We present radio observations of the afterglow of the bright \gamma -ray burst GRB 980703 made between one day and one year after the burst .
These data are combined with published late-time radio measurements and existing optical , near-infrared ( NIR ) and X-ray observations to create a comprehensive broadband dataset for modeling the physical parameters of the outflow .
While a wind-stratified medium can not be ruled out statistically , it requires a high fraction of the shock energy in the electrons , and so is not favored on theoretical grounds .
Instead , the data are consistent with a fireball model in which the ejecta are collimated and expanding into a constant density medium .
The radio data can not be fit with an isotropic shock but instead require a jet break at \approx 3.5 days , not seen at optical wavelengths due to the presence of a a bright host galaxy .
The addition of the full radio dataset constrains the self-absorption frequency , giving an estimate of the circumburst density of n \approx 30 cm ^ { -3 } , a value which differs substantially from previous estimates .
This result is consistent with the growing number of GRB afterglows for which broadband modeling yields n \simeq 0.1 - 100 cm ^ { -3 } , with a typical value \sim 10 cm ^ { -3 } .