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.

💡 Research Summary

The authors present a multi‑wavelength study of the powerful microquasar S26 in the galaxy NGC 7793, focusing on its radio lobes, X‑ray hot spots, and the surrounding ionized nebula. Using the Australia Telescope Compact Array (ATCA) they obtained high‑resolution images at 5.5 GHz and 9.0 GHz, which for the first time resolve the radio cocoon and reveal two distinct radio hot spots (lobes) at opposite sides of the core. The total 5.5‑GHz flux density of the cocoon plus lobes is ≈2.1 mJy, corresponding to a radio luminosity about three times that of the Galactic supernova remnant Cassiopeia A at the assumed distance of 3.9 Mpc. The spectral index map shows a steep index (α ≈ ‑0.8) across the lobes, consistent with optically‑thin synchrotron emission from relativistic electrons. In contrast, regions outside the lobes exhibit a flatter spectrum (α ≈ ‑0.3 to ‑0.5), suggesting an additional free‑free component or recent ejections close to the core.

No compact radio core is detected, but Chandra/XMM‑Newton observations reveal an X‑ray point source at the nucleus with a 0.3–8 keV luminosity of ≈6 × 10³⁶ erg s⁻¹, indicating a low‑luminosity accretion state. The radio hot spots have radio luminosities of order 10³⁶ erg s⁻¹ each, while the X‑ray hot spots are displaced outward by ~20 pc along the jet axis and together emit ≈2 × 10³⁷ erg s⁻¹ in the 0.3–8 keV band. Spectral fitting of the X‑ray spots favors a thermal plasma model with temperatures kT ≈ 0.5–0.8 keV, implying that the X‑ray emission originates from shock‑heated gas rather than synchrotron processes. This spatial offset between the radio and X‑ray hot spots mirrors the classic picture of a jet termination shock: the forward shock accelerates electrons that produce synchrotron radio emission, while the downstream shocked ambient medium is heated to X‑ray‑emitting temperatures.

Energy budgeting shows that the total particle energy stored in the bubble is ≈10⁵³ erg. The relativistic electron component inferred from the radio flux accounts for only a few × 10⁵⁰ erg, meaning that the bulk of the energy resides in non‑relativistic particles (protons, nuclei, and low‑energy electrons). The thermal X‑ray emitting gas in the hot spots contains ≈10⁵¹ erg, and the entire cocoon’s hot gas holds ≈10⁵² erg. Thus, the mechanical power of the jet is efficiently transferred to the surrounding interstellar medium, inflating a large, over‑pressured bubble.

The authors also compare the radio morphology with optical emission‑line images (Hα,