TITLE: GCN GRB OBSERVATION REPORT NUMBER: 207 SUBJECT: GRB 990123 r-band photometry DATE: 99/01/24 03:33:59 GMT FROM: Shri Kulkarni at Caltech GRB 990123 r-band Photometry R. R. Gal, S. C. Odewahn, J. S. Bloom, S. R. Kulkarni (CIT), D. A. Frail (NRAO) report on behalf of the Caltech-NRAO-CARA GRB Collaboration: "We report on a detailed analysis of images obtained at the Palomar 60-inch telescope, the initial results of which were reported in GCN 201. Three images of 400-s duration each were obtained of the field of GRB 990123 localized by BeppoSAX (GCN #200). These images were obtained in the Gunn r-band and not R band as reported in GCN #201. The seeing was 1.7 arcsec (FWHM) in each image. Using images obtained of two Gunn standards (Feige 34 and Feige 67), we obtained a photometric zero point for the data. The observation times and magnitudes for the optical transient reported in Odewahn, Bloom, Kulkarni (GCN #202) and the reference star from Bloom et al. (GCN #206) are: Gunn-r Mag UT (23 Jan 1999) OT Ref. Star --------------------------------------------------------- 13:37:20.3 18.70 +/- 0.04 19.10 +/- 0.04 13:51:03.6 18.78 +/- 0.04 19.11 +/- 0.04 14:02:56.5 18.75 +/- 0.06 19.07 +/- 0.06 -------------------------------------------------------- The uncertainty in the magnitudes include the systematic and statistical uncertainties, although no color term was applied. We note that the second r-band image was taken nearly simultaneous with the B-band image discussed in Bloom et al. (GCN #206). The Galactic extinction in the direction of the optical transient (l,b = 73.12, 54.64) is E(B-V) = 0.01597 (Schlegel, Finkbeiner, and Davis; 1998 ApJ, 500, 525). Thus (assuming Rv = 3.1), A_B = 0.069 and A_gunnr = 0.041. The extinction corrected magnitudes of the transient at epoch Jan 23.578 1999 UT are thus B = 18.86 and Gunn-r = 18.74. Assuming the bandpass zeropoints and effective central wavelength of the Johnson B and Gunn-r bandpasses from Fukugita, Shimasaku, and Ichikawa (1995 PASP, 107, 945) we get F(B)= 115 microJy F(r) = 94 microJy. Thus the power law slope beta = 0.5 where beta = log F(nu/log(nu). the transient is blue with a spectral index of beta = 0.5 (with beta =dlog F(nu)/dlog nu). This spectral behavior has not been seen in previous GRB afterglows. In the framework of the afterglow models, this result can be interpreted to mean that the flux from the afterglow did not peak in the optical bands by the epoch of our second r-band image. This message may be cited."