GRB 000418: A hidden jet revealed

We report on optical, near-infrared, and centimeter radio observations of G RB 000418 that allow us to follow the evolution of the afterglow from 2 to 200 days after the gamma -ray burst (GRB). In modeling these broadband data , we find that an isotropic explosion in a constant-density medium is unabl e to simultaneously fit both the radio and optical data. However, a jetlike outflow into either a constant density or wind-stratified medium with an o pening angle of 10 degrees -20 degrees provides a good description of the d ata. The evidence in favor of a jet interpretation is based on the behavior of the radio light curves, since the expected jet break is masked at optic al wavelengths by the light of the host galaxy. We also find evidence for e xtinction, presumably arising from within the host galaxy, with A(V)(host) = 0.4 mag, as well as host flux densities of F-R = 1.1 mu Jy and F-K = 1.7 mu Jy. These values supersede previous work on this burst as a result of th e availability of a broadband data set allowing a global fitting approach. A model in which the GRB explodes isotropically into a wind-stratified circ umburst medium cannot be ruled out by these data. However, in examining a s ample of other bursts (e.g., GRB 990510, GRB 000301C), we favor the jet int erpretation for GRB 000418.