Minimalist model of the W50/SS433 "Extended X-ray Jet": anisotropic wind with recollimation shocks

Abstract

W50 is a radio nebula around hyper-accreting Galactic microquasar SS~433. Here we focus on one peculiar feature of W50 - a pair of so-called "extended X-ray jets" (EXJs). These "jets" have a size of $\sim20\, {\rm pc}$, a sharp inner boundary, and their spectra are well represented by a featureless X-ray continuum. We argue that EXJ could be an outcome of a powerful {\it anisotropic} wind produced by a super-critical 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 aligned with the binary system rotation axis (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 accelerated at the shocks arising in the polar wind. At variance with many other studies, in this model, the EXJ structures are not directly related to the highly collimated and precessing $0.26\;\!c$ baryonic jets. Instead, the EXJ and the W50's ears are produced by the part of the wind with an Eddington-level kinetic luminosity confined to a half-opening angle of 5-10 degrees, which is not necessarily a recollimated version of the $0.26\;\!c$ jets.