TITLE: GCN GRB OBSERVATION REPORT NUMBER: 128 SUBJECT: GRB980703: radio source/optical transient DATE: 98/07/06 23:08:34 GMT FROM: Dale A. Frail at NRAO D. A. Frail, (NRAO), J. P. Halpern (Columbia), J. S. Bloom (CIT), S. R. Kulkarni (CIT), S. G. Djorgovski and the BeppoSAX GRB Team report: We observed the entire RXTE error circle of GRB 980703 (IAU Circ. 6966, GCN #126) with the VLA on 1998 July 4.4 UT at a wavelength of 6 cm. A single, weak radio source is detected coincident within the 50 arcsec radius error circle of a fading X-ray source seen by BeppoSAX (GCN #127). The 6-cm flux density is 135 microJy and the rms noise is 26 microJy/beam, with a 1.6 arcsec beam. The approximate position of this source is 23h59m6.67s, +08d35'6.7" (equinox J2000). Further radio observations are planned and to test for any variability. Keck II images obtained by J. B. Oke (DAO) and K. D. Horne (U. St Andrews) on 1998 July 6.60 UT in B and R with 0.5 arcsecond seeing reveals a point-like object at position RA = 23:59:06.69, Decl = +08:35:07.26 (J2000, uncertainty 0.4"), consistent with the position of the radio source. Comparison with images obtained at the Palomar 60in by R. O. Marzke (Carnegie) and D. R. Patton (U. Victoria) on July 4.48 UT show the object to have faded by 0.7 mag in R based on differential photometry with respect to nearby stars. An approximate photometric calibration can be made assuming that a nearby star at RA = 23:59:05.94, Decl = +08:34:57.4 has R = 16.5 as listed in the USNO A1.0 catalog. In this system, the optical transient faded from R = 21.2 on July 4.48 to R = 21.9 on July 6.60. This suggests a power-law decay constant of 0.75 between these dates. A finding chart can be found at http://astro.caltech.edu/~jsb/grb980703_find.ps" This report can be cited.