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