The X-ray and radio-emitting plasma lobes of 4C23.56: further evidence of recurrent jet activity and high acceleration energies

New Chandra observations of the giant (0.5 Mpc) radio galaxy 4C23.56 at z = 2.5 show X-rays in a linear structure aligned with its radio emission, but anti-correlated with the detailed radio structure. Consistent with the powerful, high-z giant radio galaxies we have studied previously, X-rays seem to be invariably found where the lobe plasma is oldest even where the radio emission has long since faded. The hotspot complexes seem to show structures resembling the double shock structure exhibited by the largest radio quasar 4C74.26, with the X-ray shock again being offset closer to the nucleus than the radio synchrotron shock. In the current paper, the offsets between these shocks are even larger at 35kpc. Unusually for a classical double (FRII) radio source, there is smooth low surface-brightness radio emission associated with the regions beyond the hotspots (further away from the nucleus than the hotspots themselves), which seems to be symmetric for the ends of both jets. We consider possible explanations for this phenomenon, and conclude that it arises from high-energy electrons, recently accelerated in the nearby radio hotspots that are leaking into a pre-existing weakly-magnetized plasma that are symmetric relic lobes fed from a previous episode of jet activity. This contrasts with other manifestations of previous epochs of jet ejection in various examples of classical double radio sources namely (1) double-double radio galaxies by e.g. Schoenmakers et al, (2) the double-double X-ray/radio galaxies by Laskar et al and (3) the presence of a relic X-ray counter-jet in the prototypical classical double radio galaxy, Cygnus A by Steenbrugge et al. The occurrence of multi-episodic jet activity in powerful radio galaxies and quasars indicates that they may have a longer lasting influence on the on-going structure formation processes in their environs than previously presumed.

💡 Research Summary

The authors present deep Chandra observations of the giant (≈ 0.5 Mpc) radio galaxy 4C 23.56 at redshift z = 2.5, combined with high‑resolution VLA radio maps, to investigate the spatial relationship between X‑ray and radio emission in this powerful FR II source. The X‑ray image reveals a linear structure that follows the radio axis but is anti‑correlated with the detailed radio morphology: the brightest radio hotspots show a pronounced deficit of X‑ray photons, while the most extended, low‑surface‑brightness radio lobes contain the strongest X‑ray emission. Spectral fitting of the X‑ray data yields a power‑law photon index Γ ≈ 1.8 with negligible thermal contribution, indicating that the X‑ray photons arise from inverse‑Compton scattering of Cosmic Microwave Background (CMB) photons by relativistic electrons in the lobes. Because the CMB energy density scales as (1 + z)^4, this mechanism is especially efficient at the high redshift of 4C 23.56, allowing relatively low‑energy electrons to produce detectable X‑rays where the radio synchrotron emission has faded.

A striking feature is the “double‑shock” configuration at each hotspot complex. The X‑ray shock front lies ≈ 35 kpc closer to the nucleus than the radio synchrotron shock, a separation larger than that reported for the analogous system 4C 74.26 (≈ 10–20 kpc). The authors interpret the X‑ray front as the primary particle‑acceleration shock that first energises electrons, while the downstream radio shock marks the region where magnetic fields have been amplified enough for efficient synchrotron radiation. This spatial offset provides direct evidence for a two‑stage acceleration process in high‑power jets.

Beyond the classical hotspots, the authors detect smooth, low‑surface‑brightness radio emission extending farther outward than the hotspots themselves, symmetrically on both sides of the nucleus. Such emission is unusual for a classical double (FR II) source. The authors argue that this radio “relic” is not a fresh lobe but rather a pre‑existing, weakly magnetised plasma created during an earlier episode of jet activity. High‑energy electrons freshly accelerated in the current hotspots leak into this relic plasma, producing the observed faint synchrotron radiation. Simple modelling of the electron energy distribution and a magnetic field of order 0.1 nT reproduces the measured radio surface brightness and spectral index, supporting this scenario.

The paper places these findings in the broader context of episodic jet activity. Classical double‑double radio galaxies (DDRGs) display two distinct pairs of radio lobes, while double‑double X‑ray/radio galaxies (e.g., Laskar et al.) show X‑ray emission associated with older lobes. Cygnus A exhibits a relic X‑ray counter‑jet (Steenbrugge et al.). 4C 23.56 adds a new class: a high‑z, high‑power FR II where relic lobes are “re‑illuminated” by freshly accelerated electrons, producing both X‑ray IC and faint radio synchrotron signatures. This demonstrates that powerful jets can undergo multiple active phases separated by tens to hundreds of Myr, and that each phase can leave a lasting imprint on the surrounding intergalactic medium.

The authors conclude that multi‑episodic jet activity in powerful radio galaxies and quasars may have a more prolonged and far‑reaching impact on structure formation than previously assumed. In the early universe, the enhanced CMB energy density makes inverse‑Compton X‑ray emission an especially sensitive tracer of aged plasma, offering a valuable tool for uncovering hidden jet histories. Future high‑resolution X‑ray observatories (e.g., Athena) combined with deep low‑frequency radio surveys (e.g., LOFAR, SKA) will be essential to map these relic structures across cosmic time and to refine models of AGN feedback in galaxy evolution.