X-ray panorama of the SS433/W50 complex by SRG/eROSITA

TL;DR

SS433/W50 is a Galactic microquasar where hyper-Eddington outflows energize and sculpt the surrounding ISM, forming the expansive W50 nebula. Using SRG/eROSITA, the paper presents the first full, high-dynamic-range X-ray map of W50, revealing a three-component energy flow: a 'dark' inner wind from $0.1$–$25$ pc, a prominent 'non-thermal' EXJ flow out to ~60 pc, and a surrounding thermal shock-heated ISM. Spectroscopy across eight regions shows soft NEI plasma in the lobes and hard, often non-thermal EXJs with sharp edges, illustrating distinct physical regimes within the same system. The findings support a picture in which a hyper-Eddington engine drives anisotropic outflows that accelerate particles to very high energies, reshape the nebula, and offer key constraints for future multiwavelength studies and numerical simulations of jet–ISM interactions.

Abstract

Galactic microquasar SS433 and the radio nebula W50 surrounding it present a prototypical example of a hyper-Eddington binary system shaping its ambient interstellar medium via energetic outflows. In this paper, we present X-ray observations of the SS433/W50 complex by the eROSITA telescope onboard the SRG space observatory. These data provide images of the entire nebula characterized by a very large dynamic range and allow spectral analysis of the diffuse X-ray emission. In particular, these data illustrate a close connection between the thermal and non-thermal components of W50 on scales ranging from sub-parsecs, represented by narrow X-ray bright filaments, to the entire extent $\gtrsim 100\,{\rm pc}$ of the nebula. These data also allow us to fully characterize a pair of nearly symmetric, sharp-edged, elongated structures aligned with the orbital axis of the binary system, which lack radio counterparts, but are prominent in very high energy gamma-ray emission. The resulting multifaceted picture of the interaction between energetic outflows and the surrounding medium paves the way for future focused multiwavelength observations and dedicated numerical simulations.

Figures (9)

• Figure 1: Global radio and X-ray views of the $4\times4$ degrees region centered on SS 433 (both images are in Galactic coordinates). Left: Radio image (square-root scale) by Green Bank telescope at 6cm (4.85 GHz, 1996ApJS..103..427G). Right: SRG/eROSITA (particle background-subtracted exposure-corrected) 0.3-2.3 keV X-ray image (log scale, two orders of magnitude range) of the W50 field, which combines the PV and all-sky data accumulated over four consecutive scans after smoothing with a Gaussian beam with $\sigma=90$ arcsec. The light-blue areas in this image (where the X-ray flux is low) correspond to regions with high Galactic column density of gas and dust, which attenuate X-rays. Locations of supernova remnants 3C396, 3C397 and G038.7-01.3 2025JApA...46...14G, as well as of a HII region Sharpless 74 1959ApJS....4..257S are marked on both images.
• Figure 2: Broad-band high-dynamic-range 0.5-4 keV X-ray image of the giant radio nebula W50 by $SRG$/eROSITA (PV data). The size of the image is 2.2 degrees by 1.1 degrees, corresponding to $\sim200\times100$ pc at the distance of W50, $\sim5$ kpc. The image is rotated so that the longer side of W50 is aligned with the horizontal axis. Colour-coding corresponds to the X-ray surface brightness on the square-root scale spanning two orders of magnitude. The bright spot in the center is the SS 433. It appears extended due to the wings of the telescope's Point Spread Function around SS 433, which itself is heavily saturated in this image. The white contours show the surface brightness of the radio emission from the nebula at 4.85 GHz 1996ApJS..103..427G. With this deeper data (compared to the all-sky survey's data), a faint diffuse emission is seen between bright EXJs and the radio boundary of W50.
• Figure 3: A composite X-ray image of the radio nebula W50. Colour-coded is the surface brightness of the X-ray emission (on a linear scale) in 0.5-1 keV (red), 1-2 keV (green), and 2-4 keV (blue) energy bands. The white arrows depict the projection of the precession cone of the SS 433 jets extrapolated to distances $\sim 100\,{\rm pc}$. Hard and soft X-ray diffuse emission splits convincingly into two components: softer filamentary emission (red-yellow) and harder (green-blue) emission of EXJs. On top comes a multitude of nearby (active stars and accreting white dwarfs) and distant (mostly AGN) compact sources. For distant sources, the absorption by the Milky Way gas suppresses emission below 1 or 2 keV, giving them a blueish color.
• Figure 4: The comparison of the Eastern and Western Lobes of W50 (see the full image in Fig. \ref{['f:rgb']}). The Western Lobe has been rotated by 180 degrees for the sake of comparison. The white circles show the Half-Power-Diameter of the eROSITA PSF corresponding to the physical scale of 0.7 pc. Both EXJs emerge at essentially the same distance from the central source with a remarkably sharp edge and continue for $\sim 30$ pc. On the contrary, the more diffuse extensions have different lengths, plausibly associated with much higher ambient density in the Western direction.
• Figure 5: "Anatomy" of the diffuse X-ray emission inside W50. The image shows a composite map of the X-ray surface brightness in 0.5-1, 1-2, and 2-4 keV energy bands (similar to Fig. \ref{['f:rgb']}) after masking bright foreground and background point sources, and the central source SS 433. Overlaid in white are markers of the distance from the central source (assuming $d_{\rm SS~433}=5$ kpc) and angle with respect to its orbital plane axis, coinciding with the precession axis of its narrow relativistic jets. The box regions, numbered from 1 to 6, are used for spectrum extraction and represent fiducial spectral components composing the image.