High-resolution view of non-thermal emission from SS 433/W50 west region with the Chandra X-ray Observatory

The X-ray binary SS 433 embed in W50 has east-west jets with a mildly relativistic velocity of 0.26c. Past X-ray observations discovered spatially extended structures including compact bright structures, so-called "knots", along the jets (Watson et al. 1983; Safi-Harb et al.1997). Recently, >25 TeV gamma-ray emission is reported from innermost knots, indicating particle acceleration up to ~ 100 TeV (Abeysekara et al. 2018). However, details of these non-thermal X-ray emissions, such as the origin of the knots and extended structures, are still veiled in mystery. We analyze archival X-ray data obtained with the Chandra X-ray Observatory from the west jet. We could not reveal any detectable emission between the SS 433 and the innermost knot. In the jet beyond this boundary, there are three knots as well as fainter diffuse emission. From the spectral variation, we find a tendency for spectral slope steepening along the jet axis. We develop a one-dimensional model that describes the emission evolution along the jet with variable magnetic field strength. We conclude that the extended structure originates from the synchrotron emission by the electrons, which are accelerated at the innermost knot. Our calculations also indicate that the outermost knot corresponds to a region of the enhanced magnetic field, which results in the efficient cooling of high-energy electrons. Comparing the X-ray morphology to the radio continuum observations (e.g., Dubner et al. 1998; Broderick et al. 2017), we find that the outermost knot position correlates with a part of the central shell of W50, which suggests that the jet interaction with the shell leads to the magnetic field compression. We also detect a new X-ray knot between the innermost and outermost knots. This knot possibly correlates with a molecular cloud (Yamamoto et al. 2008; Su et al. 2018), which is presumably related to the origin of the knot