Spatially resolved study of the SS 433/W 50 west region with Chandra: X-ray structure and spectral variation of non-thermal emission

Abstract

The X-ray binary SS 433, embedded in the W 50 nebula (or supernova remnant W 50), shows bipolar jets that are ejected with mildly relativistic velocities and which extend toward the east and west out to scales of tens of parsecs. Previous X-ray observations revealed twin lobes along the jet precession axis that contain compact bright knots dominated by synchrotron radiation, which provide evidence of electron acceleration in this system. Particle acceleration in this system is substantiated by the recently detected gamma rays with energies up to at least 25 TeV. To elucidate the origin of the knots and particle acceleration sites in SS 433/W 50 further, we report here on detailed, spatially resolved X-ray spectroscopy of its western lobe with Chandra. We detect synchrotron emission along the jet precession axis, as well as optically thin thermal emission that is more spatially extended. Between the two previously known knots, w1 and w2, we discover another synchrotron knot, which we call w1.5. We find no significant synchrotron emission between SS 433 and the innermost X-ray knot (w1), suggesting that electrons only begin to be accelerated at w1. The X-ray spectra become gradually steeper from w1 to w2, and then rapidly so immediately outside of w2. Through comparison with a model taking into account electron transport and cooling along the jet, this result indicates that the magnetic field in w2 is substantially enhanced, which also explains its brightness. We discuss possible origins of the enhanced magnetic field of w2 as well as scenarios to explain the other two knots.

Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433

“…This is compatible with the velocity ratio expected for a subrelativistic shock, which is χ ≈ 0.25 ( 39 , 40 ). A shock at this location is consistent with the spatial coincidence between the position of the highest-energy gamma-ray emission and the location where the x-ray emission reappears ( 16 , 17 ). This region has previously been interpreted as the acceleration site, but without involving shocks ( 16 ).…”

“…X-ray emission reappears 25 pc from the binary (Fig. 1), indicating collimated flows (the outer jets) on larger scales, which emit x-ray photons through nonthermal processes ( 14 – 17 ).…”

“…Several attempts have been made to use x-ray observations to probe the nonthermal emission mechanisms and internal dynamics of the eastern ( 16 , 24 ) and western ( 17 ) outer jets. However, observations of the x-ray synchrotron emission alone cannot resolve variations in the distribution of accelerated particles.…”

Cosmic Ray Processes in Galactic Ecosystems

“…The emission regions are ∼30 pc away from the location of the binary. Both the eastern and western emission lobes are found to coincide with these regions, from which X-ray emission is non-thermal [406][407][408][409][410][411]. These observations suggest that the jet downstream regions are sites of high-energy particle acceleration that produce the TeV γ-rays and non-thermal X-rays [412,413].…”