Minimalist model of the W50/SS433 extended X-ray jet: Anisotropic wind with recollimation shocks

Abstract

W50 is a radio nebula around the hyperaccreting Galactic microquasar SS 433. We focus on one peculiar feature of W50, that is, on a pair of so-called extended X-ray jets (EXJs). These jets have a size of $ pc $ and a sharp inner boundary, and their spectra are well represented by a featureless X-ray continuum. We argue that EXJs could be an outcome of a powerful anisotropic wind produced by a supercritical accretion disk. In the simplest version of this model, the wind itself consists of two components. The first component is a nearly isotropic outflow that subtends most of the solid angle as seen from the compact source and creates the quasi-spherical part of the W50 nebula. The second component is a more collimated wind that is aligned with the rotation axis of the binary system (polar wind). The isotropic outflow passes through the termination shock, and its increased thermal pressure creates a sequence of recollimation shocks in the polar wind, giving it the appearance of an extended X-ray structure. In this model, the EXJ continuum spectrum is due to synchrotron emission of electrons that are accelerated at the shocks that arise in the polar wind. At variance with many other studies, the EXJ structures in this model are not directly related to the highly collimated and precessing $0.26\;\!c$ baryonic jets. Instead, the EXJ and the ears of W50 are produced by the part of the wind whose Eddington-level kinetic luminosity is confined to a half-opening angle of 5-10 degrees. This is not necessarily a recollimated version of the $0.26\;\!c$ jets.