We present the Rossi X-Ray T iming Explorer (RXT E) observations of the SNR W50, associated with SS 433. The observations cover the X-ray lobe found between SS 433 and the outer eastern radio "" ear ÏÏ of W50. The X-ray spectrum measured with the proportional counter array and the High-Energy Timing Experiment in the 3È100 keV range favors nonthermal over thermal models for this part of W50. We combine the RXT E spectrum with the ROSAT and ASCA spectra, thus covering D3 orders of magnitude in energy. The spectrum of the harder X-ray emission is well represented by a power law with a photon index of D1.45 and a total X-ray luminosity of D1.5 ] 1035 ergs s~1 for a source distance of 5.5 kpc. We thus conÐrm the nonthermal nature of the inner X-ray lobe. We discuss its implications in the context of the interaction between the jets from SS 433 and the surroundings. Subject headings : ISM : individual (W50) È stars : individual (SS 433) È supernova remnants È X-rays : starsW50 is a large, nonthermal Galactic radio source, with a 2¡ ] 1¡ ellipsoidal shape (Geldzahler, Pauls, & Salter 1980 ;Downes, Pauls, & Salter 1981, 1986 Elston & Baum 1987). At a distance of 5.5 kpc (Vermeulen et al. 1993), W50 represents one of the largest Galactic supernova remnants (SNRs). The discovery of the Galactic microquasar SS 433 at its center (Clark, Green, & Caswell 1975 ;Seward et al. 1976) makes it one of the most exciting objects, since its study will help in the understanding of extragalactic radio jets. SS 433 is a binary source, with an orbital period of 13.1 days, known for its two-sided jets moving at a speed of 0.26c and precessing in a cone of half-angle 20¡, with a period of D163 days (Abell & Margon 1979). While the nature of SS 433 remains a mystery, the elongation of W50 in the eastwest direction and its alignment along the symmetry axis of the jets from SS 433 provides clear evidence that its morphology results from the jets interacting with the surroundings.Optical maps of W50 (Zealy, Dopita, & Malin 1980 ;van Den Berg 1980 ;Kirshner & Chevalier 1980) show Ðlaments at D30@ east and west from SS 433, with an unusually high [N II]/Ha ratio. All the Ðlaments lie well inside the eastern and western boundaries of W50 and within the precession cone of the SS 433 jets. This led to the suggestion that they result from the propagation of shock waves into the preexisting dense Ðlaments of the SNR along the direction of the impinging jets.Infrared observations of W50 (Band 1987 ;Wang et al. 1990) reveal the presence of knots aligned with the jets of SS 433. Wang et al. (1990) argue that these knots result from dust grains and gas being heated by the fast electrons and ions in the jets.In X-rays, Watson et al. (1983) found, using the Einstein Observatory, two bright di †use lobes symmetrically displaced east and west of SS 433 and aligned along the axis of W50. Each lobe is visible from D15@ to D40@ from SS 433, with emission peaking at about 35@, coinciding with the optical Ðlaments, and lying near the base of its respecti...