TITLE: GCN GRB OBSERVATION REPORT NUMBER: 136 SUBJECT: GRB980703 Transient Optical Follow-Up DATE: 98/07/08 01:09:23 GMT FROM: Josh Bloom at CIT GRB 980703 Transient Optical Follow-Up J. S. Bloom, S. J. Djorgovski, S. R. Kulkarni (CIT) and D. A. Frail (NRAO) report on behalf of the Caltech-NRAO GRB collaboration: "In the GRB 980703 field (IAUC #6966; GCN #126; GCN #127) of the radio/optical transient discovered by Frail et al. (GCN #128), R-band images (10 min) were obtained at the Palomar 60 inch on July 4,5 by R. O. Marzke (Carnegie) and D. R. Patton (U. Victoria) and at the Keck-II 10-m on LRIS by J. B. Oke (DAO), K. D. Horne (U. St Andrews), and R. Gomer. Photometry, based on an approximate LRIS zero-point (which is uncertain to ~0.2 mag), results in the following derived magnitudes for the transient: Date (UT) Intr Mag OT Rc ----------------------------------- Jul 4.477 P60 21.3 +/- 0.2 Jul 5.482 P60 21.8 +/- 0.3 Jul 6.607 LRIS 22.03 +/- 0.02 (+ zero point uncertainty 0.2mag for all three points) The zero-point may change with expected calibrations but the slope will not. The fading a power-law decline of -0.74, consistent with the I-band fading (-0.84) found from data in Vreeswijk et al. (GCN 132), would make this OT the slowest fading counterpart to a GRB yet. However, since the decay appears somewhat stronger at earlier epochs, we believe that the light may be contaminated by an underlying host. If true, the light curve should show even more flattening. This hypothesis appears corroborated by the fact that the spectral indices derived from the I/R/H-band data are not consistent with a simple fireball model prediction from the decay constant. A summary light curve with all reported I/R-band data may be found at http://astro.caltech.edu/~jsb/grb980703_ltcurve.ps" This message is citeable.