandpasses is small\ncompared to the statistical uncertainties.  These measurements have not\nbeen corrected for Galactic extinction which would amount to A_B = 0.069\nand A_r = 0.041 (GCN #207).  With the Galactic extinction correction, the\ntransient still blue (ie. has a positive spectral index beta ~= 0.3) at\nthis second epoch (see GCN #207).\n\nAssuming a power-law decay (f_nu[nu] = const*t^alpha) we find\n\n    alpha_B = -0.90 +/- 0.11    (between Jan 23.58 and Jan 23.95)\n    alpha_r = -1.04 +/- 0.14    (between Jan 23.56 and Jan 24.00)\n\nThis decay is significantly flatter than alpha = -1.5 found at earlier\ntimes (ie. t <= Jan 23.5; GCN #208). A recalibration of the transient\nmagnitude reported by Garnavich et al. (GCN #215) using the reference star\nfrom Gal et al. (GCN #207), gives r = 21.0 (errors not reported) on Jan\n24.547 UT.  This implies a decay of alpha = -1.26, consistent with that\nfound above.  \n\nAn updated light-curve may be obtained at \nhttp://astro.caltech.edu/~jsb/GRB/grb990123.html\n\nThis message may be cited.\"",
  "circularId": 218,
  "createdOn": 917260448000,
  "email": "jsb@astro.caltech.edu",
  "subject": "GRB 990123: New Optical Observations, Decay Measure",
  "submitter": "Josh Bloom at CIT  <jsb@astro.caltech.edu>",
  "eventId": "GRB 990123"
}