Hard X-ray emission from the eastern jet of SS 433 powering the W50 `Manatee' nebula: Evidence for particle re-acceleration

We present a broadband X-ray study of W50 (`the Manatee nebula'), the complexregion powered by the microquasar SS 433, that provides a test-bed for severalimportant astrophysical processes. The W50 nebula, a Galactic PeVatroncandidate, is classified as a supernova remnant but has an unusual double-lobedmorphology likely associated with the jets from SS 433. Using NuSTAR,XMM-Newton, and Chandra observations of the inner eastern lobe of W50, we havedetected hard non-thermal X-ray emission up to $\sim$30 keV, originating from afew-arcminute size knotty region (`Head') located $\lesssim$ 18$^{\prime}$ (29pc for a distance of 5.5 kpc) east of SS 433, and constrain its photon index to1.58$\pm$0.05 (0.5-30 keV band). The index gradually steepens eastward out tothe radio `ear' where thermal soft X-ray emission with a temperature$kT$$\sim$0.2 keV dominates. The hard X-ray knots mark the location ofacceleration sites within the jet and require an equipartition magnetic fieldof the order of $\gtrsim$12$\mu$G. The unusually hard spectral index from the`Head' region challenges classical particle acceleration processes and pointsto particle injection and re-acceleration in the sub-relativistic SS 433 jet,as seen in blazars and pulsar wind nebulae.<br