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PSR B1259-63 is in a highly eccentric 3.4 yr orbit around the Be-star SS2883.The system is located in the direction of the Sagittarius-Carina spiral arm ata distance of $\sim$1.5 kpc. We have made scintillation observations of thepulsar far from periastron at 4.8 GHz and 8.4 GHz, determining the diffractivebandwidth, $\Delta \nu_{\rm d}$, and timescale, $\Delta t_{\rm d}$, at bothfrequencies. We find no dependency on orbital phase until within 30 days ofperiastron. The data indicate that the scintillation is caused, not by thecircumstellar environment, but by an HII region within the Sagittarius-Carinaspiral arm located at least three-quarters of the way to the pulsar. Close toperiastron, when the line-of-sight to the pulsar intersects the disk of the Bestar, the electron densities within the disk are sufficient to overcome the`lever-arm effect' and produce a reduction in the scintillation bandwidth bysix orders of magnitude.Comment: Accepted for publication in Astrophysical Journal Letters, 1 figur

Diffractive Scintillation of the Pulsar PSR B1259−63