Radio lobes and X-ray hot spots 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 x 10^{36} erg/s.
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 3 times
the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hot
spots (combined 0.3-8 keV luminosity ~2 x 10^{37} erg/s) are located ~20 pc
outwards of the radio hot spots (ie, 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 ~10^{53} erg: from the observed radio flux, we estimate that only
about a few 10^{50} erg are stored in the relativistic electrons; the rest is
in protons, nuclei and non-relativistic electrons. The X-ray-emitting component
of the gas in the hot spots contains ~10^{51} erg, and ~10^{52} 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.
