Hard X-Ray Emission from the Eastern Jet of SS 433 Powering the W50 “Manatee” Nebula: Evidence for Particle Reacceleration

Abstract

We present a broadband X-ray study of W50 (the “Manatee” nebula), the complex region powered by the microquasar SS 433, that provides a test bed for several important astrophysical processes. The W50 nebula, a Galactic PeVatron candidate, is classified as a supernova remnant but has an unusual double-lobed morphology likely associated with the jets from SS 433. Using NuSTAR, XMM-Newton, and Chandra observations of the inner eastern lobe of W50, we have detected hard nonthermal X-ray emission up to ∼30 keV, originating from a few-arcminute-sized knotty region (“Head”) located ≲18′ (29 pc for a distance of 5.5 kpc) east of SS 433, and constrained its photon index to 1.58 ± 0.05 (0.5–30 keV band). The index gradually steepens eastward out to the radio “ear” where thermal soft X-ray emission with a temperature kT ∼ 0.2 keV dominates. The hard X-ray knots mark the location of acceleration sites within the jet and require an equipartition magnetic field of the order of ≳12 μG. The unusually hard spectral index from the “Head” region challenges classical particle acceleration processes and points to particle injection and reacceleration in the subrelativistic SS 433 jet, as seen in blazars and pulsar wind nebulae.

X-Ray Polarization of the Eastern Lobe of SS 433

“…The normalization for DU1 was fixed to unity and those for the other DUs were allowed to vary. We find a good spectropolarimetric fit (χ 2 /dof = 137.0/123) with a photon index of Γ = 1.57 ± 0.14, consistent with that found by Safi-Harb et al (2022). The observed flux is 5.8 × 10 −13 erg cm −2 s −1 in the 2-5 keV band.…”

“…We modeled the spectrum using an absorbed power law with constant polarization multiplied by a constant allowed to vary between the three DUs. The column density for the tbabs absorption model using the Wilms et al (2000) abundances was fixed to 1.18 × 10 22 cm −2 as for the "Cone" region in Safi-Harb et al (2022). The normalization for DU1 was fixed to unity and those for the other DUs were allowed to vary.…”

“…The target coordinates (R.A. = 288°.27799, decl. = 4°.93573) were selected to cover the region of hard X-ray emission identified by Safi-Harb et al (2022). The target is ¢ 5.…”

“…The IXPE image in this band is shown in Figure 1 along with an XMM-Newton mosaic in the 0.3-10 keV band (Safi-Harb et al 2022). We define an IXPE source region, that we refer to as "C11," by drawing a contour corresponding to 11 counts per pixel in the IXPE map using the SAOImage DS9 image display tool with the smoothing parameter set to 16 (Smithsonian Astrophysical Observatory 2000).…”

High-energy Neutrinos from Gamma-Ray-faint Accretion-powered Hypernebulae

“…ff at all times t < t active for all  m. Thus motivated, in what follows, we shall focus on the interaction of ions accelerated at the JTS with the thermal disk emission and its nonthermal Comptonized counterpart. The densities of the other sources of hard X-ray and/or gamma-ray photons, e.g., from the jet knots (Safi-Harb et al 2022) are also negligible compared to n X th nth .…”