Letter | Published:

Very-high-energy particle acceleration powered by the jets of the microquasar SS 433


SS 433 is a binary system containing a supergiant star that is overflowing its Roche lobe with matter accreting onto a compact object (either a black hole or neutron star)1,2,3. Two jets of ionized matter with a bulk velocity of approximately 0.26c (where c is the speed of light in vacuum) extend from the binary, perpendicular to the line of sight, and terminate inside W50, a supernova remnant that is being distorted by the jets2,4,5,6,7,8. SS 433 differs from other microquasars (small-scale versions of quasars that are present within our own Galaxy) in that the accretion is believed to be super-Eddington9,10,11, and the luminosity of the system is about 1040 ergs per second2,9,12,13. The lobes of W50 in which the jets terminate, about 40 parsecs from the central source, are expected to accelerate charged particles, and indeed radio and X-ray emission consistent with electron synchrotron emission in a magnetic field have been observed14,15,16. At higher energies (greater than 100 gigaelectronvolts), the particle fluxes of γ-rays from X-ray hotspots around SS 433 have been reported as flux upper limits6,17,18,19,20. In this energy regime, it has been unclear whether the emission is dominated by electrons that are interacting with photons from the cosmic microwave background through inverse-Compton scattering or by protons that are interacting with the ambient gas. Here we report teraelectronvolt γ-ray observations of the SS 433/W50 system that spatially resolve the lobes. The teraelectronvolt emission is localized to structures in the lobes, far from the centre of the system where the jets are formed. We have measured photon energies of at least 25 teraelectronvolts, and these are certainly not Doppler-boosted, because of the viewing geometry. We conclude that the emission—from radio to teraelectronvolt energies—is consistent with a single population of electrons with energies extending to at least hundreds of teraelectronvolts in a magnetic field of about 16 microgauss.

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Data availability

The datasets analysed during this study are available at a public repository maintained by the HAWC Collaboration: https://data.hawc-observatory.org/.

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Change history

  • 27 November 2018

    In this Letter, owing to a production error, the penultimate version of the PDF was published. The HTML version was always correct. The PDF has been corrected online.


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We acknowledge support from: the US National Science Foundation (NSF); the US Department of Energy Office of High-Energy Physics; the Laboratory Directed Research and Development programme of Los Alamos National Laboratory; Consejo Nacional de Ciencia y Tecnología, México (grants 271051, 232656, 260378, 179588, 239762, 254964, 271737, 258865, 243290, 132197 and 281653) (Cátedras 873, 1563); Laboratorio Nacional HAWC de rayos gamma; L’OREAL Fellowship for Women in Science 2014; Red HAWC, México; DGAPA-UNAM (Dirección General Asuntos del Personal Académico—Universidad Nacional Autónoma de México; grants IG100317, IN111315, IN111716-3, IA102715, IN109916 and IA102917); VIEP-BUAP (Vicerrectoría de Investigación y Estudios de Posgrado-Benemérita Universidad Autónoma de Puebla); PIFI (Programa Integral de Fortalecimiento Institucional) 2012 and 2013; PRO-FOCIE (Programa de Fortalecimiento de la Calidad en Instituciones Educativas) 2014 and 2015; the University of Wisconsin Alumni Research Foundation; the Institute of Geophysics, Planetary Physics, and Signatures at Los Alamos National Laboratory; Polish Science Centre grant DEC-2014/13/B/ST9/945 and DEC-2017/27/B/ST9/02272; and Coordinación de la Investigación Científica de la Universidad Michoacana. We thank S. Delay, L. Díaz and E. Murrieta for technical support. We thank R. Mushotzky for providing the spectrum of the XMM-Newton data in the HAWC detection region.

Reviewer information

Nature thanks A. Achterberg and M. Bowler for their contribution to the peer review of this work.

Author information

C.D.R. and H. Zhou analysed the data and performed the maximum likelihood analysis. Multiwavelength modelling of the leptonic and hadronic emission was carried out by K.F. and H. Zhang. S.Y.B. and B.L.D. helped to prepare the manuscript. The entire HAWC Collaboration contributed through the construction, calibration and operation of the detector, the development and maintenance of reconstruction and analysis software, and the vetting of the analysis presented in this manuscript. All authors reviewed, discussed and commented on the results and the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to C. D. Rho or H. Zhou.

Extended data figures and tables

  1. Extended Data Fig. 1 VHE γ rays from MGRO J1908+06 and SS 433/W50.

    The colour scale indicates the statistical significance of the excess counts above the background of contaminating cosmic rays and γ-rays before accounting for statistical trials. The bright extended γ-ray source MGRO J1908+06 is shown at the centre of the left panel with SS 433/W50 at the bottom. The dark contours show X-ray emission from SS 433 and its jets41. The semicircular area indicates the region of interest used to fit the γ-ray observations. The right panel shows the γ-ray excess measured after the fitting and subtraction of γ-rays from the spatially extended source MGRO J1908+06. The dashed box indicates the region shown in Fig. 1. The jet termination regions e1, e2, e3, w1 and w2 observed in the X-ray data are indicated, as well as the location of the central binary.

  2. Extended Data Fig. 2 Distribution of pixel significance in the region of interest of the fit.

    The significance is defined as deviations from the background expectation, in the HAWC sky map (left panel), after fitting and subtraction of emission from MGRO J1908+06 (middle panel), and after fitting and removal of emission from MGRO J1908+06 and the γ rays from w1 and e1 (right panel).

  3. Extended Data Fig. 3 Fraction of jet power needed to produce the observed VHE γ-rays in the hadronic scenario.

    The blue-shaded region shows the energy injection rate of protons, in units of the kinetic luminosity of the jet, required to produce the observed VHE γ-rays by interacting with ambient gas, as a function of the proton confinement time. A gas density of 0.05 cm−3 is adopted for the source vicinity16,27. Most hadronic models require >100% jet power (above the red solid line) and are thus not allowed. Even when the diffusion coefficient is extremely small (for reference, the dashed grey lines show the source age and the confinement time of 200 TeV protons in a 30-pc region in the ISM with Kraichnan- and Kolmogorov-type diffusion) and when the spectral index is much harder than 2, the hadronic scenario still requires a large energy input from the jet.

  4. Extended Data Table 1 Fits to teraelectronvolt emission from SS 433 using nested point source models
  5. Extended Data Table 2 Dependence of measured HAWC flux at 20 TeV on spectral assumption, assuming a power law in energy parameterized by a spectral index and an exponential cutoff parameterized by Ecutoff
  6. Extended Data Table 3 Systematic uncertainties on the flux of VHE γ-rays from SS 433 measured by HAWC

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Fig. 1: VHE γ-ray image of the SS 433/W50 region in Galactic coordinates.
Fig. 2: Broadband spectral energy distribution of the eastern emission region e1.
Extended Data Fig. 1: VHE γ rays from MGRO J1908+06 and SS 433/W50.
Extended Data Fig. 2: Distribution of pixel significance in the region of interest of the fit.
Extended Data Fig. 3: Fraction of jet power needed to produce the observed VHE γ-rays in the hadronic scenario.


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