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Radio lobes and X-ray hotspots in the microquasar S26

We have studied the structure and energetics of the powerful microquasar/shock-ionized nebula S26 in NGC 7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free–free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3–8 keV luminosity 6 × 1036  erg s−1. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is 2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about three times the luminosity of Cas A. The total 9.0-GHz flux is 1.6 mJy. The X-ray hotspots (combined 0.3–8 keV luminosity 2 × 1037  erg s−1) are located 20 pc outwards of the radio hotspots (i.e. downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ∼1053 erg: from the observed radio flux, we estimate that only approximately a few times 1050 erg is stored in the relativistic electrons; the rest is stored in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hotspots contains ∼1051 erg, and ∼1052 erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate.