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.
We examined the X-ray and radio spatial structure at the eastern ear of the W 50/SS 433 system to clarify a characteristic feature of the termination region of the SS 433 jet, and found that a hot spot ahead of the filament structure, which is considered to be a terminal shock of the SS 433 eastern jet, is clearly different from a single point source. The detailed spatial structure of the X-ray emission is finely resolved by Chandra observations, showing that there are two sources. By comparing the point-spread function of Chandra with the radial profiles of the two sources, the northern one is clearly more extended than a point source while the other seems marginally extended. Since there are no point sources nearby, the northern hot spot is likely a localized diffuse source. The northern hot spot spatially corresponds to the peak of the radio emission. Its spatial correlation is confirmed by an X-ray image using XMM-Newton. The X-ray spectra of the two sources are reproduced by a single absorbed power-law but the column density of the northern part is larger by a factor of ∼3. When a radiation model comprising synchrotron emission and inverse Compton emission is applied to the spectral energy distribution of the northern hot spot, the emission from this spot can be explained by the radiation from an electron population accelerated up to 30 TeV in a magnetic field strength of B ≲ 50 μG. This model also agrees with the radio and X-ray data, as well as the upper limit of gamma-ray emission obtained by the Fermi satellite.
more » « less- NSF-PAR ID:
- 10367891
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Publications of the Astronomical Society of Japan
- Volume:
- 74
- Issue:
- 3
- ISSN:
- 0004-6264
- Format(s):
- Medium: X Size: p. 510-520
- Size(s):
- ["p. 510-520"]
- Sponsoring Org:
- National Science Foundation
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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 μ G. 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.more » « less
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Abstract Microquasar SS 433 located at the geometric center of radio nebula W 50 is a suitable source for investigating the physical process of how galactic jets affect the surrounding interstellar medium (ISM). Previous studies have searched for evidence of the interaction between the SS 433 jet and ISM, such as neutral hydrogen gas and molecular clouds; however, it is still unclear which ISM interacts with the jet. We looked for new molecular clouds that possibly interact at the terminal of the SS 433 eastern jet using the Nobeyama 45 m telescope and the Atacama Submillimeter Telescope Experiment (ASTE). We identified two molecular clouds, comprising many small clumps, in the velocity range of 30.1–36.5 km s−1 for the first time. These clouds have complex velocity structures, and one of them has a density gradient toward SS 433. Although it is difficult to conclude the relation between the molecular clouds and the SS 433/W 50 system, there is a possibility that the eastern structure of W 50 constructed by the SS 433 jet swept up tiny molecular clumps drifting in the surroundings and formed the molecular clouds that we identified in this study.
