Owens Valley interferometric observations of the gamma-ray bursts GRB 970228 and GRB 970508

We have obtained new observations of the gamma-ray bursts GRB 970228 and GRB 970508 with the Owens Valley Radio Observatory. GRB 970228 was observed in the 1 and 3 mm continuum bands. The held was centered on the position of the optical counterpart approximately 17 days after the initial GRB burst. GRB 970228 was not detected at either wavelength, which places 3 sigma upper limits of 2.16 mJy at 92.7 GHz and 38.1 mJy at 242.4 GHz. GRB 970508 was observed in the 3 mm continuum band, centered first on the position of the X-ray source SAX J0653.8+7916 and then on the position of the optical counterpart and the transient source detected by the Very Large Array. A series of five observations were made from 1997 May 10 (similar to 2 days after initial burst detection) to 1997 May 16 (similar to 8 days after burst). GRB 970508 was not detected during any observation. We combine data from May 12 to May 16 to place a solid 3 sigma upper limit of 3.0 mJy at 86.8 GHz. Assuming the gamma-ray burst can be modeled by a cosmological fireball, our nondetection supports the interpretation by Frail and coworkers that large fluctuations seen in the centimeter-wave radio fluxes during this time are not due to changes in the source itself but are instead due to interstellar scintillation when the early fireball is nearly a point source.